which are totally devoid of skeleton. Similarly it does not appear to be a matter of great significance that some forms (Polycyttaria) form colonies, instead of the central capsules separating from one another after fission has occurred.

It is important to note that the skeleton of silex or acanthin does not correspond to the shell of other Gymnomyxa, which appears rather to be represented by the membranous central capsule. The skeleton does, however, appear to correspond to the spicules of Heliozoa, and there is an undeniable affinity between such a form as Clathrulina (Fig. VII. 2) and the Sphærid Peripylæa (such as Heliosphæra, Fig. XIV. 14). The Radiolaria are, however, a very strongly marked group, definitely separated from all other Gymnomyxa by the membranous central capsule sunk in their protoplasm. Their differences inter se do not affect their essential structure. The variations in the chemical composition of the skeleton and in the perforation of the capsule do not appear superficially. The most obvious features in which they differ from one another relate to the form and complexity of the skeleton, a part of the organism so little characteristic of the group that it may be wanting altogether. It is not known how far the form-species and form-genera which have been distinguished in such profusion by Haeckel as the result of a study of the skeletons are permanent (i.e., relatively permanent) physiological species. There is no doubt that very many are local and conditional varieties of a single Protean species. The same remark applies to the species discriminated among the shell-bearing Reticularia. It must not be supposed, however, that less importance is to be attached to the distinguishing and recording of such forins because we are not able to assert that they are permanent species.

The yellow cells (of spherical form, 005 to 0.15 of a millimetre in diameter) which occur very generally scattered in the extracapsular protoplasm of Radiolaria were at one time regarded as essential components of the Radiolarian body. Their parasitic nature is now rendered probable by the observations of Cienkowski (43), Brandt (44), and Geddes (45), who have established that each cell has a cellulose wall and a nucleus (Fig. XIV. 6 to 13), that the protoplasm is impregnated by chlorophyll which, as in Diatoms, is obscured by the yellow pigment, and that & starchlike substance is present (giving the violet reaction with iodine). Further, Cienkowski showed, not only that the yellow cells multiply by fission during the life of the Radiolarian, but that when isolated they continue to live; the cellulose envelope becomes softened; the protoplasm exhibits amoeboid movements and escapes from the envelope altogether (Fig. XIV. 13) and multiplies by fission. Brandt has given the name Zooxanthella nutricola to the parasitic unicellular Alga thus indicated. He and Geddes have shown that a similar organism infests the endoderm cells of Anthozoa and of some Siphonophora in enormous quantities, and the former has been led, it seems erroneously, to regard the chlorophyll corpuscles of Hydra viridis, Spongilla, and Ciliata as also parasitic Algae, for which he has coined the name Zoochlorella. The same arguments which Brandt has used to justify this view as to animal chlorophyll would warrant the creation of a genus "Phytochlorella" for the hypothetical Alga which has hitherto been described as the chlorophyll corpuscles" of the cells of ordinary green plants. Zooxanthella nutricola does not, for some unknown reason, infest the Acanthometridea, and it is by no means so universally present in the bodies of the Silico-skeleta as was supposed before its parasitic nature was recognized.

The streaming of the granules of the protoplasm has been observed in the pseudopodia of Radiclaria as in those of Heliozoa and Reticularia; it has also been seen in the deeper protoplasm; and granules have been definitely seen to pass through the pores of the central capsule from the intracapsular to the extracapsular protoplasm. A feeble vibrating movement of the pseudopodia has been occasionally noticed.

The production of swarm-spores has been observed only in Acanthometra and in the Polycyttaria and Thalassicollida, and only in the two latter groups have any detailed observations been made. Two distinct processes of swarm-spore production have been observed by Cienkowski (43), confirmed by Hertwig (46)-distinguished by the character of the resulting spores which are called "crystalligerous" (Fig. XIV. 15) in the one case, and "dimorphous" in the other (Fig. XIV. 16). In both processes the nucleated protoplasm within the central capsule breaks up by a more or less regular cell-division into small pieces, the details of the process differing a little in the two cases. In those individuals which produce crystalligerous swarm-spores, each spore encloses a small crystal (Fig. XIV. 15). On the other hand, in those individuals which produce dimorphous swarm-spores, the contents of the capsule (which in both instances are set free by its natural rupture) are seen to consist of individuals of two sizes

66

spores and microspores," neither of which contain crystals (Fig. XIV. 16). The further development of the spores has not been observed in either case. Both processes have been observed in the same species, and it is suggested that there is an alternation of sexual and asexual generations, the crystalligerous spores

| developing directly into adults, which in their turn produce in their central capsules dimorphous swarm-spores (macrospores and microspores), which in a manner analagous to that observed in the Volvocinean Flagellata copulate (permanently fuse) with one another (the larger with the smaller) before proceeding to develop. The adults resulting from this process would, it is suggested, produce in their turn crystalligerous swarm-spores. Unfortunately we have no observations to support this hypothetical scheme of a life-history.

Fusion or conjugation of adult Radiolaria, whether preliminary to swarm-spore-production or independently of it, has not been observed-this affording a distinction between them and Heliozoa, and an agreement though of a negative character, with the Reticu laria.

Simple fission of the central capsule of adult individuals and subsequently of the whole protoplasmic mass has been observed in several instances, and is probably a general method of reproduction in the group.

The siliceous shells of the Radiolaria are found abundantly in certain rocks. They furnish, together with Diatoms and Spongespicules, the silica which has been segregated as flint in the Chalk formation. They are present in quantity (as much as 10 per cent.) in the Atlantic ooze, and in the celebrated "Barbados earth" (a Tertiary deposit) are the chief components.

GRADE B. CORTICATA, Lankester, 1878 (64). Characters.-Protozoa in which the protoplasm of the cell-body, in its adult condition, is permanently differentiated into two layers, an outer denser cortical substance and an inner more fluid medul lary substance (not to be confused with the merely temporary distinction of exoplasm and endoplasm sometimes noted in Gymnomyxa, which is not structural but due to the gravitation and self-attraction of the coarser granules often embedded in the uniformly fluid protoplasm).

Since the Corticata have developed from simple Gymnomyxa exhibiting both amoeboid and flagellate phases of form and activity, it results (1) that the forms of the body of many Corticata are traceable to modifications of these primitive forms; (2) that the young stages of the Corticata are in the lower classes of that group typical flagellule or amœbulæ; and (3) that there are certain archaic forms included in those lower classes whose position there is doubtful, and which might be with almost equal propriety assigned to the Gymnomyxa, since they are transitional from that lower grade to the higher grade of Corticata.

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CLASS I. SPOROZOA, Leuckart (47); Syn. Gregarinida, Auct. Characters.-Corticata parasitic in almost all classes and orders of animals, imbibing nutriment from the diffusible albuminoids of their hosts and therefore mouthless. In typical cases there is hatched from a chlamydospore one or more modified nucleate or non-nucleate flagellula (falciform young, drepanidium phase). The flagellula increases in size and differentiates cortical and medullary substance. Fission is common in the younger stages of growth. The movements now become neither vibratile nor amoboid but definitely restrained, and are best described as euglenoid" (cf. Flagellata, Fig. XX. 27, 28). The nucleus is single, large, and spherical. No contractile vacuole and rarely any vacuole is present. A size of th inch may be attained in this phase, which may be definitely spoken of as the euglena phase corresponding to the amoeba phase of Gymnomyxa. It is usually of oblong form, with sac-like contractile wall of cortical substance, but may be spherical (Coccidiidea) or even amoeboid (Myxosporidia). Conjugation, followed directly or after an interval by sporulation, may now ensue. The conjugated individuals (two), or sometimes a single individual, become encysted. The contents of the cysts now rapidly divide (by a process the details of which are unknown) into minute ovoid nucleated (?) bodies; sometimes a portion of the protoplasm is not converted into spores but may form sporoducts lef. capillitium of Mycetozoa). Each piece acquires a special chitin-like colourless coat, and is then a chlamydospore. Rarely one spore only is formed from the whole contents of a cyst. The spore-coat is usually thick, and remarkable for processes and other accessory developments. The included protoplasm of the chlamydospore frequently divides into several pieces before hatching. These usually, when set free from the spore-coat, have the form of modified nucleated flagellulæ, i.e., flagellule in which the protoplasm is not drawn out into a thread-like flagellum but exhibits an elongate form, uniformly endowed with vibratile activity. With few (if any) exceptions, the falciform young thus characterized penetrates a cell of some tissue of its host and there undergoes the first stages of its growth (hence called Cytozoa). In some forms the pre-cystic phase never escapes from its cell host. In other cases it remains connected with the hospitable cell long after it has by growth exceeded by many hundred times the bulk of its quondam entertainer; often it loses all connexion with its cell host and is carried away to some other part of the infested animal before completing its growth and encysting.

The Sporozoa are divided into four sub-classes, differing from one another according to the form and development attained by the euglena phase. We shall place the most highly developed first, not only because our knowledge about it is most complete, but because it is possible that one at least of the other sub-classes is derived by degeneration from it.

SUB-CLASS I. Gregarinidea, Bütschli (9). Characters.-Sporozoa in which the euglena phase is dominant, being relatively of large size, elongate in form, definitely shaped, having contractile but not viscid cortex, and exhibiting often active nutritional and locomotor phenomena. Though usually if not invariably cell-parasites in early youth, they become free before attaining adult growth, and inhabit either the body-cavity or the intestine of their hosts. Many spores are produced in the encysted phase. The spores have an oblong, sometimes caudate coat, and produce each one or several falciform young. At present only known as parasites of Invertebrata.

tris, L.). Two individuals, which are implanted by one extremity at ò in two epithelial cells of the rosette of the spermatic duct. a, nucleus of the Monocystis. 5. Tailed chlainydospores of Monocystis senuridis, Köll. 6. Two M. agilis encysted, spores forming on the surface of the protoplasm. 7. A similar cyst further advanced in spore-formation (see Fig. XVIII.). 8. Spore of M. agilis, now elongated but still naked. a, nucleus. X 1400. 9. The spore has now encased itself in a naviculashaped coat. a, nucleus. 10. The spore protoplasm has now divided into several falciform swarm-spores, leaving a portion of the protoplasmi unused. b, Schneider's residual core. 11. Optical transverse section of a completed spore. b, Schneider's residual core. 12. Chlamydospore 13,

18.

of Klossia chitonis, nov. sp., from the liver of Chiton (original.) 14. Chlamydospore of Monocystis nemertis, Köll., liberating falciform young. b, Schneider's residue. 15. Monocystis pellucida, Koll. (from Nereis); x 150; to show the very thiok cortical substance and its fibrilla tion (after Lankester, 54). 16. Monocystis sænuridis, Köll., two indivi duals adhering to one another (a'syzygium). For spores see 5. 17. Monocystis aphrodite, Lankester (55); x 60; remarkable among Monocystids for its long proboscis resembling the epimerite of some Septata. Klossia helicina, Aim. Schn., from the kidney of Helix hortensis. A single cell of the renal epithelium in which a full-grown Klossia is embedded. a, nucleus of the Klossia; a', nucleus of the renal cell. 19. Cyst of Klossia helicina, the contents broken up into spherical chlamydospores. 20. Single spore from the last, showing falciform young and a Schneider's residue b. 21. The contents of the same spore. 22. A small renal cell of Helix containing two of the youngest stage of Klossia. 23. Monocystis sagittata, Leuck., from the intestine of Capitella capitata; X 100. 24 to 31. Coccidium oviforme, Leuck., from the liver of the Rabbit: -24, adult individual encysted; 25, the protoplasm contracted-a, nucleus; 26, 27, division into four spores, as yet naked; 28, 29, the spores have acquired a covering, i.e., are chlamydospores, and each contains a single falciform young; 30, 31, two views of a chlamydospore more highly nragnified so as to show the single falciform young (from Leuckart). 32. Klossia octopiana, Aim. Schn., from Cephalopoda. a, nucleus; b, cyst-membrane. X 200 diam. 33. Single spherical spore of the same; x 1400 diam; showing numerous falciform young, and b, Schneider's residue. Myxidium Lieberkühnii, Bütschli, one of the Myxosporidia, from the bladder of the Pike (Esox); creeping euglena phase, showing strongly lobed amoeboid character (pseudopodia and undifferentiated (?) cortex); x 60 diam. 35-39. Eimeria falciformis, Eimer sp., from the Mouse:35, an adult non-encysted individual inhabiting an epithelial cell of the intestine of the mouse; 36, encysted phase; 37, clear corpuscles appear in the encysted protoplasin; 38, the protoplasm now forms a single spore containing several falciform young; b, Schneider's residue; 39, isolated spore showing falciform young, and b, Schneider's residue. 40. Chlamydospore of Myxobolus Mülleri, Bütschli, one of the Myxosporidia from the gills of Cyprinoid Fishes. a, nucleus; b, refringent corpuscle; c, polar body or thread-capsule. 41. A similar chlamydospore which has ejected the filaments from its thread capsules. Chlamydospore of a Myxosporidium infesting the kidney of Lota vulgaris. c, polar body (psorosperm of authors). 43, 44. Chlamydospores of

84.

42.

45

a Myxosporidium from the gills of Perca (psorosperm of authors). Compare with the tailed chlamydospore of Monocystis sænuridis, 5. -47. Drepanidium ranarum, Lankester, the falciform young of an unascertained Coccidiide infesting the Frog (supposed by Gaule to be produced by the blood corpuscles):-45, specimen stained by iodine; 46, redblood corpuscle of Frog, showing b, two contained Drepanidia, and a, the nucleus of the blood corpuscle; 47, living Drepanidium. 48. Chlamydospore of Lieberkühn's Coccidium of the Frog's kidney, perhaps belonging to the life-cycle of Drepanidium ranarum. The spore contains two falciform young (Drepanidia?) and a Schneider's residue. 49. Chlamydospore of Monocystis thalassemæ, Lankester, containing numerous falciform young. 50, 51. Sarcocystis Miescheri, Lankester:-50, falciform young escaped from chlamydospores; 51, adult euglena phase inhabiting a striated muscle fibre of the Pig.

ORDER 1. HAPLOCYTA, Lankester.

Characters.-Gregarinidea in which there is never at any time a partition of the medullary substance into two or more chambers. The euglenoid is always a single contractile sac with one mass of medullary substance in which floats the large vesicular transparent nucleus. Spores larger than in the next group, each producing several falciform young.

Genus unicum.-Monocystis, Stein, 1848. The various generic subdivisions proposed by Aim. Schneider (48), and accepted by Bütschli, appear to the present writer to have insufficient characters, and serve to complicate rather than to organize our knowledge of the subject. We do not yet know enough of the sporulation and subsequent development of the various monocystic Gregarinides to justify the erection of distinct genera.

Monocystis agilis, Stein, Fig. XVII. 1, 2, 3, 6, 7, 8, 9, 10, 11, and Fig. XVIII. is the type. The other species of Monocystis occur chiefly (and very commonly) in marine Annelids, Platyhelminthes, Gephyræa, and Tunicata; not in Arthropoda, Mollusca, nor Vertebrata. The only definite differences which they present of possibly more than specific worth, as compared with M. agilis, are in the form of the chlamydospores, which are sometimes tailed, as in M. sænuridis (Fig. XVII. 5), and in M. nemertis (Fig. XVII. 13) and M. sipunculi, and further also certain differences in the general form, as for instance the anchor-like M. sagittata (Fig. XVII. 23), and the proboscidiferous M. aphrodita (Fig. XVII. 17). The fine parallel striation of the cuticule in sne species (M. serpulæ, &c.) might also be made the basis of a generic or subgeneric group.

On the whole it seems best to leave all the species for the present in the one genus Monocystis, pending further knowledge. It seems probable that more than one species (at least two, M. agilis and M. magna) infest the common Earthworm.

ORDER 2. SEPTATA, Lankester. Characters.-Gregarinidea in which in the adult the medullary substance is separated into two chambers-a smaller anterior (the

protomerite) and a larger posterior (the deutomerite), in which lies the nucleus. There is frequently if not always present, either in early growth or more persistently, an anterior proboscis-like appendage (the epimerite) growing from the protomerite. The epimerite serves to attach the parasite to its host, and may for that purpose carry hooklets. It is always shed sooner or later. The phase in which it is present is called a "cephalont," the phase after it has broken off a "sporont" (see Fig. XIX. 22, 23). The spores are smaller than in the preceding group, often very minute, and sometimes the cyst is complicated by the formation of sporoducts, and by a kind of "capillitium" of residual protoplasm (Fig. XIX. 2). Spores producing each only a single (?) falciform young.

Genera.-Gregarina, Dufour; Hoplorhynchus, Von Carus.

[The numerous genera which have been proposed at different times by Hammerschmidt and others, and more recently by Aimé Schneider, appear to the present writer to be unserviceable, owing to the fact that our knowledge is as yet very incomplete. A good basis for generic or family distinctions might probably be found in the greater or less elaboration of the cyst and the formation or not of sporoducts. But of the majority of Septata we do not know the cysts or the history of sporulation; we merely know that some have simple cysts with complete sporulation leaving no residue of protoplasm, and that others form cysts with double walls and elaborate tubular ducts, whilst a part of the protoplasm is not sporulated but forms a capillitium (Fig. XIX. 2).

>

Another possible basis for generic division of the Septata may be found in the characters of the epimerite. This may be present or absent altogether. It may exist only in the young condition or persist until growth is completed. It may be simple, short, elongate, or provided with hooklets. The presence of hooklets on the epimerite is the only character which at present seems to serve conveniently for generic distinction. With regard to the other points mentioned we are not sufficiently informed, since we know the complete history of development from the young form set free from the spore in only one or two cases.]

The Septata are found exclusively in the alimentary canals of Arthropoda (Insects, Myriapods, Crustacea, not Arachnida). See Fig. XIX. for various examples of the group.

FIG. XVIII.-Cyst of Monocystis agilis, the common Gregarinide of the Earthworm; x 750 diam.; showing ripe chlamydospores and complete absence of any residual protoplasm or other material in the cyst (original).

SUB-CLASS II. Coccidiidea, Bütschli (9).

Sporozoa in which the euglena phase remains of relatively minute size, of spherical shape and simple egg-cell-like structure. It is not locomotive, but continues, until the cyst is formed, to inhabit a single cell of the host. Many, few, or one single chlamydospore are formed in the cyst. One or more falciform young escape from each spore, and exhibit active movements (flagellulalike) leading to a penetration of a tissue-cell by the young form as in Gregarinidea. Many are parasites of Vertebrata.

ORDER 1. MONOSPOREA, Aim. Schn.

Characters.-The whole content of the cyst forms but a single

spore.

Genus unicum.-Eimeria (in the intestinal epithelium of Triton, Frog, Sparrow, Mouse, and the Myriapods Lithobius and Glomeris, Fis. XVII. 35 to 39).

2

FIG. XIX.-Sporozoa (Septata). 1. Gregarina blattarum, Siebold, from the intestine of Blatta orientalis; x 80. A syzygium of two individuals Each animal consists of a small anterior chamber, the protomerite, and a large posterior chamber, the deutomerite, in which is the nucleus a. Over-ripe cyst of Gregarina blattarum, with thick gelatinous envelope e and projecting sporoducts d. The spores have been nearly all discharged, but a mass of tnem still lies in the centre of the cyst b. The specimen has been treated with dilute KHO, and the granular contents of the cyst dissolved. Around the central mass of spores is rendered visible the net work of protoplasmic origin in which the ejected spores were embedded, This distinctly resembles in origin and function the capillitium of Mycetozoa (Fig. III.). u, the plasmatic channels leading to the everted sporoducts; b, the still remaining spores; c, the proper cyst-wall; d, the everted sporoducts; e, the gelatinous envelope. 3. A ripe spore (chlamydo spore) of Gregarina blattarum, a long time after its escape from the cyst; > O diam. 4. Commencing encystment of a syzygium of G. blatt a, protomerite of one individual; b, gelatinous envelope; c, pro rite of the second individual. 5. Three epithelial cells of the mid-gut of Blatta orientalis, into the end of each of which an extremely young Gregarina blattarum has made its way. 6. Further development of the young Gregarina; only the epimerite a is now buried in the substance of the epithelial cell, and this will soon break off and set the Gregarina free. It is now a "cephalont"; it will then become a "sporont." 7. Basal part of an everted sporoduct of Gregarina blattarum. a, granu lar-fibrous mass investing the base of the duct; b, commencement of the plasmatic channel in the interior of which the sporoduct was produced as an invaginated cuticular formation before its eversion. 8. Gregarina gigantea, E. Vaa Ben., from the intestine of the Lobster; x 150. a, nucleus.

9. Anterior end of the same more highly magnified. a, protomerite ; b, layer of circular fibrille lying below the cuticle; c, cortical substance of the deutomerite; d, medullary substance of the deutomerite. 10. Two spores of Gregarina gigantea (after Bütschli), showing the very thick coat of the spore. 11-15. Stages in the development of Gregarina gigantea:-11, recently escaped from the spore-coat, no nucleus; 12, still no nucleus, one vibratile and one motionless process; 13, the two processes have divided; one here drawn has developed a nucleus; 14, further growth; 15, the dentonierite commences to develop. 16. Cysts of Gregarina gigantea, from the rectum of the Lobster. The double contents are believed by Ed. Van Beneden to be due not to conjugation previous to encystment to subsequent 18.

Stell, from the intestine of Blaps mortisaga:-17, cephalont phase, with a leng proboscis-like epimerite a, attached to the protomerite b; 18, sporont phase, the epimerite having been cast preliminarily to syzygy and encystment.- 19. Gregarina Manieri, Aim. Schneider, from the intestine of Timarcha knebricosa, to show the network of anastomosing fibres beneath the cuticle, similar to the annular fibrilla of G. gigantea shown in 9. 20. Gregarina (Hoplorhynchus) obligacanthus, Stein, from the intestine of the larva of Agrion. Cephalont with spine-crowned epimerite a. 21. Spores of Gregarina oligacanthus. 22, 23. Gregarina (IIoplorhynchus) Dujardini, Aim. Schneider, from the intestine of Lithobius forficatus:-22, specimen with epimerite a, therefore a "cephalont"; 23, specimen losing its epimerite by rupture and becoming a "sporont."

ORDER 2. OLIGOSPOREA, Aim. Schn. Characters.--The cyst-content develops itself into a definite and constant but small number of spores.

Genus unicum.-Coccidium, Leuck. (in intestinal epithelium and liver of Mammals, and some Invertebrates, Figs. XVII. 24 to 31). ORDER 3. POLYSPOREA.

Characters.-The cyst-content develops itself into a great number of spores (sixty or more).

Genus unicum.-Klossia, Aim. Schn. Three species of Klossia are found in Mollusca-viz., in Helix, in Cephalopods, and in Chiton. Schneider's genus, Adelea, from Lithobius, appears to belong here. Kloss (49) discovered the parasite of the renal cells of Helix hortensis represented in Fig. XVII. 18, 19, 20, 21, and 22; Schneider that of Cephalopods, Fig. XVII. 32, 33. In Chiton Dr Tovey has discovered a third species with very remarkable spores, which are here figured for the first time (Fig. XVII. 12).

The Drepanidium Ranarum (Fig. XVII. 45, 46, 47), discovered by Lankester (50) in the Frog's blood, is probably the falciform young of a Coccidium parasitic in the Frog's kidney, and discovered there by Lieberkühn (51). A spore of this Coccidium is shown in Fig. XVII. 48; whilst in 46 two Drepanidia which have penetrated a red-blood corpuscle of the Frog are represented.

The Polysporous Coccidiidea come very close to the Gregarinide genus Monocystis, from which they may be considered as being derived by an arrest of development. The spores and falciform young of the Coccidiidea are closely similar to those of Monocystis, and the young in both cases penetrate the tissue-cells of their host; but in Monocystis this is only a temporary condition, and growth leads to the cessation of such "cell-parasitism." On the other hand, growth is arrested in the Coccidiidea, and the organism is permanently a cell-parasite.

Since the parasitism is more developed in the case of a cell-parasite than in the case of a parasite which wanders in the body cavity, it seems probable that the Coccidiidea have been derived from the Gregarinidea rather than that the reverse process has taken place.

SUB-CLASS III. Myxosporidia, Bütschli. Characters.-Sporozoa in which the euglena-phase is a large multinucleate amoeba-like organism (Fig. XVII. 34). The cysts are imperfectly known, but appear to be simple; some attain a diameter of two lines. The spores are highly characteristic, having each a thick coat which is usually provided with a bifurcate process or may have thread capsules (like nematocysts) in its substance (Fig. XVII. 40, 41, 42, 43, 44).

The spores contain a single nucleus, and are not known to produce falciform young, but in one case have been seen to liberate an ambula. The further development is unknown. The Myxosporidia are parasitic beneath the epidermis of the gills and fins, and in the gall-bladder and urinary bladder of Fishes, both freshwater and marine.

Genera.-Myxidium, Bütschli (Pike, Fig. XVII. 34); Myxobolus, Bütschli (Cyprinoids); Lithocystis, Giard (the Lamellibranch Echinocardium).

The Myxosporidia are very imperfectly known. They present very close affinities to the Mycetozoa, and are to be regarded as a connecting link between the lower Gymnomyxa and the typical Sporozoa. Possibly their large multinucleate amoeba phase is a plasmodium formed by fusion of amoebulæ set free from spores, though it is possible that the many nuclei are the result of a division of an original single nucleus, preparatory to sporulation.

Their spores are more elaborate in structure than those of any other Protozoa, and are more nearly paralleled by those of some species of Monocystis than by those of Mycetozoa. The threadcapsules of the spores are identical in structure with those of Hydrozoa, and probably serve as organs of attachment, as do the furcate processes of the spore-case. It is not certain that a definite

|

cyst is always or ever formed, but as occurs rarely in some Gregarinidea, the spores may be formed in a non encysted amoeba form. Although pseudopodia, sometimes short and thread-like, have been observed in the amoeba phase, yet it is also stated that a distinction of cortical and medullary substance obtains.

The "psorosperms" of J. Müller are the spores of Myxosporidia.

SUB-CLASS IV. Sarcocystidia, Bütschli.

(This division is formed by Bütschli for the reception of Sarcocystis, parasitic in the muscular fibres of Mammals, and of Amæbidium, parasitic in Crustacea. Both are very insufficiently known, but have the form of tubular protoplasmic bodies in which numerous ovoid spores are formed from which falciform young escape.)

Genera. Sarcocystis, Lankester; Amabidium, Cienkowski (52). Sarcocystis (Fig. XVII. 50, 51, S. Miescheri, Lank.), was first observed by Miescher in the striated muscle-fibres of the Mouse; then by Rainey in a similar position in the Pig, and taken by him for the youngest stage in the development of the cysts of Taenia solium; subsequently studied by Beale and others in connexion with the cattle-plague epidemic, and erroneously supposed to have a causal connexion with that disease. It is common in healthy butcher's meat. See Leuckart (47).

Further remarks on the Sporozoa.-The Sporozoa contrast strongly with the large classes of Gymnomyxa, the Heliozoa, Reticularia, and Radiolaria, as also with the Ciliate and Tentaculiferous Corticata, by their abundant and rapidly recurrent formation of spores, and agree in this respect with some Proteomyxa, with Mycetozoa, and some Flagellata. Their spores are remarkable for the firm, chitin-like spore-coat and its varied shapes, contrasting with the cellulose spherical spore-coat of Mycetozoa and with the naked spores of Radiolaria and Flagellata.

The protoplasm of the more highly developed forms (Gregarinidea) in the euglenoid phase exhibits considerable differentiation. Externally a distinct cuticle may be present, marked by parallel ruga (Monocystis serpulæ) or by fine tubercles (Monocystis sipunculi). A circlet of hooks may be formed by the cuticle at one end of the body. Below the cuticle is sometimes developed a layer of fibrils running transversely to the long axis of the body (Fig. XIX. 9 and 19), which have been regarded as contractile, but are probably cuticular. The cortical layer of protoplasm below these cuticular structures is dense and refringent and sometimes fibrillated (Monocystis pellucida, Fig. XVII. 15). It is the contractile substance of the organism, and encloses the finely granular more liquid medullary substance. The granules of the latter have been shown by Bütschli (9) to give a starch-like reaction with iodine, &c. Probably the protoplasm in which they lie is finely reticulate or vacuolar, and when the granules are few it is actually seen to be so. No contractile vacuole is ever present. In Myxosporidia the medullary protoplasm is coloured yellow by hæmatoidin derived from the blood of its host or by absorbed bile-pigment, and also contains small crystals.

The nucleus of the Gregarinidea is a large clear capsule, with a few or no nucleolar granules. It has never been seen in a state of division, and it is not known what becomes of it during sporulation, though sporulating Gregarinidea have been observed with many minute nuclei scattered in their protoplasm, presumably formed by a breaking up of the single nucleus.

The habit of attaching themselves in pairs which is common in Gregarinidea is perhaps a reminiscence of a more extensive formation of aggregation plasmodia (compare Mycetozoa). The term "syzygium" is applied to such a conjunction of two Gregarinidea; it is not accompanied by fusion of substance. The formation of cysts is not connected with this pairing, since the latter occurs in young individuals long before encystment. Also cysts are formed by single Gregarinidea, as is always the case in the non-motile Coccidiidea.

The encystment always leads to the formation of spores, but in rare cases sporulation has been observed in unencysted Gregarinidea, and it occurs perhaps normally without true cyst-formation in the Myxosporidia.

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The cell-parasitism of the young Sporozoa, and their flagellulalike (falciform) young and active vibratile movement, are points indicating affinity with the lower Gymnomyxa, and especially with those Proteomyxa, such as Vampyrella and Plasmodiophora, which are cell-parasites. Indeed it is probable that we have in this fact of cell-parasitism, and especially of parasitism in animal cells, a basis for the theoretical association of several unicellular organisms. The Haplococcus of Zopf (regarded by him as a Mycetozoon) is parasitic in the muscular cells of the Pig, and is probably related to Sarcocystis. Recently Von Lendenfeld (53) has described in Australia an amoeba-like organism as parasitic in the skin of Sheep, which will probably be found to be either a Sporozoon or referable to those parasitic spore-producing Proteomyxa which are separated from Sporozoa only by their negative characters (see previous remarks on the negative characters of Proteomyxa).

The application of the name "Gregarines" has sometimes been

made erroneously to external parasitic organisms, which have nothing in common with the Sporozoa. This was the case in regard to a fungoid growth in human hair-the so-called "chignon Gregarine. The Silk-worm disease known as "pebrine" has also been attributed to a Gregarine. It seems probable that the parasitic organism which causes that disease is (as is also the distinct parasite causing the disease known as "flaccidozza" in the same animals) one of the Schizomycetes (Bacteria). No disease is known at present as due to Sporozoa, although (e.g., the Klossia chitonis) they may lead to atrophy of the organs of the animals which they infest, in consequence of their enormous numbers. Coccidia and Sarcocystis are stated to occur in Man.

CLASS II. FLAGELLATA,1 Ehrenberg. Characters.-Corticata in which the dominant-phase in the lifehistory is a corticate flagellula, that is, a nueleated cell-body provided with one or a few large processes of vibratile protoplasm. Very commonly solid food particles are ingated through a distinct cell-mouth or aperture in the cortical protopiasm, though in some an imbibition of nutritive matter by the whole surface and a nutritional process chemically resembling that of plants (holophytic), chlorophyll being present, seems to occur.

Conjugation followed by a breaking up into very numerous minute naked spores is frequent in some; as also a division into small individuals (microgonidia), which is followed by their conjugation with one another or with big individuals (macrogonidia) and subsequent normal growth and binary fission.

Many have a well-developed cuticle, which may form collar-like outgrowths or stalk-like processes. Many produce either gelatinous or chitin-like shells (cups or coenoecia), which are connected so as to form spherical or arborescent colonies;. in these colonies the protoplasmic organisms themselves produce new individuals by fission, which separate entirely from one another but are held together by the continuity, with those already existing, of the new shells or jelly-houses or stalk-like supports produced by the new individuals. A single well-marked spherical nucleus, and one or more contractile vacuoles, are always present in the full-grown form.

Often, besides ingested food-particles, the protoplasm contains starch granules (amylon nucleus), paramylum corpuscles, chromatophors and chlorophyll corpuscles, some of which may be so abundant as to obscure the nucleus. One or two pigment spots (stigmata or so-called eye-spots) are often present at the anterior end of the body.

SUB-CLASS I. Lissoflagellata, Lankester.

Never provided with a collar-like outgrowth around the oral pole.

ORDER 1. MONADIDEA, Bütschli.

Characters.-Lissoflagellata of small or, very small size and simple structure; often naked and more or less amoeboid, sometimes forming tests. Usually colourless, seldom with chromatophors. With a single anterior large flagellum or sometimes with two additional paraflagella. A special mouth-area is often wanting, sometimes is present, but is never produced into a well-developed pharynx.

Fam. 1. RHIZOMASTIGINA, Bütschli. Simple mouthless forms with 1 to 2 flagella; either permanently exhibiting a Gymnomyxalike development of pseudopodia or capable of passing suddenly from a firm-walled into a Gymnomyxa-like condition, when the flagella may remain or be drawn in. Ingestion of food by aid of the pseudopodia.

Genera.-Mastigamaba, F. E. Schultze; Ciliophrys, Cienkowski (65); Dimorpha, Gruber; Actinomonas, Kent; Trypanosoma, Gruby (parasitic in the blood of Frogs and other Amphibia and Reptiles, Fig. XX. 21, 22). The Rhizomastigina might all be assigned to the Proteomyxa, with which they closely connect the group of Flagellata. The choice of the position to be assigned to such a form as Ciliophrys must be arbitrary.

Fam. 2. CERCOMONADINA, Kent. Minute oblong cell-body which posteriorly may exhibit amoeboid changes. One large anterior flagellum. Mouth at the base of this organ. Reproduction by longitudinal fission and by multiple fission producing spores in the encysted resting state.

Genera.-Cercomonas, Duj. (Fig. XX. 32, 33); Herpetomonas, S. Kent; Oikomonas, Kent (Monas, James Clark; Pseudospora, Cienkowski, Fig. XX. 29, 30, 31); Ancyromonas, S. K.

Fam. 3. CODONECINA, Kent. Sinall colourless monads similar to Ofkomonas in structure, which secrete a fixed gelatinous or membranous envelope or cup.

Genera.-Codonaca, James Clark; Platythaca, Stein.

Fam. 4. BIKECINA, Stein. Distinguished from the last family by the fact that the monad is fixed in its cup by a contractile thread-like stalk; cup usually raised on a delicate stalk. Genera.-Bicosoca, J. Cl.; Poteriodendron, Stein.

1 Butschli's work (9) has been pretty closely followed in the diagnosis of the groups of Flagellata and the enumeration of genera here given.

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FIG. XX.-Flagellata. 1. Chlamydomonas pulvisculus, Ehr. (=Zygoselmis, From.); one of the Phytomastigoda; free-swimming individual. a. nucleus; b, contractile vacuole; c, starch corpuscle; d, cellulose investment; e, stigma (eye-spot), 2. Resting stage of the same, with fourfold division of the cell contents. Letters as before. 3. Breaking up of the cell-contents into minute biflagellate swarm-spores, which escape, and whose history is not further known. 4. Syncrypta volvox, Ehr.; one of the Phytomastigoda. A colony enclosed by a common gelatinous test c. a, stigma; b, vacuole (non-contractile). 5. Uroglena volvox, Ehr.; one of the Monadidea. Half of a large colony, the flagellates embedded in a common jelly. 6. Chlorogonium euchlorum, Ehr.; one of the Phytomastigoda. a, nucleus; b, contractile vacuole; c, starch grain; d, eye-spot. 7. Chlorogonium euchlorum, Elir., one of the Phytomastigoda. Copulation of two liberated microgonidia. a, nucleus; b, contractile vacuole; d, eye-spot (so-called). 8. Colony of Dinobryon sertularia, Ehr.; 200; one of the Monadidea. 9. Hæmatococcus palustris, Girod (= Chlamydococcus, Braun, Protococcus Cohn) one of the Phytomastigoda; ordinary individual with widely separated test. a, nucleus; b, contractile vacuole; c, amylon nucleus (pyrenoid). 10. Dividing resting stage of the same, with eight fission products in the common test e. 11. A microgonidium of the samer Phalansterium consociatum, Cienk., one of the Choanoflagellata; x 325. Disk-like colony. 13. Euglena viridis, Ehr.; x 300; one of the Euglenoidea. a, pigment spot (stigma); b, clear space; c, paramylum granules; d, chromatophor (endochrome plate. 14. Gonium pectorale, O. F..Müller; one of the Phytomastigoda. Colony seen from the flat side. x 300. a, nucleus; b, contractile vacuole; c, amylon nucleus. Dinobryon sertularia. Ehr.; one of the Monadidea. a, nucleus; b, con

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