Fig. 130.-Archimul acris Acadicus (after Dawson). From the In the Secondary period, remains of Insects are much more abundant than in any Palæozoic deposit. The Jurassic Rocks have yielded the earliest examples of the orders Hymenoptera and Hemiptera, whilst the orders Neuroptera, Orthoptera, and Coleoptera are well represented. In the Tertiary Rocks, again, the remains of Insects become still more abundant, and in some deposits they are found in the greatest profusion. Whilst all the above-mentioned orders are represented, the Tertiary Rocks have also yielded the first traces (with doubtful exceptions) of the orders Diptera and Lepidoptera. Amber, which is, geologically speaking, a very modern product, has yielded the remains of a vast number of insects, all of which belong to extinct forms. Carboniferous Rocks of The following are the names of the Orders of Insects which are known in a fossil condition, with the date of their first appearance: 1. Neuroptera (Dragon-flies, White Ants, May-flies, &c.) Devonian. 2. Orthoptera (Cockroaches, Crickets, Locusts, &c.) Carboniferous. 3. Coleoptera (Beetles). Carboniferous. 4 Hymenoptera (Bees, Wasps, Saw-flies, Ants). Jurassic. 5. Hemiptera (Aphides, Field-bugs, Cicadas, &c.) Jurassic. 6. Lepidoptera (Butterflies and Moths). Tertiary. 7. Diptera (House-flies, Flesh-flies, Gnats, Crane-flies, &c.) Tertiary. 8. Thysanura (Spring-tails). Late Tertiary (in amber). CHAPTER XVIII. SUB-KINGDOM MOLLUSCA. POLYZOA. SUB-KINGDOM MOLLUSCA.-The Mollusca comprise the animals ordinarily known as Shell-fish, from their commonly possessing an exoskeleton or shell. The Molluscs are soft-bodied and destitute of any evident segmentation. Commonly the integument secretes a hard calcareous or horny envelope, but this may be absent. The alimentary canal is always present, and never communicates with the body-cavity. The nervous system consists typically of three pairs of ganglia, disposed in a characteristically scattered manner; but in the lower forms a single ganglion alone is present. A heart may or may not be present, and there may or may not be distinct respiratory organs. As a matter of course, it is only with the shell of the Mollusca that the paleontologist has to deal, and those forms which are destitute of this structure are wholly unknown in the fossil condition. The special characters of the shell will be treated of in speaking of the separate classes. In the meanwhile it is sufficient to draw attention to some general considerations. In the Sea-mosses and Sea-mats (Polyzoa), the animal is compound, and the hard structures secreted by the colony would not come under the common designation of "shell." In these cases the investment of the colony would rather be termed a "polypidom," and when of a horny nature, it does indeed show a very close resemblance to the "polypary" of the Sertularian Zoophytes. In the Ascidian Molluscs or Sea-squirts (Tunicata), the animal is simply enclosed in a leathery or cartilaginous case, in which calcareous matter is very rarely developed. Hence we need feel no surprise that the Tunicaries, with one or two very problematical exceptions, are not known in the fossil state. The Lamp-shells and their allies (Brachiopoda) possess a bivalve shell, consisting of two pieces or "valves," which are more or less highly calcareous. Coming to the higher Mollusca, the true bivalve Shell-fish (Lamellibranchiata), as their common name implies, have also a bivalve shell; but this is distinguished from the shell of the Brachiopods by sufficiently good characters. No Lamellibranch is destitute of a shell, and the remains of this class occur more or less abundantly in all deposits except the most ancient. The ordinary univalve Shell-fish (Gasteropoda), as indicated by their common name, have usually a shell composed of a single piece or "valve." In many Gasteropods, however, there is either no shell at all, when the animal is said to be "naked" (as in the Sea-slugs), or the shell is quite rudimentary, and is concealed within the mantle (as in the ordinary slugs). In other Gasteropods again (viz. in the Chitons), the shell is "multivalve," consisting of eight pieces or valves placed one behind the other. Most, however, of the "multivalve" shells of older writers are really referable to the Cirripedia. In the minute Oceanic Molluscs which form the class Pteropoda, the animal is sometimes naked, but is more usually protected by a symmetrical glassy shell, which is always univalve. In the class of the Cephalopoda, finally, great diversity exists in the character of the skeleton. All the ordinary Cuttle-fishes have an internal skeleton, embedded in the mantle, and not visible externally. This internal skeleton may be calcareous or horny, and it may be of a very complicated nature; but it merely serves to support the soft parts of the animal, and it does not form an external case in which the animal lives. In one Cuttle-fish only (viz., the Argonaut or Paper Nautilus), is there an external shell, but the nature of this is quite peculiar, and it cannot be compared with the shell of any of the ordinary Molluscs. In another group of the Cephalopoda, represented at the present day by the Pearly Nautilus, there is a well-developed external shell, which is always composed of a single piece, and is always chambered, the animal living in the last and largest chamber of the shell. In composition the shell of the higher Mollusca consists of carbonate of lime-usually having the atomic arrangement of calcite-with a small proportion of animal matter. In the Pholadida, however, the calcareous matter exists in the allotropic condition of arragonite, which is very much harder than calcite. As regards their texture, three principal varieties of shells may be distinguished-viz., the "porcellanous," the nacreous," and the "fibrous." In the nacreous or pearly shells, as seen in "mother-of-pearl," the shell has a peculiar lustre, due to the minute undulations of the edges of alternate layers of carbonate of lime and membrane. The "fibrous" shells are composed of successive layers of prismatic cells. The "porcellanous" shell has a more complicated structure, and is composed of three layers or strata, each of which is made up of very numerous plates, "like cards placed on edge." The direction in which these vertical plates are placed, is sometimes transverse in the central layer, and lengthwise in the two others; or longitudinal in the middle, and transverse in the outer and inner strata. From their so commonly possessing hard structures, whether external or internal, no fossils are more abundant or important than Molluscs. As regards the general distribution of the Mollusca in time, the sub-kingdom commences its existence in the Cambrian period, and there is no reason to suppose that this is really its first appearance. In the Cambrian Rocks, the classes of the Polyzoa, Brachiopoda, Pteropoda, Gasteropoda, and Cephalopoda are certainly represented, and the Lamellibranchiata existed in Lower Silurian times, if not earlier. Speaking generally, the chief representatives of the Mollusca in Paleozoic time are the chambered Cephalopods (Tetrabranchiata) and the Brachiopoda; in Mesozoic time, the Cuttlefishes (Dibranchiate Cephalopods), the chambered Cephalopods, and the Polyzoa; in Kainozoic time, the Lamellibranchs and Gasteropods. The Polyzoa are comparatively poorly represented in Paleozoic Rocks, and attain their maximum towards the close of the Mesozoic period. The Brachiopods are vastly more abundant in Palæozoic deposits than in Mesozoic, and have gradually declined to the present day. The Lamellibranchiata seem to have been gradually increasing in importance since their first appearance in the Lower Silurian seas, and they have attained their maximum at the present day. The Gasteropods, upon the whole, like the Bivalves, seem to have reached their culminating point in recent seas; whilst the Pteropods seem to have been as abundant in Silurian seas as they are at present. The history of the Cephalopoda is a remarkable one. The Tetrabranchiate forms, with chambered shells, attained their maximum in the earlier portion of the Silurian period, as regards their simpler types; but the more complex types of the group swarmed in the seas of the Secondary period, and finally disappeared at the close of this epoch. This group at the present day is represented solely by the Pearly Nautilus. The Dibranchiate Cephalopoda, on the other hand, represented at the present day by the Cuttle fishes, did not make their appearance till the commencement of the Secondary period, and seem to have reached their maximum in existing seas. The sub-kingdom Mollusca is divided into two great divisions, termed respectively the Molluscoida and the Mollusca Proper. The division Molluscoida comprises the three classes. of the Polyzoa, Tunicata, and Brachiopoda, characterised by having a nervous system consisting of a single ganglion or prin cipal pair of ganglia, whilst there is either no distinct organ of the circulation or an imperfect heart. In the division of the Mollusca Proper are comprised the classes of the Lamellibranchiata (Bivalves), Gasteropoda (Univalves), Pteropoda, and Cephalopoda. All these classes are distinguished by having a nervous system composed of three principal pairs of ganglia; whilst there is a well-developed heart, consisting of at least two chambers. CLASS POLYZOA OR BRYOZOA. Each Animal composite, forming colonies, all the members of which are produced by budding from a primitive being (zooid). member of the colony (zooid) is enclosed in a double-walled sac, the outer coat of which is mostly hardened by horny or calcareous matter. There is no heart, and the mouth is surrounded by a circle or crescent of hollow ciliated tentacles. The colonies are all but invariably fixed to some foreign object, and are in many cases plant-like in form. All the Polyzoa live in an associated form in colonies or "polyzoaria," which are sometimes foliaceous, sometimes branched and plant-like, sometimes encrusting, and very rarely are free. Each "polyzoarium" consists of an assemblage of distinct but similiar zoöids arising by continuous gemmation from a single primordial individual. The colonies thus produced are in very many respects closely similar to those of many of the Hydroid Polypes, with which, indeed, the Polyzoa were for a long time classed. The "polyzoarium," however, of a Polyzoon differs from the polypidom of a composite Hydroid in the general fact that the separate cells of the former do not communicate with one another otherwise than by the continuity of the external integument; whereas the zooids of the latter are united by an organic connecting medium, or "coenosarc," from which they take their origin. On this point Mr Busk observes : "It has been before said that the Polyzoa are always associated into compound growths, made up of a congeries of individuals, which, though distinct, yet retain some degree of |