in the Crab being concentrated into a single large ganglion, from which nervous filaments are sent to all parts of the body. In the Land-crabs (Gecarcinus) re

spiration is by branchiæ, but there is almost always an aperture behind the carapace for the admission of air. They are distributed over the warm countries of the Old and New Worlds, as well as Australia. They are essentially terrestrial in their habits, and migrate in large bodies to the sea, in order to lay their eggs. Besides the true Gecarcini, members of other very different families live more or less constantly on dry land, and have air admitted directly into the branchial chamber. Amongst these are the Calling-crabs (Gelasimus) and the Sandcrabs (Ocypoda).

Fig. 143-Larva (Zoea) of Crab (Pirimela denticulata), magnified. (After Kinahan.)

Reproduction in the Crabs is the same as in the Macrura, but the larva is exceedingly unlike the adult, and approximates closely to the type of the Macrura, another proof that the Brachyura stand higher in the Crustacean scale. The larval Crab was originally described as a distinct animal, under the name of Zoea (fig. 143), presenting in this condition a long and well-developed abdomen. It is only after several successive moults that the young Crab assumes its characteristic Brachyurous form, and acquires by gradual changes the features which distinguish the adult. The Zoea of the Crabs are usually distinguished by the possession of long spines developed from the carapace. When first liberated from the egg, the Zoea is enveloped in a larval skin or membrane, which is shed in a few hours.

CHAPTER XXXV.

DISTRIBUTION OF THE CRUSTACEA.

DISTRIBUTION OF CRUSTACEA IN SPACE.-The Crustacea are distributed over the whole globe, some forms being terrestrial in their habits, but the majority inhabiting the sea or fresh water. As a rule, the development of the Crustacean fauna is in proportion to the temperature, the higher and larger forms

being most abundant in warm regions. The groups of the Cirripedia, Rhizocephala, Xiphosura, and Lamodipoda, are only found in salt water. On the other hand, the Ichthyophthira, Ostracoda, Copepoda, Phyllopoda, Eurypterida (?), Amphipoda, Stomapoda, Isopoda, and Decapoda, are found both in fresh and in salt water. Of these, however, the Phyllopods are principally fresh-water forms, and the Stomapods and Decapods are essentially inhabitants of the sea; whilst the Eurypterids are certainly mainly a salt-water group, though some forms may perhaps have lived in fresh water as well. The Isopoda and Decapoda also include terrestrial forms.

DISTRIBUTION OF CRUSTACEA IN TIME.-As regards the general distribution of the Crustacea in time, remains of the class are comparatively abundant in all formations except the very oldest; as might have been expected from the generally chitinous or sub-calcareous nature of their integuments and their aquatic habits. Owing also to their habit of periodically casting their shell, a single individual may leave repeated traces of himself, and the number of fossils may considerably exceed that of the individuals which actually underwent fossilisation. The Crustaceans appear to have commenced their existence in the Cambrian period, remains of members of this class being tolerably abundant in the higher portion of this formation. The Paleozoic formations, taken as a whole, are characterised by the predominance of the orders Trilobita, Eurypterida, Ostracoda, and Phyllopoda, of which the two former are exclusively confined to this period. All the other orders of Crustacea which have left any traces of their past existence at all, appear to have come into existence before the close of the Paleozoic period. Upon the whole, however, there has been a marked progression in proceeding from the older formations to the present day. The Trilobites and Eurypterids of the older Paleozoic Rocks, though highly organised so far as their type is concerned, are in many respects inferior to later forms, whilst they present some striking points of resemblance to the larval forms of the higher groups. The great group of the Stalk-eyed Crustaceans-undoubtedly the highest of the entire class is not represented at all till we reach the Carboniferous Rocks; and it is not till we come into the Secondary period that we find any great development of this group, whilst its abundance increases to a marked extent in the Tertiary period, and it attains its maximum at the present day. Similarly, of the two orders of the Merostomata, the Eurypterida are confined to the earlier portion of the Paleozoic period, whilst the more highly organised and less larval King-crabs

(Xiphosura) hardly made their appearance till the Eurypterids. had disappeared, at the close of the Carboniferous period.

1. Cirripedia.-The Cirripedes are hardly known as Palæozoic fossils, but valves of a singular member of this order (Turrilepas) have been found in the Silurian Rocks. With this exception, the Cirripedes are entirely confined in past time to the Secondary and Tertiary epochs. The Balamida are the most common, commencing, with the doubtful exception of a Liassic form, in the Chalk, and attaining their maximum in recent seas. The Verrucide commence in the Chalk, and the Lepadida, with one exception, begin in the Jurassic Rocks, and attain their maximum of development in the Cretaceous epoch. The Upper Silurian genus Turrilepas, above mentioned, is also referable to the Lepadoids.

2. Ostracoda.-Small Ostracode Crustacea are extremely abundant as fossils in many formations, and extend from the Cambrian period up to the present day.

3. Phyllopoda. Remains of Crustaceans supposed to belong to this order are found in the Palæozoic Rocks. Hymenocaris is found in the Upper Cambrian, Caryocaris in the Lower Silurian, Ceratiocaris in the Upper Silurian, and Dithyrocaris in the Carboniferous Limestone. All these forms, with other similar ones, are believed to be most closely allied to the recent Apus and Nebalia. The genus Estheria, represented by many forms from the Devonian period to the present day, is also to be referred here.

4. Trilobita.-The Trilobites are exclusively Palæozoic fossils. In the Upper Cambrian rocks-the so-called "primordial zone"-there occurs a singular group of Trilobites-the so-called primordial Trilobites — distinguished by the possession of many larval characters. In the Lower and Upper Silurian Rocks the Trilobites attain their maximum of development. They are still well represented in the Devonian Rocks; but they die out completely before the close of the Carboniferous epoch, being represented in the Mountain Limestone by three genera only (Phillipsia, Brachymetopus, and Griffithides).

5. Eurypterida.-These, like the last, are entirely Paleozoic, attaining their maximum in the Upper Silurian and Devonian formations, and dying out in the Carboniferous Rocks. Pterygotus, Eurypterus, and Slimonia are the most characteristic genera.

6. Xiphosura-The genus Limulus commenced, as far as is yet known, in the Permian period, and has survived up to the present day. Its first appearance, therefore, was just at the close of the Paleozoic epoch. Of the remaining genera, which constitute with Limulus this sub-order, Belinurus, Euproops, and Prestwichia are Paleozoic, and are not known to occur out of the Carboniferous Rocks. The genus Neolimulus is Upper Silurian.

7. Amphipoda-The oldest known Amphipod is the Necrogammarus of the Upper Silurian.

8. Isopoda.-The earliest known Isopod is the Præarcturus of the Devonian Rocks.

9. Stomapoda.-This order is represented in the Carboniferous Rocks by the genus Gampsonyx.

10. Decapoda.-The Macrurous Decapods commence their existence in the Carboniferous period, with a few Prawn-like forms. The Decapoda are, however, well represented, in all their three tribes, in the Secondary and Tertiary epochs, attaining their maximum at the present day. The London Clay (Eocene) is especially rich in the remains of Macrura and Brachyura.

CHAPTER XXXVI.

ARACHNIDA.

CLASS II. ARACHNIDA.-The Arachnida-including the Spiders, Scorpions, Mites, &c.-possess almost all the essential characters of the Crustacea, to which they are very closely allied. Thus, the body is divided into a variable number of somites, some of which are always provided with articulated appendages. A pair of ganglia is primitively developed in each somite, and the neural system is placed ventrally. The heart, when present, is always situated on the opposite side of the alimentary canal to the chain of ganglia. The respiratory organs, however, whenever these are differentiated, are never in the form of branchiæ as in the Crustacea, but are in the form either of pulmonary vesicles or sacs, or of ramified tubes, formed by an involution of the integument, and fitted for breathing air directly. Further, there are never "more than four pairs of locomotive limbs, and the somites of the abdomen, even when these are well developed, are never provided with limbs;" the reverse being the case amongst the Crustacea. Lastly, "in the higher Arachnida, as in the higher Crustacea, the body is composed of twenty somites, six of which are allotted to the head; but in the former class, one of the two normal pairs of antennæ is never developed, and the eyes are always sessile; while, in the higher Crustacea, the eyes are mounted upon movable peduncles, and both pairs of antennæ are developed" (Huxley).

The head of the Arachnida is always amalgamated with the thorax, to form a "cephalothorax;" the integument is usually chitinous, and the locomotive limbs are mostly similar in form to those of insects, and are usually terminated by two hooks.

In many of the Arachnida the integument remains soft over the entire body; in others, as in the majority of Spiders, the abdomen remains soft and flexible, whilst the cephalothorax is more or less hard and chitinous; in the Scorpions, again, the integument over the whole body forms a strong chitinous shell. The cephalothorax may be segmented (Solpugida); and the abdomen may or may not be segmented. Though four pairs of legs are present, the first is certainly homologous with the labial palpi of the Insecta.

The typical somite of the Arachnida is constituted upon exactly the same plan as that of the Crustacea, consisting

essentially of a dorsal and ventral arc; the former composed of a central piece, or "tergum," and of two lateral pieces, or

Fig. 144-A, The male of the common House-Spider (Tegenaria civilis), considerably magnified: Front portion of the body, consisting of the amalgamated head and thorax; Maxillary palpi; a Abdomen. B, Front portion of the head of the same, showing the eight eyes (f), and the mandibles (n). C, Under side of the head and trunk, showing the true jaws (m), the lower lip (2), and the horny plate to which the legs are attached. D, Diagram of one of the air-chambers or breathing-organs. (Figs A, B, and C are after Blackwall.)

"epimera;" whilst the latter is made up of a median "sternum" and of two lateral "episterna."

As regards the composition of the cephalothorax of Spiders, "the tergal elements of the coalesced segments are wanting, and the back of the thorax is protected by the elongation, convergence, and central confluence of the epimeral pieces; the sternal elements have coalesced into the broad plate in the centre of the origins of the ambulatory legs, from which it is separated by the episternal elements. . . . The non-development of the tergal elements explains the absence of wings" (Owen).

The mouth is situated, in all the Arachnida, in the anterior segment of the body, and is surrounded by suctorial or masti