Thus the circulating fluid is brought into direct contact with all the viscera, and fills up the abdominal cavity, so that not until after it has passed through the respiratory apparatus does it again find itself enclosed in vessels properly so called.

* Milne-Edwards, loc. cit.

(1024). As might be anticipated from an examination of the external configuration of the different families comprised in the extensive class we are now considering, the nervous system is found to pass through all those gradations of development which we have found gradually to present themselves as we have traced the Homogangliata from the lowest to the most highly organized types of structure. In the most imperfect Crustacea, indeed, we find a simplicity of arrangement greater than any hitherto pointed out even in the humblest Annelida - a disposition of parts which, theoretically, might have been expected to exist, but has only been distinctly recognized in the class before us.

(1025). We have all along spoken of the nervous centres of the Articulata as arranged in symmetrical pairs, although in no example which has yet occurred to our notice have we been able strictly to point out the accuracy of such a view of the subject. The two lateral masses of the supra-oesophageal ganglion are found united into one brain in the humblest forms of annulose animals; and even in the ganglia forming the ventral series, although we might presume each to be composed of two symmetrical halves, the divisions are most frequently so intimately blended, that their distinctness is not susceptible of anatomical demonstration. In some of the Crustacea, however, among those species which have the segments of their external skeleton most perfectly separate and distinct, the nervous system is found to present itself in such a condition that the division into lateral halves is perfectly evident; and from this condition their progressive coalescence may be traced, step by step, until we arrive at a state of concentration as remarkable as that already noticed in the most elevated of the Arachnidans. It is to Milne-Edwards and Audouin that we are indebted for the interesting particulars connected with this part of our subject; and the results of their investigations are of such great physiological importance*, thae the following condensed account of their labours cannot be omitted in this place.

In Talitrus, every pair of ganglia consists of two separate nuclei of nervous substance, united by a transverse band so disposed as to bring them into communication with each other, while an anterior and posterior nervous filament derived from each unites it with the preceding and following ganglia of the same side of the body: even the encephalic mass is composed of two lateral portions united by a cord passing between them. All these pairs of ganglia (thirteen in number, corresponding with the number of the segments of the body) are exact counterparts of each other both in size and figure, so that none seems to preponderate in energy over the rest; but the anterior or encephalic pair alone communicates with the eyes and antennae, the only organs of the senses as yet discernible.

(1026). In Oniscus Asellus, a concentration of the elements composing the nervous system above described is discernible; and this is found to be indicated by incipient approximation, which takes place in two directions, one longitudinal, the other transverse. In the first place, the entire number of pairs of ganglia is reduced to ten, three pairs having become obliterated by coalescence; and moreover, while the central portions still consist of two lateral masses each, the first and last pairs are united into single ganglia. As we rise to higher forms the coalescence still proceeds; all the pairs of ganglia soon become united in a transverse direction, and gradually the whole chain becomes shorter by the fusion of several pairs into larger and more powerful masses.

(1027). In the Crab (which, from its terrestrial habits, holds a position among the Crustacea equivalent to that which the Spiders occupy among other Ar-ticulata), this centralization is carried to the utmost extent, and all the abdominal and thoracic ganglia become agglomerated into one great centre, from which nerves radiate to the parts of the mouth and instruments of locomotion (fig. 205).

Nervous system of the Crab: from the ventral aspect.

Fig. 205. Nervous system of the Crab: from the ventral aspect.

* "Recherches Anatomiques sur le Systeme Nerveux des Crustacea," Ann. des Sci. Nat. t. xiv.

(1028). But this change in the condition of the nervous system is not only observable as we proceed from species to species, as they rise higher in the scale of development; similar phenomena are met with in watching the progress of any individual belonging to the more perfect families, as it advances from the embryo to its mature condition. Thus, in the Cray-fish (Astacus fluviatilis), Rathke* observed that, when first perceptible, the nervous system consisted of eleven pairs of ganglia, perfectly distinct from each other, and situated on each side of the mesial line of the body. The first six pairs then unite transversely, so as to form as many single masses, from which the nerves of the mandibles and foot-jaws emanate; while the five posterior, from which the nerves of the ambulatory extremities are given off, remain separate. Such is the state at birth, or on leaving the egg; but further changes occur before the Cray-fish arrives at maturity. The four anterior ganglia, which supply nerves to the mandibles and foot-jaws, are, by degrees, all consolidated into one mass, and the fifth and sixth likewise coalesce, while the other pairs continue permanently distinct.

The reader will at once recognize the resemblance between these changes and those already described as taking place during the progress of evolution in the Caterpillar: the same great law is, in fact, in operation in both cases, and the same results are obtained from the completion of the process 1.