The infra-œsophageal ganglia which are the first to fuse are those which supply the most anterior portion of the animal with nerves, and include always those anterior appendages which are modified for mastication purposes. To this part the name prosoma has been given; in many cases it forms a well-defined, distinct portion of the animal.
Succeeding this prosoma or masticatory region, there occurs in all gill-bearing arthropods a respiratory region, in many cases more or less distinctly defined, which has received the name of mesosoma. The rest of the body is called the metasoma.
In accordance with this nomenclature the central nervous system of many of the Arthropoda may be divided as follows:—
1. Pre-oral, or supra-œsophageal ganglia.
2. Infra-oral, or infra-œsophageal ganglia and ventral chain, which consist of three groups: prosomatic, mesosomatic, and metasomatic ganglia.
The infra-œsophageal ganglion-mass, then, in most of the Arthropoda may be spoken of as formed by the fusion of the prosomatic or mouth-ganglia, the mesosomatic and metasomatic remaining separate and distinct. The number of ganglia which have fused may be observed by examination of the embryo, in which it is easy to see indications of the individual ganglia or neuromeres, although all such indication has disappeared in the adult; thus the infra-œsophageal ganglia of the cray-fish have been shown to be constituted of six prosomatic ganglia.
In Fig. [25] I give figures of the central nervous system (with the exception of the abdominal or metasomatic ganglia) of Branchipus, Astacus, Limulus, Scorpio, Androctonus, Thelyphonus, and Ammocœtes. In all the figures the supra-œsophageal ganglia are lined horizontally, and their nerves shown, viz. optic (lateral eyes (II) and median eyes (II′)), olfactory (I) (first antennæ, camerostome, nose); then come the prosomatic ganglia (dotted), with their nerves (A) supplying the mouth parts, and the second antennæ or cheliceræ; then the mesosomatic (lined horizontally), with their nerves (B) supplying respiratory appendages. These figures show that the concentrated brain mass around the œsophagus of an arthropod which has arrived at the stage of Astacus, is represented by the supra-œsophageal ganglia and the fused prosomatic ganglia.
The next stage in the evolution of the brain is seen in the gradual inclusion of the mesosomatic ganglia, one after the other, into the infra-œsophageal mass of the already fused prosomatic ganglia. With this fusion is associated the loss of locomotion in these mesosomatic appendages, and their entire subservience to the function of respiration. Dana urges that cephalization is a consequence of functional alteration in the appendages, from organs of locomotion to those of mastication and respiration. Whether this be true or not, it is certainly a fact that in Limulus, the ganglion supplying the first mesosomatic appendage has fused with the prosomatic, infra-œsophageal mass. It is also a fact that the prosomatic appendages are the organs of mastication, their basal parts being arranged round the mouth so as to act as foot-jaws, while the mesosomatic appendages, though still free to move, have been reduced to such an extent as to consist mainly of their basal parts, which are all respiratory in function, except in the case of the first pair, where they carry the terminal ducts of the genital organs. In the next stage, that, of the scorpion, in which the mesosomatic appendages have lost all power of free locomotion, and have become internal branchiæ, another mesosomatic ganglion has fused with the brain mass, while in Androctonus two of the branchial mesosomatic ganglia have fused; and finally, in Thelyphonus and Phrynus, all the mesosomatic ganglia have coalesced with the fused prosomatic ganglia, while the metasomatic ganglia have themselves fused together in the caudal region to form what is known as the caudal brain.
Fig. 25.—Comparison of Invertebrate Brains from Branchipus to Ammocœtes.