348 COMPARATIVE ANATOMY chap. 



Isopoda. — Among the true Isopoda several genera (such as Sphmroma, 

 Idoihea, Ghjiitonotus) are, in the rich segmentation of their nervous 

 systems, closely connected with Apseudes. In Sphceroma we even find a 

 7th abdominal ganglion. The double nature of the central nervous 

 system is everywhere more or less distinctly marked. The transverse 

 commissure corresponding with the ganglia of the 2d pair of antennae 

 seems to be more or less completely fused with the mandibular 

 commissure. 



In many Isopoda there is fusion in the ventral chord and displacement and re- 

 duction of the pairs of ganglia. The mandibular, maxillary, and maxillipedal ganglia 

 in the first place fuse to form an infra-cesophageal ganglionic mass. Then a reduction 

 in the number of the ganglia takes place chiefly in those of the abdomen. In some 

 Isopoda 5 abdominal ganglia occur, in others {PorceUio, Oniscus, Asellus, Fig. 236, E) 

 we find as the remains of the abdominal ventral chord only one ganglionic swelling 

 attached to the last thoracic ganglion, and in others even this is wanting. The 

 number of the separate thoracic ganglia is less frequently reduced. 



In those Entoniscidcc {Poriunion mcenadis] which are specially strongly modified 

 through parasitism we find, besides the brain (which is everywhere retained), 2 

 thoracic ganglia and one abdominal ganglion at some distance from them, under 

 the heart ; while in the not less strongly modified parasitic Bopyridce 7 thoracic 

 ganglia are said to occur. 



Ampliipoda. — Here in all cases we have a fusing of the anterior 

 ganglia of the ventral chord, so that the nervous system no longer 

 shows at any point that segmentation which we meet with in many 

 Isopoda. In the nervous system of the Gammaridce we can still 

 distinguish, apart from the brain, an infra-cesophageal ganglion 

 consisting of several fused ganglia, and further 7 thoracic ganglia in 

 the 7 free thoracic segments, and 4 abdominal ganglia in the 4 

 anterior abdominal segments. 



In Phronima the centralisation goes still further. There are only 5 thoracic 

 ganglia behind the infra-cesophageal ganglionic mass, which consists of 6 fused 

 ganglia. The last of the 4 abdominal ganglia is formed by the fusion of 3 gangBa 

 which are still separate in the embryo. Not only the points of divergence of the 

 nerves for the 2d antennfe, but also those of the nerves of all the oral appendages 

 are shifted forward on to the cesphageal commissures. The nervous system of 

 many Sypcrida, is similar to that of Phronima. In others, however, concentration 

 goes still further, as not only the last 2 thoracic but also the last 2 abdominal 

 ganglia may fuse together. In the most extreme cases we thus find only an infra- 

 cesophageal ganglionic mass, 4 thoracic and 3 abdominal pairs of ganglia. In the 

 Caprellidce the reduced abdomen retains no ganglia. Besides the brain and the 

 sub-cesophageal ganglion which supplies the mandibles, maxillffi, and maxillipedes, 

 we find 7 thoracic ganglia, the 7th of which lies behind the 6th in the last thoracic 

 segment but one. Three small ganglia belonging to the abdomen follow close 

 behind the 7th thoracic ganglion. In young animals 4 more pairs of abdominal 

 ganglia begin to form, and then fuse with the 3 small ganglia of the adult animal 

 above mentioned. 



III. Thoraeostraea. — In many ScUzopoda {Euphavsia, Fig. 236, 

 A, Bm-eomysis) all the ganglia for the oral and thoracic limbs, 11 in 

 number, seem to have remained separate. 



