216 CRUSTACKA. 



into feet, sometimes simple and at others constituting a claw, but 

 almost always with a single toe or hook. 



According to the observations of Messrs. Audouin and Edwards, 

 the two ganglionary cords of the spinal marrow are perfectly sym- 

 metrical and distinct throughout the whole of their length, and from 

 those of the Baron Cuvier it would appear that the Onisci are only 

 removed from them because these cords do not present the same 

 uniformity in all the segments of the body, and because there are 

 some ganglions less *. Thus, according to them, the nervous system 

 of the Crustacea is the simplest of all ; in the Cymothoae and Idoteae 

 the two ganglionary chains are no longer distinct, and those ganglions 

 which immediately follow the two cephalics, form as many small 

 circular masses situated on the median line of the body ; but the 

 cords of communication which serve to connect them, remain 

 isolated and attached to each other. It would appear from these facts 

 that the latter Crustacea are higher in the animal scale than the 

 preceding ones, but other considerations seem to us to require a con- 

 siderable separation between the Talitri and Onisci, and the 

 arrangement of the Cymothoae and Idotese in an intermediate rank. 



The organs of generation are situated inferiorly near the origin of 

 the tail. The two first appendages with which it is furnished beneath, 

 and which are analogous to those presented to us by the same part in 

 the preceding Crustacea, but more diversified, and always, as it 

 appears, supporting the branchiae, differ in this respect, according to 

 the sex. The coitus takes place like that of insects, the male placing 

 himself on the back of his female ; the latter carries her ova under 

 the thorax, between scales which form a sort of pouch. There they 

 are developed, and the young remain attached to the feet or other 

 parts of the body of their mother, until they have acquired the 

 strength requisite for natation, and providing for their wants. All 

 these Crustacea arc small, and mostly inhabit the sea-coast or fresh 

 water. Some are terrestrial, and others are known which are 

 parasitical. 



They are divided into three orders : those whose mandibles are 

 furnished with a palpus, appear to be naturally connected with the 

 preceding Crustacea such are the Amphipoda ; those in which these 

 organs are deprived of them will constitute the two following orders 



the Laemodipoda and the Isopoda. The Cyami, a genus of the 



second one, being parasitical, naturally lead us to the Bopyri and 

 Cymothoae, with which we commence the Isopoda. 



*See ONISCUS. 



