•216 
('UUSTACKjS. 
into feet, sometimes simple and at otliers constituting a claw, bnt 
almost always with a sing'le toe or hook. 
According to the observations of Messrs. Audouin and Edwai'ds, 
the two ganglionary cords of the spinal marrow are perfectly sym- 
metrical and distinct throughout the wliole of their length, and from 
those of the Baron Cuvier it Avould appear that the Onisci are only 
removed from tliem 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 iiervoi.is 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 ; bnt 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 
})receding ones, but other considerations seem to us to require a con- 
siderable separation between the Talitri and Onisci, and the 
arrangement of tlie Cymothoae and Idotea? in an intermediate rank. 
'J’he 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 tliose 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 
arc developed, and the young remain attached to the feet or other 
parts of the body of their mother, until they have acquired tlie 
strength requisite for natation, and providing for their Avaiits. All 
these Crustacea arc small, and mostly inhabit the sea-coast or fresh 
water. Some are terrestrial, and others are known which are 
parasitical. 
I'hey are divided into three orders : those whose mandibles are 
furnished with a palpus, appear to be naturally connected Avith the 
preceding Crustacea — such are the Amphipoda ; those in Avhich these 
organs are deprived of them Avill constitute the tavo following orders 
— the Liemodipoda and the isopoda. 'I’he Cyami, a genns of the 
second one, being jiarasitical, naluially lead us to the Boj)yri and 
Cymothooe, Avith AA'hich Ave commence the Isopoda. 
'h>ce Onjsccs. 
