ISOPODA. 
227 
spects. The two anterior feet are not attached to the head, and belong 
as well as the following ones, to a particular segment. They are 
always fourteen in number, unguiculated, and without any vesicular 
appendage at their base. The under part of the tail is furnished with 
very apparent appendages resembling leaflets or vesicular bursae, the 
two first or external of which, either partially or wholly, usually 
cover the others. The body is generally flattened, or is wider than it 
is thick: The mouth consists of the same pieces as in the preceding 
Crustacea; but here, those which correspond to the two superior 
foot-jaws of the Decapoda, exhibit an appearance of a lower lip 
terminated by two palpi, still more than in the latter. The two 
mediate antennae arc almost obliterared in the last Crustacea 
of this order, which are all terrestial and also differ from the 
others in their respiratory apparatus. The male organs of gene- 
ration are usually announced by linear or filiform appendages, 
and sometimes by hooks, situated at the internal origiir of the first 
sub-caudal laminae. The females carry their ova under the thorax, 
either between scales, or in a pouch or membranous sac, which they 
open in order to allow a passage to their young, which are produced 
with the form of parts peculiar to their species, merely changing 
their skin as they increase in size. Most of tliem are aquatic. Those 
which are terrestrial, like all other Crustacea which live out of water, 
still require a certain degree of atmospheric humidity to enable them 
on that of the Ligiae in particular. The heart resembles a long vessel extended 
above the dorsal surface of the intestine. From its anterior extremity arise three 
arteries, similar to those of the Decapoda. Lateral branches are also to be observed 
running from the heart towards the feet. On a level with the two first segments of 
tlie abdomen (the tail), that organ receives, from the right and left, small canals 
(branchio-cardiac vessels) which seem to proceed from the branchiae. From their 
experiments on the Ligiae, it would appear that the venous system is less complete 
than in the Decapoda macroura, and that the blood driven from the heart into 
various parts of the body, passes into lacunae formed between the organs in the infe- 
rior part of the body which communicate freely with the afferent vessels of the 
branchiae. The blood having traversed the respiratory apparatus, returns to the 
heart through the branchio-cardiac vessels. This disposition would form the tran- 
sition from the circulating system of the Decapoda to that of certain Branchiopoda. 
According to Cuvier, the two anomalous cords which form the mediate portion of the 
nervous system of the Onisci — and, probably, of the other Isopoda and even of the 
Amphipoda — are not in complete juxtaposition, and may be distinguished throughout 
their whole course. There are nine ganglions without counting the brain, but the 
two first and two last are so closely approximated that we may reduce the number to 
seven. The second and six subsequent ones furnish nerves to the seven pairs of feet; 
the four anterior, although, by the order of the parts, analogous to the four last 
foot-jaws of the Decapoda, are true feet. The segments which immediately follow, 
or those which form the tail, receive their nerves from the last ganglion ; these seg- 
ments may be considered as simple divisions of one segment represented by this gang- 
lion ; thus we find that the number of these posterior segments varies. 
Q 2 
