ISO POD A. 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. Tin; under part of the tail is furnished with 

 very apparent appendages resembling leaflets or vesicular bursae, the 

 i\vo first or external of which, cither 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 

 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 are 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 origin 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 them 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 

 the 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 brauchio-cardiac vessels. This disposition would form the tnui- 

 i 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 

 Ampuipoda are not in complete juxtaposition, and may be distinguished throughout 

 thru \\hnk- ''m;r-i -. I iun are nine ganglions without counting the brain, but the 

 two first and two last arc 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 lumber of these posterior segments varies. 



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