ISOPODA. 99 



ORDER V. 

 ISOPODA(l). 



T]ie Isopoda approach the Loemodipoda by the palpi of 

 the mandibles being absent, but are removed from them in 

 several other respects. The two anterior feet are not at- 

 • tached to the head, and belong, as well as the following ones, 

 to a particular segment. They are always fourteen in num- 

 ber, 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 bursse, 

 the two first or external of which, either partially or wholly, 

 usually cover the others. The body is generally flattened, 



(1) The Polygonata, Fab., with the exception of the genus Monoculus. 



Messrs Audouin and Edwards — Ann. des Sc. Nat., Aout 1827, p. 379, 381 — 

 have published some interesting obsei'vations on the circulation of the Isopoda, and 

 on that of the Ligise in particular. The heart resembles a long vessel extended 

 above the doi'sal 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 branchize. From their 

 experiments on the Ligis, it would appear that the venous system is less complete 

 than in the Decapoda macroura, and that the blood driven from the heart into va- 

 rious 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 Branchio- 

 poda. 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 dis- 

 tinguished 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 fur- 

 nish 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 segments may be considered as simple divi- 

 sions of one segment represented by this ganglion; thus we find that the number 

 of these posterior segments varies. 



