CRUSTACEANS. 601 



The innermost part of the first pair of modified feet or accessory 

 jaws is still fleshy and lobed, like the jaws that precede them ; but 

 the auxiliary jaws of the second and especially of the third pair, 

 indicate by their form that they correspond to feet, and bear small 

 gills at their base. In the short-tailed ten-footed crustaceans the 

 third pair has two very broad joints (the two joints of the femur, 

 according to SAVIGN Y) , so that it covers the oral organs on its under 

 urface. 



The auxiliary jaws of the two last pairs, which, as we have 

 aid, indicate by their form even in the decapod crustaceans most 

 lanifestly their true nature of feet, remain in many crustaceans, as 

 n Gammarus and Squilla, unchanged feet. Hence these crustaceans 

 ave fourteen unchanged feet, and not ten like lobsters and crabs. 



The genus Apus amongst the Entomostraca possesses those oral 

 arts alone, which also occur in hexapod insects. To these succeed 

 umerous feet, of which the first pair terminates in filaments con- 

 isting of many joints, but contributes nothing to mastication. In 

 jimulus there are six pairs of feet at the cephalothorax, without 

 ny jaws ; the broad basal pieces, armed with spines, of the ten 

 ast feet surround the mouth and perform the office of jaws, whilst 

 he first pair of feet, scarcely a third of the length of the other feet, 

 placed in front of the mouth. It forms two false jaws that may 

 )e compared with the mandibulce of the Arachnoidea, whilst the 

 oxa is membranous, and unites with that of the opposite side to 

 orm a kind of upper-lip 1 . If we compare the second pair of feet 

 vith the so-named under-jaws of Arachnids, we shall observe the 

 p-eatest agreement between Limulus and them. Behind the last 

 >air of feet also in Limulus there are even found two small 

 ippendages, which may be compared with the pectinated organs of 

 he scorpions. 



The body of crustaceans in the more restricted sense or the 

 Tunk, the anterior portion of the abdomen, is almost always 

 livided beneath by transverse indentations into sections, but its 

 ipper part in the decapod crustaceans is covered by a continuous 

 shield, named the shell (testa, in French carapace 2 ). In the short 



1 J. VAN DER HOEVEN, Recherches sur VHist. not. et I'Anat. des Limules. Leide, 

 [838, folio, p. 12. 



3 In the shell DESMAREST has distinguished certain parts by particular terms, from 

 the position, relative size and determinate form of which, the situation, magnitude 



