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 SAVIGNY), so that it covers the oral organs on its under 
surface. 
The auxiliary jaws of the two last pairs, which, as we have 
said, indicate by their form even in the decapod crustaceans most 
manifestly their true nature of feet, remain in many crustaceans, as 
in Gammarus and Squilla, unchanged feet. Hence these crustaceans 
have fourteen unchanged feet, and not ten like lobsters and crabs. 
The genus Apus amongst the Hntomostraca possesses those oral 
parts alone, which also occur in hexapod insects. To these succeed 
numerous feet, of which the first pair terminates in filaments con- 
sisting of many joints, but contributes nothing to mastication. In 
Limulus there are six pairs of feet at the cephalothorax, without 
any jaws; the broad basal pieces, armed with spines, of the ten 
last feet surround the mouth and perform the office of jaws, whilst 
the first pair of feet, scarcely a third of the length of the other feet, 
is placed in front of the mouth. It forms two false jaws that may 
be compared with the mandibule of the Arachnoidea, whilst the 
coxa is membranous, and unites with that of the opposite side to 
form a kind of upper-lip’. If we compare the second pair of feet 
with the so-named under-jaws of Arachnids, we shall observe the 
greatest agreement between Limulus and them. Behind the last 
pair of feet also in Limulus there are even found two small 
appendages, which may be compared with the pectinated organs of 
the scorpions. 
The body of crustaceans in the more restricted sense or the 
trunk, the anterior portion of the abdomen, is almost always 
divided beneath by transverse indentations into sections, but its 
upper part in the decapod crustaceans is covered by a continuous 
shield, named the shell (testa, in French carapace*). In the short 
1 J. VAN DER HorEvEN, Recherches sur Hist. nat. et Anat. des Limules. . Leide, 
1838, folio, p. 12. 
? In the shell DesmAREsT has distinguished certain parts by particular terms, from 
the position, relative size and determinate form of which, the situation, magnitude 
