ox CRUSTACEA. 231 



In some others the head is distinct, but the firstlings of the 

 body are united above, so as to form a sort of buckler of no 

 great extent. 



In some others flimulcBj, the segmentary division of the 

 body is only apparent underneath, while the head, above, pre- 

 sents a vast buckler, and the trunk and abdomen are con- 

 founded, and covered by a second large plate, terminating in 

 an ensiform appendage. 



Sometimes we find the head more or less distinct, and the 

 body not divided neatly into trunk and abdomen, but exhibit- 

 ing scarcely any trace of segments, and comprised in a bivalve 

 testa, formed by a hardened expansion of the dorsal skin. 



The general form of the antennae is that of a thread or lash, 

 that is, they are longitudinally conical, or diminished insensibly 

 in thickness, from a round base to a very attenuated extremity. 

 They are composed of small hollow cylinders of corneo-calca- 

 reous substance, or of articulations superadded one to the 

 other, and whose cavity encloses muscles, nerves, and without 

 doubt ramifications of the circulating system. Each antenna 

 has its peduncle and thread. The peduncle (a term borrowed 

 from botany) is a sort of stem or stalk, composed of three or 

 four articulations much thicker than the rest, and frequently 

 affording an attachment to certain appendatory leaflets. The 

 thread is single, double, or triple, varying in the number of its 

 articulations, but often composed of a multitude of small ones. 



The antennae, in certain genera, assume anomalous forms, 

 which assimilate them to organs of locomotion. At other 

 times their peduncle alone exists, and is transformed into very 

 broad and crenulated plates. In the decapod Crustacea, the 

 base of the external antennae presents a little rounded sub- 

 triangular body, strong in the short-tailed, a little membrana- 

 ceous in the long-tailed species, which closes the external 

 issue of a cavity traversing the testa or shell, and which is 

 considered to be the auricular organ. 



