ON 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 (limulce), 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. 
