X PHYLUM ARTHROPODA 215 



The ventral and lateral regions of the thoracic exoskeleton 

 are produced into the interior of the body in the form of a 

 segmental series of calcified plates, so arranged as to form a 

 row of lateral chambers in which the muscles of the limbs 

 lie, and a median tunnel-like passage or sternal canal, con- 

 taining the thoracic portion of the nervous system. The 

 entire endophragmal system, as it is called, constitutes a kind 

 of internal skeleton. 



The head exhibits no segmentation : its sternal region is 

 formed largely by a shield-shaped plate, the epistoma, nearly 

 vertical in position. The ventral surface of the head is, in 

 fact, bent so as to face forwards instead of downwards. The 

 cephalic region of the carapace is produced in front into a 

 large median spine, the rostrum (Fig. 114, r.) : immediately 

 below it is a plate from which spring two movably articu- 

 lated cylindrical bodies, the eye-stalks, bearing the eyes at 

 their ends. 



Among the appendages the most conspicuous are the 

 long feelers (Fig. 114, a. 1, a. 2} attached to the head, the 

 five pairs of legs (9-13) springing from the thorax, and the 

 little fin-like structures arising from the sterna of the abdomen. 

 It will be convenient to begin with the last-named region. 



The third, fourth, and fifth segments of the abdomen bear 

 each a pair of small appendages, the abdominal feet or plea- 

 pods (Fig. 115, 10). Each consists of an axis ot protopoditc, 

 consisting of a very short proximal (pr. 1) and a long distal 

 (pr. 2) podomere, and bearing at its free end two jointed 

 plates, fringed with setae, the endopodite (en.) and exopodite 

 (ex.). These appendages act as fins, moving backwards and 

 forwards with a regular swing, and probably aiding in the 

 animal's forward movements. 



In the female a similar appendage is borne on the second 

 segment, while that of the first is more or less rudimentary. 



