XXVII APPENDAGES  321 



is immovable, and shows no distinction into segments either 

 on its dorsal (tergal) or lateral (pleural) aspect. But on the 

 ventral surface the sterna of the thoracic segments are 

 clearly marked off by transverse grooves, and the hindmost 

 of them is slightly movable Altogether eight thoracic 

 segments can be counted. 



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 (Fig. 83, b). 



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. 81, 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. 



The appendages have very various forms, and are all, like 

 the abdomen, jointed or segmented, being divisible into 

 freely articulated limb-segments or podomeres. The observer 

 is at once struck by the long feelers attached to the head, the 

 five pairs of legs springing from the thorax, and the little 

 fin-like bodies 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 swimming-feet 

 or pleopods. A pleopod (Fig. 82, 10) consists of an axis or 



Y 



