vii EXOSKELETON 363 



pleuron, and formed by a little peg-like process of one 

 segment fitting into a depression or socket in the other. A 

 line drawn between the right and left joints constitutes the 

 axis of articulation, and the only possible movement is in a 

 plane at right angles to this axis. 



Owing to the presence of the carapace the thoracic region 

 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 lie the muscles of the 

 limbs and a median tunnel-like passage or sternal canal, 

 containing the thoracic portion of the nervous system 

 (Fig. 94). The entire endophragmal system, as this 

 series of plates 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 upwards, so as to face forwards instead of down- 

 wards. The cephalic region of the carapace is produced in 

 front into a large median spine, the rostrum : immediately 

 below it is a plate from which spring two movably 

 articulated 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. You will 



