VI EXOSKELETON 349 



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

 ment 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 the muscles of the limbs 

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

 taining the thoracic portion of the nervous system (Fig. 

 87). 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 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 : 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 

 at once notice the long feelers attached to the head, the 



