vii EXOSKELETON 363 



the junction of the tergum and 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 exo- 

 skeleton 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 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 



