PHYLUM ARTHROPODA 



499 



junction. of the tergum and pleuron, and formed by a little peg- 

 l&e process of one segment fitting into a depression or socket in 

 the other. A line drawn between the right and left hinges con- 

 stitutes the avis of artic/'/ction, 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 

 imm<fyable, 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 mar^ke'd 

 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 



Fio. 396. Astacus fluviatilus, side view of male, a 1 , antemmle ; '-', antenna ; alt, abdomen 

 ct It., cephalothorax ; kd, gill-cover; r. rostrum; 8, third m axillipede ; 9, first leg; 10 13, 

 remaining legs ; 19, uropod ; XIV, first abdominal segment ; XIX, sixth abdominal segment. 

 (From Lang's Coniparatifc Anatomy.) 



I 



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, containing the thoracic portion 

 of the nervous system. The entire endopkragmal system, as it is 

 called, constitutes a kind of internal skeleton: its anteriorend is 

 formed by a plate, the cephalic apodeme, having the same anatomical 

 relations as the similarly named structure in Apus. 



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 epistoma is bounded 

 laterally by the free edge of the carapace instead of passing 

 insensibly into it like the sub-frontal area of ApUs, with 

 Inch however it agrees in having the labrum attached to the 



K K 2 



