THE LANCELET 



329 



General 

 Anatomy, 



The skin is covered with a columnar epithelium, ciliated 

 only within the oral hood and in parts of the 

 atrium. The connective tissue is scanty, and 

 consists of fibrillated ground substance with 

 some cells. There is a thick muscular body-wall, divided, 

 as we have seen, into segments, which are V-shaped and 

 fit into one another so that several are cut in a transverse 

 section. Within the body-wall lies a perivisceral ccelom, not 



A-f- ' 



est. 



at.fl. 

 Fig. 237. — Amphioxus, 



ph 



The forepart of the body cut in half 

 longitudinally. 



a/., "Atrium; at.jl., atrial floor; c.c, central canal of nerve cord; c.v., cerebral 

 vesicle ; d.f.r. t dorsal fin rays ; est. , endostyle ; n.c, nerve cord ; nch., notochord ; 

 or.c, oral cirri ; or.h., oral hood ; p.ph.b., peripharyngeal band ; pg., anterior 

 pigment spot ; pk., pharynx ; sk.c, skeleton of cirri ; sk.r., skeleton ring in oral 

 hood ; v.t., velar tentacles ; vm. , velum ; 'w.o., part of wheel organ. 



divided by septa, but greatly complicated by the presence of 

 the gill slits, which reduce it in the region of the pharynx to 

 a number of canals presently to be described. There are 

 numerous other ccelom ic cavities, of which the most 

 important are those in the region in front of the mouth, 

 in the velum, in the metapleural folds of the larva, and in 

 the gonads. As in the frog, the dorsal body-wall is much 

 thicker than the ventral. In it there lies a longitudinal, 

 hollow central nervous system, comparable to that of the 

 frog, but at the front end not enlarged into a brain, though 



