28 



CHORDATE ORGANIZATION 



ii. 4- 



Amphioxus probably does not often swim free in the water and the 

 body is not adapted for fast movements. It has no elaborate fins such 

 as those of later fishes, which ensure static stability like the feathers 

 on an arrow, or are movable, to allow active control of the direction 



Fig. 6. Transverse section through amphioxus in the region of the pharynx. 



atr. atrium; d.a. dorsal aorta; d. coel. dorsal portion of coelom; div. intestinal diverticulum; 

 d.n. 1 and d.n. 2 , branches of the dorsal nerve-root; end. endostyle; ep.gr. epipharyngeal groove; 

 /. fin-ray box; g. gonad; I y.b. primary gill bar containing coelom; my. myotome; metapl. 

 nietapleural fold; n. notochord; n.c. nerve-cord; ph. pharynx; sub. end. coel. subendostylar 

 coelom; t.b. tongue bar; v. a. ventral aorta; v.n. ventral nerve-root. (After Krause.) 



of swimming (p. 136). There is a low dorsal ridge, which continues 

 behind as a small caudal fin. There are no definite paired fins, but the 

 metapleural folds might perhaps be considered comparable to the 

 lateral fin folds from which all vertebrate limbs are probably derived. 

 They are distended with coelomic fluid and, with the dorsal ridge, 



