CHAPTER II 



AMPHIOXUS, A PRIMITIVE CHORD ATE 



A thorough knowledge of the type which forms the subject 

 of this chapter is fundamental for the study of vertebrates. 



Externals. — Amphioxus lanceolatus is a small animal 

 (about 2 inches long) found in shallow seas with a sandy 

 bottom in which it burrows. It is elongated and pointed at 

 each end, from which fact it derives its name. The body is 

 compressed from side to side and is capable of rapid move- 

 ment by swimming, though it usually stays embedded in 

 sand, feeding with only the mouth protruding. Both ends of 



be. n. gs. g -^ i f. 



Fig. 3. — Amphioxus seen from the left side. 

 a, anus ; ap, atriopore ; be, buccal cirrhi ; g, gonad ; gs, gill-slits ; n, 

 notochord ; t, tail. 



the animal are expanded into thin vertical fins, which are 

 joined by a shallow fin running all along the middle line of 

 the back. The fin also extends a short distance forwards 

 from the hind end on the ventral side. This median fin is of 

 simple structure ; it is supported by a row of stifTeners known 

 as " fin-ray boxes." These are made of connective tissue, 

 and they are more numerous than the segments of the body. 

 Along the dorsal fin the fin-ray boxes are arranged in a single 

 row, but along the short ventral fin there is a double row 

 of fin-ray boxes. 



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