394 COLLEGE ZOOLOGY 



of the chordates in a simple condition. Furthermore it is prob- 

 ably similar to the ancestors of the VERTEBRATA. 



Amphioxus is several inches long. The semi-transparent 

 body is pointed at both ends and laterally compressed. It is 

 found near the shore, where it burrows in the clean sand with 

 its head or tail, and conceals all but the anterior end. It some- 

 times leaves its burrow at night and swims about by means of 

 rapid lateral movements of the body. When it ceases to move, 

 it falls on its side. 



External Features (Fig. 341). Although Amphioxus is 

 shaped like a fish, it differs from the latter in many important 

 respects both externally and internally. There are no lateral 



msz. 





FIG. 341. An adult specimen of Branchiosloma lanceolatus, seen from the 

 left side as a transparent object, an., anus ; atp., atriopore c., caudal fin ; 

 ci., buccal cirri; df, dorsal fin; e, eye-spot; fr, fin-rays; g 1 , g 26 , twenty-six pairs 

 of gonadial pouches; m l , m 36 , m, myotomes; n, neural tube; nch., notochord; 

 vel., velum; ves., vestibule; vf., ventral fin. (From Bourne.) 



fins and no distinct head. Along the mid-dorsal line is a low 

 dorsal fin (df) extending the entire length of the body and widen- 

 ing at the posterior end into a caudal fin (c.). The caudal fin 

 extends forward on the ventral surface (vf.). Both dorsal and 

 ventral fins are strengthened by rods of connective tissue, called 

 fin-rays (fr). In front of the ventral fin the lower surface of 

 the body is flattened, and on each side is an expansion of the 

 integument called the metapleural fold (Fig. 342, mp). 



The body-wall is divided into a number (62) of V-shaped muscle 

 segments, the myotomes (Fig. 341, m l , w 36 , w 52 ); these are sepa- 

 rated from one another by septa of connective tissue. The myo- 

 tomes on one side of the body alternate with those on the other 

 side. The muscle fibers contained in them are longitudinal, and, 



