20 



THE LANCELET 



HABITS AND EXTERNAL FEATURES 



The common lancelet Br anchio stoma lanceolatitm {=Amphioxiis 

 lanceolatus) , is a little, fish-like creature found on most European 

 coasts, including those of Britain, living in shallow water on a 

 sandy bottom. It passes most of its time buried in the sand, with 

 its long axis upright and the fore-end projecting, gathering small 

 organisms for food by a ciliated apparatus round the mouth. 



>^<^^' ,n.c. 



•>r.c. 



Fig. 224. — Br anchio stoma, from the left side, with the atrial floor contracted. 



;^y^-,v.j. r. 



Fig. 225. — Branchiostoma. from the ventral side. 



an Anus ; at.fl., floor of atrium ; at.fl'., the same cut through and turned back ; aip., atriopore ; atp'., line 

 indicating position of same in side view ; c.f., caudal tin ; d.f.r., rays of dorsal fin ; est., endostyle ; 

 gon. gonads; Ir., liver; m.f., metapleural fold; mym., myomeres; myc, myocommata or septa of 

 connective tissue between the myomeres ; n.c, nerve cord ; nch., notochord ; or.c, oral cim ; or.h., oral 

 hood ; ph., pharynx ; v.f.r., rays of ventral fin. 



From time to time, usually at night, it leaves the sand, and then 

 swims by movements of its muscular body, turning clockwise (as 

 seen from behind) as it does so. It is about an inch and a half 

 long, lustrous but translucent, slender, pointed at each end. and 

 flattened from side to side. The external features, and so much of 

 the general structure as can be seen without dissection, are shown 

 in Figs. 224 and 225. The head is in no way marked off from the 

 rest of the body, and there are no ears, nostrils, or limbs. The 



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