CHAPTER XIX 



THE LANCELET. CHORDATA 



The Common Lancelet, AtnphioxuS lanceolatus, is a little, 

 fish-like creature found on most European 

 Habits and coasts, including those of Britain, living in shal- 

 Features. low water on a sandy bottom. It passes most 

 of its time buried in the sand, with its length 

 upright and the fore end projecting, gathering small 

 organisms for food by a ciliated apparatus around the 

 mouth. From time to time, usually at night, it leaves the 

 sand, and then shows that it can swim swiftly by movements 

 of its muscular body. It is about an inch and a half long, 

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

 flattened from side to side. The head is in no way marked 

 off from the rest of the body, and there are no ears, nostrils, 

 or limbs. A low dorsal fin runs along the middle of the 

 back from end to end, becoming deeper at the hinder end 

 as the upper lobe of a caudal fin, which passes round the 

 end of the tail. The under lobe of this is continuous with 

 a low, median ventral fin which extends along the hinder 

 third of the body. In front of the ventral fin the belly is 

 flattened and bears at each side a continuous lateral fin or 

 metapleural fold. The sides of the body are marked by a 

 series of about sixty v-shaped lines, with their apices 

 forwards, due to septa of connective tissue known as 

 myocommata, which divide the muscles of the body-wall 

 into segments called myomeres, Certain of the internal 

 organs are repeated in correspondence with these, so that 

 the body is segmented, though not so completely as that of 

 the earthworm. The segmentation is peculiar in that the 

 myotomes of opposite sides alternate. About seven 



myotomes lie in front of the mouth. The anus lies against 



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