GENERAL CHARACTERS 



419 



and a hepatic portal system ; usually two pairs of limbs, and 

 never more than two pairs ; and a series of body-muscles 

 which are divided into segments 

 or myomeres (p. 203), at any rate in 

 early stages, and are composed of 

 striped fibres. 



Now in various parts of the 

 world, occurring only rarely off the 

 English coasts, certain small fish- 

 like animals called lancelets are 

 found, the commonest species of * I 

 which is known as Amphioxus 

 lanceolatus (Fig. 106). This animal, 

 which possesses a median fin like 

 that of the tadpole (p. 207), is not 

 more than a couple of inches in 

 length, and lives in the sea near the 

 shore, burrowing in the sand ; it is 

 especially interesting owing to the 

 fact that it presents certain charac- 

 teristics indicating a near relation- 

 ship to the primitive ancestors of 

 Vertebrates. It possesses a noto- 

 chord, a dorsal hollow nervous 

 system, a pharynx perforated by 

 gill-slits, a hollow outgrowth of the 

 intestine representing a simple 

 liver, with a hepatic portal system, 



and a series of nephridia. But it differs from all the 

 higher Vertebrates in the following important points. 



The epiderm consists of a single layer of cells. There 

 is no distinct head and no skull ; the persistent noto- 



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