CHAP. IX 



GENERAL CHARACTERS 



419 



red blood-corpuscles ; a liver, 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 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 interest- 

 ing owing to the fact that it pre- 

 sents certain characteristics indi- 

 cating a near relationship to the 

 primitive ancestors of Vertebrates. 

 It possesses a notochord, a dorsal 

 hollow nervous system, a pharynx 

 perforated by gill-slits, a hollow 

 outgrowth of the intestine repre- 

 senting a simple liver, with a 

 hepatic portal system, and a 

 series of nephridia. But it differs from all the 

 Vertebrates in the following important points. 



The epiderm consists of a single layer of cells. 



higher 



There 



is no distinct head and no skull ; the persistent notochord 



