CHAP. IX 



419 



red blood-corpuscles ; a liver, and a hepatic portal system ; 



usually hi'o pairs of limbs, and never more than two pairs ; 



and a series of body-muscles which 



are divided into segments or 



myotneres (p. 203), at any rate in 



early stages, and are composed of 



striped fibres. 



5- 



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 ^.^f ^ ^.-3 



outgrowth of the intestine repre- " s 5 



senting 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 notochord 



= '=:=. 

 * 'C . 



m 



^3 ..? 



lii 



: - - - 



