SECTION V 



THE LANCELETS— 

 CEPHALOCHORDA 



General position. External appearance. General characters. Functions 

 and habits. Development and life-history. Relationships. Classification. 

 Distribution. Features of special evolutionary interest. 



NEAR the base of the great series or phylum of back- 

 boned animals (Vertebrata) a place must be found for 

 a number of small semi-transparent widely distributed 

 marine animals known as lancelets. One of the best known is 

 called Amphioxus lanceolatus or Branchiostoma lanceolatum. 

 It is a matter of opinion whether they should be included within 

 the Vertebrata, or kept by themselves in the vicinity of Verte- 

 brata, and grouped along with Tunicates and acknowledged 

 Vertebrata under the general heading Chordata. We shall 

 return to this question ; in the meantime it is enough to say 

 that the Lancelets are either simple Vertebrates, or are allied to 

 simple Vertebrates and at a somewhat lower grade. Their 

 chief interest for the zoologist is their threefold simplicity, (a) 

 There is simplicity in some parts of their organisation — but 

 this is combined with a remarkable complexity in other parts ; 

 (d) there is, in certain respects, an interesting simplicity of 

 function, e.g., as regards digestive system, sensory system, circula- 

 tory system. But again it will be seen that the physiology of 

 the whole pharyngeal region is much more complex than in 

 higher animals; (c) thirdly, there is simplicity in many re- 

 spects as regards the development. Several considerations lead 

 one to think that the Lancelets are much less primitive than they 

 at first sight appear. They are probably derivatives of a pri- 

 mitive stock which have specialised on a line of their own — a 

 cul de sac as regards higher forms. 



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