12 ANATOMY OF AMPHIOXUS. 



the number of muscle-segments, the latter forming one 

 of the chief diagnostic characters for a given species. 



The extensive geographical distribution of Amphioxus, 

 combined with the fact that it is a shore-dweller and not 

 a roving pelagic animal, and also with its remarkably 

 constant features and, as a rule, trifling specific differ- 

 ences, shows that we have to do with an extremely 

 archaic form. 



EXTERNAL FORM. 



A good idea of the external appearance an propor- 

 tions of AuipJiioxiis lanceolatits can be obtained from the 

 accompanying figure (Fig. i). Its actual length varies 



Wesssspe-f^^ 

 TlWJM^SZ-~~y- 



Fig. I. Amphioxus Lanceolatus from the left side, about twice natural size. 

 (After LANKESTER.) The gormdic pouches are seen by transparency through the 

 body-wall ; the atrium is expanded so that its floor projects below the metapleural 

 fold ; the fin-chambers of the ventral fin are indicated between atriopore and anus. 

 The dark spot at the base of the fifty-second myotome represents the anus. 



from about four to as much as eight centimetres. In 

 the fresh condition it is semi-transparent, so that some 

 of the internal organs can be seen through the skin, which 

 is often iridescent. 



The figure shows the pointed extremities of the body 

 and the circlet of tentacles or buccal cirri round the mar- 

 gin of the mouth, or more accurately, the oral hood, 

 because the mouth proper is covered over by a hood-like 

 fold of the integument, from the margin of which these 

 processes grow out. Extending from near the anterior 

 extremity of the body to the posterior end are seen some 

 sixty-two oblique parallel lines, each bent upon itself in 



