406 



ZOOLOGY. 



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Bifi 



Hi , 



CLASS II. LEPTOCAEDII (Lancelet). 



The lancelet is the only type of this class. From ite 

 worm-like form it was regarded as a worm by some authors, 

 and as a mollusk ("Limax") by 

 Pallas. The body is four or five cen- 

 timetres in length, slender, com- 

 pressed, pointed at each end, hence 

 the generic name (Ampliioxus, acjjfpi, 

 both, o%v s, sharp), the head-end be- 

 ing thin, compressed. The muscu- 

 lar segments are distinct to the 

 naked eye. From the mouth to the 

 vent is a deep ventral furrow, and 

 a slight fin extends along the back 

 and ventrally as far front as the vent. 



The lancelet, A. lanceolatus (Pal- 

 las), lives in sand just below low- 

 water mark, ranging on our coast from 

 the mouth of Chesapeake Bay to 

 Florida; it also occurs on the South 

 American coast, and in the European 

 seas and the East Indies, the species 

 being nearly cosmopolitan. 



As this is the lowest Vertebrate, its 

 structure and mode of development 

 merit careful study. 



The mouth is oval, surrounded 

 with a circle of ciliated tentacles 

 supported by semi-cartilaginous pro- 

 cesses arising from a circu moral ring. 

 The mouth leads directly into a large 

 broad pharynx or "branchial sac" 

 (Fig. 387, d), protected at the en- 

 trance by a number of minute cili- 

 ated lobes. 



The walls of this sac are perforated 

 by long ciliated slits, comparable with those of the bran- 



