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Class IL Leptocaedii {Lancelet). 



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

 worm-like form it was regarded as a worm bj 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, bence 

 the generic name {Ampliioxus, a/x<pi, 

 both, oSvi, sharp), the head-end be- 

 ing thin, compressed. The muscu- 

 lar segments are distinct to the 

 naked eye. From the mouth to the 

 Tent is a deep ventral furrow, and 

 a slight fin extends along the back 

 and Tentrally as far front as tliu vent. 



The lancelet, A. laiiceolafus (Pal- 

 las), lives in satid just below low- 

 water mark, ranging on our coast from 

 the mouth of Chesapeake Bay to 

 Florida ; it also occurs ou the South 

 American coast, and in the European 

 seas and the East Indies, the species 

 being nearly cosmoj^olitan. 



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 fi-om a circumoral 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 perforatei! 

 by long ciliated slits, comparable with those of the bran 



