450 



ZOOLOGY 



SECT. 



The development of the Myzostomida closely resembles that of 

 the Polychseta. A trochosphere larva is first formed, and this 

 becomes metamorphosed into a larva with provisional setae, bearing 

 a close resemblance to the larva of Nereis (p. 415). 



FIG. 351. Myzostoma. Diagrammatic view of the internal organs, c, cirri ; da, branches of 

 the stomach ; ul, hind-gut ; h, testes ; klo, aperture of cloaca ; ;, stomach ; //<", male genital 

 aperture ; o, ovaries ; p, parapodia, with hooks and supporting rod ; ph, pharynx ; jihji. 

 pharyugeal tentacles; pht, pharyngeal pouch; sb, vesicula seminalis ; u, uterus; v:o, female 

 genital aperture. (From Lang's Text-book.) 



CLASS IL GEPHYREA. 



The tiqiJit/rca. are marine Annulata devoid of any trace of segment- 

 ation in the adult condition, without parapodia, and either without 

 seise or with only a limited number ; with either an invaginable 

 anterior body region or introvert, at the extremity of which is the 

 mouth surrounded by tentacles, or with a long, highly retractile 

 proboscis representing the prse-oral lobe of the larva, and having 

 the mouth situated at the base. The anus is sometimes terminal 

 and posterior, sometimes anterior and dorsal. There is an exten- 

 sive ccelome filled with a corpusculated fluid, and not divided by 



