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 setoe, bearing 

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



V 



^ 



\ 



B 







M f 



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

 the stomach ; ed, hind-gut ; h, testes ; Uo, aperture of cloaca ; TO, stomach ; mo, male genital 

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

 pharyngeal tentacles ; pht, pharyngeal poxich ; sb, vesicula seminalis ; u, uterus ; wo, foma" 

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



CLASS IL GEPHYREA. 





| The Gepliyrea are marine Annulata devoid of any trace of segment- 

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

 setae 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 praB-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 coelome filled with a corpusculated fluid, and not divided by 



