PHARYNX 



353 



of the pharynx has very thick walls composed of bundles 

 of muscular fibres which are concerned in the move- 

 ments of a pair of laterally placed chitinous jaws. The 

 anterior part of the alimentary canal is capable of being 

 everted, as a proboscis, until the jaws are thrust forth and 

 project freely, so that they can be brought to bear on 

 some small living animal or fragment of animal matter, 

 which is thus seized and swallowed. 

 In correspondence with its different mode of life, 



FIG. 89. Trochosphere of the worm Eupomatus, from the side, highly magnified. 

 an. anus ; md. mid-gut ; n. larval head-nephridium ; sp. neural plate (brain) ; 

 st. stomodapum ; ivk. pre-oral ciliated ring ; wk.i post-oral ciliated ring. (From 

 Lang's Comparative Anatomy, after Hatschek.) 



Sfereis is much better provided with sense-organs than is 

 the earthworm. The tentacles and palps, as well as the 

 cirri, are probably organs of touch ; and as we have 

 already seen, four eyes are present on the prostomium. 

 Each eye consists of a doubly-pigmented cup, the retina, 

 :ormed as an invagination of the ectoderm, with a small, 

 rounded aperture, or pupil, and enclosing a mass of 

 gelatinous matter, the lens. The cuticle of the general 



PRACT. ZOOL. 



