PHYLUM AXNULATA\\ 



423 



It. 



The Earthworm is devoid of organs of sight or hearing. It 

 -exhibits sensitiveness to bright light, which may be due to direct 

 action on the central parts of the nervous system. The sense of 

 hearing appears to be absent ; but a faculty analogous to taste or 

 smell, enabling the animal to distinguish between different kinds 

 of food, is well developed. The 

 yoblet-shaped liu<lic$, groups of 

 narrow epidermal cells, most 

 abundant on the prostomium 

 and peristomium, have probably 

 to do with this faculty. 



The organs of excretion 

 the segmented organs or nephrid'ui. 

 (Fig. 333) are similar to those , 

 of Nereis, but somewhat more I 

 complicated. They are slender 

 tubes which occur in pairs in all 

 the segments of the body except 

 the first three and the last. Ex- 

 ternally each nephridium opens 

 by one of the small excretory 

 pores which have already been 

 mentioned as occurring on the 

 ventral surface ; internally it ends 

 in a funnel-shaped ciliated ex- 

 tremity with an aperture, the 

 ne-phrost.orm:., opening into the 

 cavity of the corresponding seg- 

 ment. The tube is thrown into 

 several loops attached to the-, 

 posterior surface of the corre- 

 sponding mesentery by a fold of 

 membrane. Two parts' are clearly 

 recognisable an inner narrow 

 and an outer thick part : in the 



former the narrow central lumen is a perforation through the 

 axis of a string of cells, and is thus in/rcrdl/dar : it is lined 

 in ] tarts with cilia arranged in two rows: in the latter the 

 ,e is lined by cells, and is thus inter-edlulwr, and there is a 

 thick muscular investment. The nephridia are abundantly 

 supplied with blood by means of nephridial branches of the 

 ventral ves 



Reproductive Organs. The Earthworm is hermaphrodite. 

 There are two pairs of very small flattened testes (Fig. 334, te, te') t 

 partly divided^into a number of digitate lobes, situated in the 

 tenth and eleventh sr/ments. A pah\of comparatively Inrg- 

 the anterior r , m. .sv ///.), lie partly in the 



FIG. 333. Nephridium of an Oligoi 



'/,.-. mi's.-ntery ; :!>. terminal dilatation 

 (absent in the Earthworm), //-. Tiody 

 wall; /'.'M, thick; <i<i\, narrow part of 

 the tube; ^.funnel. (From Lang's 

 Text-Book.) 



