354 THE EARTHWORM CHAP. 



four eyes are present on the prostomium. Each eye 

 consists of a doubly-pigmented cup, the retina, formed 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 surface passes over the eye, 

 and a continuation of the epiderm, with its cells somewhat 

 flattened, constitutes a cornea (compare p. 182) 



Almost without exception, the Polychseta further differ 

 from the Oligochseta in being dioecious, and in passing 

 through a metamorphosis. The segmented oosperm gives 

 rise to a more or less oval larva known as a trochosphere 

 (Fig. 89), which swims by means of cilia arranged in 

 circles round the body, and gradually undergoes metamor- 

 phosis into the adult. 



PRACTICAL DIRECTIONS. 



EARTHWORM. 



Select a large earthworm, and after noting its movements and 

 mode of progression, kill by immersion in spirit for a few minutes and 

 then place in a dish and let the tap run on it for a short time. 



A. External Characters : 



1. Note : a. The form and colour of the body and its division into 

 nietameres ; b. the anterior end, terminating in the prostomiuiii and fol- 

 lowed by the peristomium ; c. the clitelhim ; and d. the last or anal 

 segment. 



2. If the worm be drawn through the fingers backwards, the setcz 

 will be felt : examine with a lens and observe their position and the 

 number in each segment. 



3. Make out the following apertures : a. the mouth ; b. the anus ; 

 c. the dorsal pores (p. 333) ; d. the two apertures of the spermiducts, 

 with thickened lips, on the fifteenth segment. 



(It requires careful examination to see the other apertures, viz. 

 those of the oviducts, spermothecce, and nephridid]. 

 Sketch from below or from the side. 



