354 . THE EARTHWORM CHAP. 



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 Poly chaet a further differ 

 from the Oligochaeta in being dioecious, and in passing 

 through a metamorphosis. The segmented oosperm 

 gives rise to a minute, 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 metamorphosis 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 metameres : b. the anterior end, terminating 

 in the proslotnium and followed by the peristomium ; c. the 

 clitellum ; and d. the last or anal segment. 



2. If the worm be drawn through the fingerS'backwards, 

 the setce 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 aper- 

 tures of the sbermiducts, with thickened lips, on the 

 fifteenth segment. 



(It requires careful examination to see the other apertures, 

 viz. those of the oviducts, spermotheccs, and nephridia.) 

 Sketch from below or from the side. 



B. Dissection. 



I. Take a freshly-killed worm in the left hand, and 

 carefully insert the point of the fine scissors into the integu- 

 ment about one-third of the way along the body, close to 

 the middle dorsal line. Place a drop of the cailomic fluid 



