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 Polychaeta further differ 

 from the Oligochseta 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 

 few minutes and then place in -a dish and let the tap rur 

 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 prostomium 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, spermothecce, and nephridia.} 

 Sketch from below or from the side. 



B. Dissection. 



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

 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 ccelomic fluid 



