82 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



The sympathetic or visceral system is constituted by a slender 

 nerve running on the under surface of the crop, which it supplies 

 with numerous branches. This nerve is probably connected in 

 front with the ventral cord ; behind it bifurcates to supply the 

 last pair of crop-pouches. 



The large pear-shaped nerve cells make up the external part 

 of the ganglia. 



9. The most important sense-organs are the ten pairs of eyes 

 situated in the dorsal margin of the anterior sucker. Each of 

 these (Fig. 24) is a cylindrical cup, covered by elongated trans- 

 parent epidermal cells and perforated by a nerve near its internal 

 end. The axis of the cup is occupied by slender cells in conti- 

 nuity with nerve fibres, and it is lined with large cells containing 

 refractive masses. A pigment-layer surrounds the eye except on 

 its outer side. 



A large number of tactile organs are found on the upper side 

 of the front part of the body, and there is a ring of tactile papillae 

 on the first annulus of every segment. These all closely resemble 

 the eyes in structure, but are devoid of pigment. It is probable 

 that the eyes in this case are modified tactile organs. The leech 

 possesses a very delicate sense of touch. 



LIFE-HISTORY. 



The breeding-season is spring. The fertilized ova are laid in 

 ellipsoidal cocoons, and these are deposited in the damp earth at 

 the margin of the water. Each cocoon is about an inch long, 

 and has a thick wall composed of a firm inner layer and a spongy 

 outer layer which is supposed to prevent desiccation. The cocoon 

 contains as many as twenty eggs floating freely in an albuminous 

 liquid. The cocoon is made from the hardened secretion of the 

 clitellar glands, and in the first stage of its formation is a cylin- 

 drical band surrounding the clitellar region. The leech draws 

 its body back out of this band, the ends of which then close up. 



The course of development is in many respects similar to that 

 of the earthworm, and germinal bands of the same nature are 

 present. The embryos are soon liberated from the eggs and 

 swim about in the surrounding albumen by which they are nour- 

 ished. The young leeches escape from the cocoon after several 

 weeks have elapsed, 



