STRUCTURE OF THE LEECH. 193 



chain, by a narrow nerve-ring surrounding the beginning of 

 the gut. From the dorsal centres nerves proceed to the 

 " eyes " and anterior sense-spots, from the ventral centres the 

 general body is innervated,- and from the beginning of the 

 ventral chain special nerves supply the alimentary canal, 

 forming what is called a visceral system. 



The sense-organs of the leech are ten so-called " eyes," 

 besides numerous sense-spots usually occurring on every 

 fifth skin-ring. The eyes are distributed on the first eight 

 rings, five on each side, and look like little black spots. 

 Microscopic examination of sections shows them to be 

 definite cups, surrounded by connective tissue with black 

 pigment. At the base of each cup a nerve enters, and 

 breaks up into fibrils, which are continued into long sens- 

 ory cells in the centre of the cylinder. These are sur- 

 rounded by large clear cells, which may act like small 

 lenses, and the mouth of the cup is covered with skin. 

 Following the structure of a vertebrate eye, we may call the 

 protective outside wall a sclerotic, the pigment layer a choroid, 

 the sensitive cells in the centre a retina, and the surrounding 

 clear cells lenses. But it would be bold to say that the leech 

 really sees with its ten " eyes ;" sensory organs they certainly 

 are, but very blind eyes. The sense-spots which are distri- 

 buted over the body in regular segmental order, look like 

 simple homologues of the " eyes," differing, however, in 

 having fewer clear cells and no pigment layer, while the 

 sensitive cells in the centre are more obviously derived from 

 the epidermis. AVe can only guess as to their function, 

 whether it be tactile, olfactory, or otherwise. 



The Alimentary System. — When the leech has firmly 

 fastened itself to its prey by the hind sucker, it brings its 

 muscular mouth into action, pressing the lips tightly on the 

 skin, and protruding three chitinous tooth-plates which lie 

 within. Each of these tooth-plates is worked by muscles, and 

 is like a semicircular saw, for the edge bears from 60 to 100 

 small teeth. Rapidly these saws cut a triangular wound, 

 whence the flowing blood is sucked into the mouth by the 

 muscular pharynx. The process may be observed and felt 

 by allowing a hungry leech to fasten on your arm. As the 

 blood passes down the pharynx, it is influenced by the secre- 

 tion of salivary cells which lie among the muscles, and exude 



N 



