236 



MANUAL OF ELEMENTARY ZOOLOGY 



~ph 



A 



eighth to the twenty-third, are the openings of a pair of 

 nephridia. On the middle annulus of each segment is a 

 transverse row of minute sense-papillae. On the head a 

 pair of these in each segment are transformed into minute 

 eyes, recognisable by their pigment as dark spots. There 

 are no setae. The worm can walk 

 by looping like a caterpillar, and swim 

 by undulation of its body. 



The body is covered by a thin 

 cuticle, which is shed from time to 

 time. Under this lies an epidermis, 

 between the bases of whose cells run 

 blood capillaries, so that the skin is a 

 respiratory organ, in which the blood 

 is exposed to the surrounding water. 

 Below stand circular and longitudinal 

 muscle-layers, and within these a 

 layer of botryoidal tissue, composed 

 of branched canals, whose walls are 

 laden with black pigment, while their 

 cavity is full of blood. This tissue 

 takes the place of a perivisceral cavity 

 and imbeds the gut. The mouth is 

 provided with three jaws, which are 

 compressed cushions of muscle 

 covered with a finely toothed layer 

 of chitin : by these the skin of the 

 mentary canal of the p rey is pierced with a characteristic 



r.!n 1C whL 'rt' Z t"-radiate wound. A muscular phar- 

 seen when it is not . . , . r 



ynx succeeds the mouth; it pumps 



the juices of the prey, and into it 

 open numerous unicellular glands 

 whose secretion prevents blood from 

 clotting so that the leech's food 

 remains fluid while it is being taken and passed backward 

 to be digested. It is owing to this secretion that bleeding 

 continues for some time after the leech is removed from its 

 wound, and an extract of the heads of these worms is some- 

 times used in physiological experiments to prevent clotting. 

 After the pharynx stands the very large crop (segs. 8-18), with 

 eleven pairs of lateral caeca, of which the last is much larger 



intr 



\l 



Fig. 151. — A diagram- 

 matic view of the ali- 



gorged with blood. 



1., cr. 11., Casca of the 

 crop ; int., intestine ; 

 Ph., pharynx ; m., rec- 

 tum ; st. t stomach. 



