l82 



LEECHES. PHYLUM ANNELIDA 



-ph 



crt 



sis 



UU.\} 



■cr.u. 



toothed layer of cuticle : by these the skin of the prey is pierced 

 with a cliaracteristic tri-radiate wound. A muscular pharynx 

 (Fig. 130) succeeds the mouth ; it sucks up 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 sometimes used in physiological 

 experiments to prevent clotting. After the 

 pharynx comes the very large crop (segments 

 8-18) with eleven pairs of lateral caeca, of 

 which the last is much larger than the rest 

 and extends backwards on each side of the 

 remainder of the alimentary canal. This 

 consists of the stomach — a narrow tube, with 

 an enlargement at its start and a spiral fold 

 of its inner wall — the intestine, narrower 

 than the stomach, and the rectum, somewhat 

 wider. The blood sucked for food is stored 

 in the crop, whose caeca are more or less 

 dilated according to the amount of their con- 

 tents, and passed drop by drop into the 

 Fig. 130.— a diagram- stomach, where it at once turns green and then 



matic view of the ali- • j- -. j a x n 1 -n i j. xi, • i 



mentary canal of the IS digested. A lull meal Will last the animal 

 medicinal leech, as for several months or a year. Seventeen 



pairs of nephridia lie in segments 6-22. Each 



is a mass of glandular tissue traversed by a 



system of intracellular ductules. There is 



no internal opening, but those which lie 



in the testis somites have a swollen end 



lying in the capsule of the testis, and bearing a number of ciliated 



funnels which do not communicate v/ith the ductules. There are 



two systems of tubes which contain a fluid like blood — a red 



plasma with a few colourless corpuscles. One of these systems is 



the true blood-vascular system : its principal vessels are two 



lateral trunks, which unite before and behind, and are connected 



also by a network of capillaries. There are no hearts, but the 



lateral vessels are contractile. The other vascular system consists 



of n dorsal and a ventral longitudinal sinus, which are also 



rm.- 



seen when it is not 

 gorged with blood. 



cr.i., cr.ii., Caeca of the 

 crop; int., intestine; 

 ph., pharynx ; rm., rec- 

 tum ; St., stomach. 



