-8 



.-V-12 



FIG. 270. Diagram of the 

 internal organs of the leech. 

 i, head with eye-spots; 2, mus- 

 cular pharynx; 3, ist diverti- 

 culum of crop; 4, nth diver- 

 ticulum of crop; 5, stomach; 

 6, rectum; 7, anus; 8, cerebral 

 ganglia; 9, ventral nerve cord; 

 10, nephridium; n, lateral 

 blood vessel; 12, testis; 13, vas 

 deferens; 14, prostate gland; 

 15, penis; 16, ovary; 17, uterus. 

 (From Hegner, College Zoology, 

 after Shipley and MacBride.) 



ECONOMIC ZOOLOGY 



Leeches, Fig. 27. The leeches, of which the 

 genus Hirudo may be taken as an example, 

 are somewhat flattened annelids, with a large 

 posterior and a smaller anterior sucker; the 

 mouth is in the centre of the latter and may 

 or may not be armed with jaws or teeth. 

 Only the digestive system will be noted 

 here. It consists of a short pharynx into 

 which open certain glands whose secretion 

 is to keep the blood, on which the animal 

 feeds, from clotting. The pharynx, Fig. 

 270, 2, opens into an enormous crop, 3, 4, 

 with about n pairs of lateral pouches for 

 the storage of enough blood to last, perhaps, 

 for many months, as the leech may have 

 very infrequent opportunities for feeding. 

 Back of the crop are a small stomach, 5, 

 and intestine, 6, opening to the exterior 

 through a dorsally located anus, 7. The 

 leech is admirably adapted to its peculiar 

 parasitic mode of life. Its suckers enable 

 it to quickly attach itself to a passing ani- 

 mal, mammal, turtle, fish, etc., its mouth 

 enables it to extract blood from its host 

 without pain to the host, its glands keep 

 the blood from clotting, and its huge crop 

 enables it to store up a very large quantity 

 of blood at each of its probably infrequent 

 feeding times. 



This power of extracting a considerable 

 quantity of blood, in a painless manner, 

 has given the medicinal leech, Hirudo medi- 

 cinalis, its value to man. In the case of 

 bruises and certain other pathologic condi- 

 tions it is merely necessary to apply a 

 hungry leech they may be 12 to 15 cm. 

 long to the desired spot and it will pain- 

 lessly gorge itself with blood and then drop 



