452 TEXT-BOOK OF ZOOLOGY 



as such. After the skin has been thus sawed through, producing the 

 characteristic (triradiate) wound, the intestine, which is very extensile and 

 provided with lateral caecal pouches, is filled with blood to such an extent 

 that the leech frequently acquires four times its natural circumference. 

 It is, however, absolutely necessary that the animal should be capable of 

 thus taking in at once so large a supply of blood, which may last it for a 

 considerable time, for it may be years before it gets another opportunity 

 of thus "bleeding" a warm-blooded animal. It is to this peculiar 

 manner of feeding that the leech owes its application in medicine (hence 

 called medicinal leech). In the summer the adult females leave the 

 water, bury themselves in the damp earth near the bank, and there 

 deposit a number of eggs, protected by an envelope of mucus, which 

 hardens in the air. 



A much more frequent inhabitant of our waters is the so-called 

 Common or Horse Leech (Aulostoma gulo). In this species, however, 

 the teeth of the jaws are blunt, and cannot cut through the thick skin 

 of vertebrate animals. 



CLASS II. : ROUND OR THREAD WORMS (NEMAT- 



HELMINTHES). 



Body cylindrical, not divided into rings. 



The Trichina (Trichina spiralis). 



In 1835 an English student discovered in the muscles of a human 

 body, spirally coiled up, white, eyeless (compare with tape-worm) worms 

 about -i T inch long. They were enclosed in a small capsule or cyst, 

 and were called trichinae. Afterwards they were also discovered in the 

 muscles of the pig, rabbit, rat and several other animals ; but the 

 question how they had got to these parts was not easily answered. It 

 was not until the year 1860 that three German naturalists, by feeding 

 pigs and rabbits with meat containing trichinae, succeeded in throwing 

 some light on the life-history of this dangerous worm. This, briefly, is 

 as follows : If a portion of meat containing encysted trichinae is eaten by 

 man or another animal, the cyst is dissolved by the gastric juice. The 

 liberated trichinae find their way into the small intestine (intestinal 

 trichinae), where they rapidly grow to the length of about £ inch. They 

 next bore their way through the intestinal walls and enter the lymphatic 

 vessels (see Part I., p. 8). Here they produce a large number of living 

 young, and shortly afterwards die. The minute young worms are now 



