ZOOLOGY 



after a short free life attach themselves to the gills of a tad- 

 pole. When the gills atrophy, the larvae proceed down the 

 alimentary canal and eventually reach the bladder of the 

 young frog. Here they take five or six years to reach 

 maturity. This Trematode has at its hinder end a disc, round 

 which are grouped numerous hooks and suckers. 



Gyrodactylus elegans has a similarly -situated triangular 

 plate which bears two large hooks in the centre, and sixteen 

 smaller ones round the edge, by means of which it attaches 

 itself to the fins of sticklebacks and other freshwater fish 

 (Fig. 74). Its most remarkable feature is that it is viviparous, 

 and its embryos before they leave the body of their mother 

 have already developed their embryos inside them; and the 

 latter may contain their embryos, so that four generations may 

 be included under the cuticle of the sexually mature animal. 



B. The Digenea have always one, and usually several asexual 

 intermediate generations intercalated between the sexual, 

 and their life-history usually involves residence in two 

 distinct hosts. 



The asexual generations usually inhabit some Mollusc, more 

 rarely they attack fish. The sexual forms are found in all 

 classes of the Vertebrata. The genus Distoma includes 

 more than three hundred species, eight of which infest the 

 human race. 



One of the most dangerous human parasites is Bilharzia 

 haematohia ; it is remarkable amongst Trematodes for its sexes 

 being separate. The mature worms are found in couples, the 

 female partly enclosed in a gynaecophoric canal or groove on 

 the under side of the thicker male. They inhabit the blood- 

 vessels of the bladder and give rise to considerable disturbance 

 in the system. Their eggs escape with the urine, but their 

 future fate is not known. 



Leucochloridium paradoxum is parasitic in the body of a 

 snail, Succinea putris ; it develops two sacs which grow into 

 the tentacles of its host, which may ultimately be ruptured 

 by the increase of these structures. 



Both the Cestoda and the Teematoda have been consider- 

 ably modified by leading an endoparasitic life. They have 

 lost their locomotor organs, and are dependent on cilia or on 



