112 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 
haematobia; 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, Suceinea 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 Cesropa and the TREMATODA have been consider- 
ably modified by leading an endoparasitic life. They have 
lost their locomotor organs, and are dependent on cilia or on 
