Diseases of Fishin the Adirondacks 21 
slit. The sack is connected with the exterior by a duct extending from 
its posterior end. 
A better appreciation of the difficulties involved in unravelling the 
life history of Clinostomum may be had if we outline what is known 
about the development of some of the other distomes. This group 
of flatworms is exclusively parasitic throughout life in the early 
and mature stages. Ihe egg is enclosed in a chitinous shell and 
supplied with a mass of food cells. These eg 
the host. In one species (Cryrodactylus) the eggs develop in the body 
gs may be attached to 
of the parent. The egg of Polystomum hatches six weeks after ovipo- 
sition, producing a minute larva (.3 mm. long) which swims about 
freely; but 1f in twenty-four hours it does not become attached to a 
tadpole, it dies. After finding a suitable host it attaches itself and 
eventually reaches the bladder by traveling the length of the digestive 
canal. Here it remains three years before attaining maturity. 
The development of distomes is indirect and the life history often 
very complicated. From the egg a larva arises which enters a tem- 
porary host. Gamble says that “the larva gives rise by a peculiar 
process to numerous individuals of a second larval form, and these 
usually produce a third form from which the minute immature trema- 
tode is developed.” Thus a large number of sexual individuals may 
be derived from a single egg. The larvae usually live in molluscs, 
the mature worm in vertebrates and the immature but metamorphosed 
trematode in either host. 
The majority of distomes are hermaphroditic and many are capable 
of self-fertilization. In such cases the sperms ripen first and pass over 
into the uterus. The so-called egg consists of a fertilized ovum and 
a mass of yolk-cells. After the larva becomes attached to its host, 
it may degenerate into a sac-like structure full of germ cells and in 
this stage is designated a “sporocyst.” The masses of cells in the 
sporocyst may give rise to another generation of larvae. Certain 
structures become characteristic of this larva, such as a pharynx and 
straight digestive sac. The name “redia” has been given to this 
second larva. The rediae move about in the sporocyst and eventually 
burst the wall, thus escaping. The free rediae may give rise to a 
number of new individuals, the “ cercariae.” Gamble* states that 
“the cercaria is just visible to the naked eye and has an oval or dis- 
coidal body and usually a long tail of variable form.” The body 
of the cercaria contains in miniature all the organs of the adult. 
*Gamble, F. W. 1910-11. Trematodes. 
The Encyclop. Brit. 77 edition, Vol. XXVII. 
