148 INVERTEBRATE MORPHOLOGY. 



bile-ducts of Sheep, in which it produces what is termed the " Kot," which, 

 in the low-lying pastures of England and the Continent, is frequently the 

 cause of the destruction of large numbers of sheep. In exceptional cases 

 it has been known to occur in man. In Egypt, however, the Fellaheen 

 are not unfrequently attacked by another form, Distoimim hcematobium, 

 which is peculiar in that, contrary to the rule, the sexes are separated in 

 different individuals. The margins of the body of the male are rolled 

 inwards on the ventral surface, forming a tube within which the more slen- 

 der female lives. Associated in pairs in this way, they are found in the 

 blood of the portal vein and its connections and pass to the ureters and 

 bladder, in whose mucous membrane they deposit their ova, thus pro- 

 ducing an inflammation, accompanied by suppuration, of these organs. 



Development of the Trematodes. The ova of Trematoda 

 consist of two distinct parts, a germ-cell, the product of the 

 ovary, surrounded by a mass of food-material, the secretion 

 of the vitellaria, the whole being enclosed in a shell formed 

 by the shell-gland. In the Polystomese the development, as 

 a rule, is entirely carried on outside the body of the parent, 

 the stalked ova being attached to the body of the host, though 

 Gyrodactylus is viviparous. In the Distomese, however, the 

 reverse is the rule, the ova undergoing a certain part of their 

 development in the uterus of the parent, and leaving the egg 

 shortly after its extrusion as a larva, sometimes ciliated, 

 sometimes provided in the place of the cilia with a structure- 

 less cuticle, and furthermore in these endoparasites there 

 occurs a remarkable alternation of generations of the kind 

 already referred to as heterogony (see p. 61). 



The heterogony may be of various degrees of complexity. 

 It begins, however, in all cases w T ith the embryo (Fig. 79, A), 

 which may be a free-swimming ciliated organism provided 

 with a short pouch-shaped intestine and with a mouth, and 

 frequently possessing also a nervous system and pigment 

 eye-spot as well as excretory tubes ; in other cases, however, 

 as stated, the embryo is destitute of cilia, usually in this case 

 being provided with one or more spines at the mouth-end 

 of the body, and all gradations of degeneration of the eye- 

 spot and nervous system, as well as of the excretory tubes 

 and digestive system, may be observed. In all, however, the 

 space between the more or less developed digestive, tract and 

 the body-wall is occupied by numerous unspecialized cells 



