PLATHELMINTHES 181 



The eggs of Dishviwn hepaticum are found in great numbers 

 in the gall-bladder of the host inhabited bj the worm. From 

 liere thej pass into the intestinal canal, to be voided together 

 with the faeces. Their development begins only after thej 

 are outside the host. If by chance tlie egg reaches the 

 water, then the favourable conditions for development are 

 present. In from three to six weeks afterwards the embiyo 

 abandons the egg-shell (variations in the time of develop- 

 ment are caused by higher or lower temperatures). By 

 means of the cilia which thickly cover it, the embryo, or 

 better the larva, easily moves about in the water. It pos- 

 sesses an x-shaped eye-spot (Fig. 89 A). Under this lies a 

 ganglion. The intestine is only slightly developed. Two 

 ciliated funnels already represent the beginning of the ex- 

 cretory system. The remaining part of the body is filled 

 with the germ cells, the origin of which has already been 

 considered in treating of the embryonic development, and 

 the significance of which consists in the production of the 

 subsequent developmental stages of the Distomum. 



In this condition the larva may swim about for as many as 

 eight hours ; then it perishes, unless it meets with a snail, 

 into whose respiratory cavity it bores. In this process 

 its cephalic protuberance (also interpreted as tactile organ) 

 is said by Thomas to render good service. Limnceus minutus 

 (8. truncatulus) is now to be regarded with certainty as the 

 intermediate host of Distomum hepaticum j as the investi- 

 gations of Leuckaet, confirmed by Thomas, have shown. 

 Having arrived in the respiratory cavity or other organ 

 of the snail, the larva casts off its coat of cilia and secretes 

 about itself a cuticula-like envelope. It now grows and 

 becomes a sac-like body, which is called a spoi'ocj/st (Fig. 89 

 B). In it the germ cells become enlarged, and by 

 repeatedly dividing produce the cell-masses which give rise 

 to a new generation. The sporocyst besides has the power 

 of reproducing itself by transverse division. To this end it 

 constricts itself in the middle of the body, and produces two 

 new sporocysts. 



The generation produced in the sporocyst consists in turn 



Eres which are sac-like, but which are more highly 



