114 A. V. THOMAS. 



only I found a full-grown sporocyst in the foot of a snail ; the 

 survival of this one in so unsuitable a place was probably 

 owing to its having accidently forced its way into a connective 

 tissue space or into a venous sinus. The most natural situa- 

 tion for the development of the embryo seems to be the pul- 

 monary chamber, and this organ is, of course, from its position 

 and the' thinness of its walls, most easily accessible to the 

 embryo. Other embryos, however, may be found in the body 

 cavity of the snail. 



The average maximum duration of the embryo^s free and 

 active life in water is only about eight hours, though occasionally 

 one may live over night. During the last portion of the time its 

 movements become slower, and it will then in desperation often 

 endeavour to bore into any object which presents itself, even 

 into its own empty egg-shell. If an embryo has not succeeded 

 in finding a host, its motion becomes gradually feebler, and at 

 length ceasing the body assumes an oval or elliptical shape ; 

 the outer ciliated cells absorb water and swell up into round 

 vesicles, and the whole body disintegrates. In a feebly alkaline 

 solution of peptone I have kept them alive for three days. 

 The cilia were not lost until the third day, though their 

 motion became very sluggish ; the embryos increased a little in 

 size and remained alive even after a number of the ciliated cells 

 were detached. 



3. Sporocyst. — Arrived within the suitable snail the embryo 

 undergoes a metamorphosis, loses its organs of locomotion, and 

 degenerates into an inactive sporocyst. The outer layer of cili- 

 ated cells is lost, whilst the embryo changes in form. The 

 ciliated cells absorb water and appear as round or hemispherical 

 vesicles with the cilia standing out perpendicularly from their 

 surface (fig. 7). During the metamorphosis embryos may 

 have various irregular shapes, but sometimes retain a less 

 elongated conical form, even after they have lost the ciliated 

 cells. The conical form is, however, soon lost, and the embryos 

 take an elliptical shape such as is shown in fig. 8. The eye- 

 spots of the embryo become detached from one another and 

 lose their crescentic form ; but they, as well as the head- 



