LIFE-HISTORY. 41 



shells. Within each shell are a single ovum or germ- 

 cell and a large number of yolk-cells. After the escape 

 of the egg from the body of the host, segmentation 

 occurs, an embryo develops, and a circular operculum 

 at one end of the shell opens to allow it to escape. 



2. The free larva is conical, with a short papilla at its 



broad anterior end ; the whole surface is covered 

 with long cilia, enabling the larva to swim rapidly. 

 Two eye-spots are present, and two 'flame-cells,' 

 or excretory organs. The ectoderm is a single layer 

 of flattened cells, usually arranged in five transverse 

 bands : within is a mass of granular cells. When 

 this larva meets with Limncea truncatula, a small 

 amphibious snail, the head-papilla becomes elon- 

 gated, and by means of it the larva bores its way 

 into the snail. Unless the larva happens to come 

 across a Limncea within about eight hours after its 

 escape from the egg, it dies. 



3. Development of the sporocyst. Within the snail, usually 



in its pulmonary chamber, the ectoderm cells of the 

 larva swell, lose their cilia, degenerate, and are 

 thrown off ; the remainder grows rapidly, and in two 

 or three weeks becomes an elongated sac, '024 inch 

 long. This sac, the sporocyst, has an outer struc- 

 tureless cuticle, a thin muscular layer, and an 

 epithelial layer lining the cavity. 



The eye-spots, though losing their form, persist ; 



and ciliated excretory funnels are present. Such 



sporocysts sometimes, though rarely, multiply by 



transverse fission in the early stages of their 



development. 



B. The Second Generation consists of Rediae, which develop 



within the sporocyst, from cells that may be looked 



upon as unfertilised ova, and are themselves parthe- 



nogenetic. 



1. From the epithelium of the sporocyst, cells are budded off 



which segment to form solid masses or morulas lying 



