LIFE-HISTOEY 33 



They are ovoid bodies, 'IS mm, long and '08 mm. 

 broad, enclosed in smooth brownish ohitinous shells. 

 Within each shell are a single germ-ceU and a large 

 number of yolk-cells. The germ-cell segments 

 before the egg is laid, but no further development 

 occurs till after its escape from the body of the 

 host. Afterwards an embryo is formed, and a cir- 

 cular operculum at one end of the shell opens to 

 allow it to escape. 



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



broad anterior end ; the whole surface is covered 

 with long cilia, enabling the embryo to swim rapidly. 

 Two eye-spots are present, and two ciliated funnels, 

 probably excretory. The ectoderm is a single layer 

 of flattened cells, usually arranged in five transverse 

 bands, within which is a mass of granular cells. 

 When this embryo meets with Limncea truncatula, 

 a small freshwater snail, the head-papilla becomes 

 elongated and by means of it the embryo bores its 

 way into the snail. The free life of the embryo is 

 usually of about eight hours' duration. 



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



in its pulmonary chamber, the ectoderm cells of the 

 embryo swell and lose their cilia ; the embryo grows 

 rapidly, and in two or three weeks becomes an elon- 

 gated sac, '6 mm. long. This sac, the sporocyst, 

 has an outer structureless 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 develop- 

 ment. 

 B. The Second Generation consists of redise, which are pro- 

 duced asexually within the sporocyst, and are themselves 

 asexual. 



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

 which segment to form sohd masses or morulsB lying 



D 



