296 ERNEST WARREN. 



cyst-Avall are iiidistiiiguisliable from tliose lying deeper. 

 After a time there is formed an outer layer of apparently 

 modified protoplasm; the nuclei of the cells which have 

 produced it lie on the inner side and are quite irregularly 

 disposed, and may be connected with it only by strands of 

 protoplasm. These cells become the epidermal cells of the 

 adult. 



The course of development after the thick cercaria-cyst is 

 formed may be briefly recapitulated in the following para- 

 graphs : 



(1) The cortex-protoplasm secretes a thin cuticle studded 

 with very symmetrically arranged spinelets. 



(2) The fundament of the cirrus-sac becomes a thick-walled 

 muscular tube enclosing the embryonic vesicula seminal is, and 

 it lies obliquely across the embryo. 



(3) The cortex is continued upwards by differentiation 

 from one end of the embryo ; this will split, and will constitute 

 the ventral surface of the future adult. 



(4) Oral and ventral suckers are differentiated. 



(5) The gut-caeca are formed from verj^ flat cells, Avhich 

 become arranged into a right and left vesicle, meeting at the 

 fundament of the pharynx. 



(6) Fundaments of the gonads appear. 



(7) The genital atrium is formed as a deep pit, which 

 opens at first into the depression of the ventral sucker. 



(8; The fundament of the cirrus-sac is continued forwards 

 to the genital atrium, but does not as yet open into it. The 

 fundament of oviduct, uterus, and vagina a])pears as a short 

 tube running from the ovary to the atrium. 



(9) A strand is formed passing from the pharynx to the 

 oral sucker ; this splits, and thus the mouth and pre-pliarynx 

 originate. 



(10) The excretory vesicle is formed as a V-shaped tract, 

 which splits into a V-shaped, thin-walled vesicle opening to 

 the exterior by a pore lined by the general cuticle. 



(11) The intra-cellular ducts of excretory system are formed. 

 Yolk-glands appear. 



