73] LIFE HISTOR Y OF TREMA TODES—FA UST 73 



is pointed at the tip. It is directed ventrad by about ten degrees more 

 than the shank. The stylet has a length of 44^t and a breadth at the base 

 of the shank of 14/1. 



Cercaria dendritica was secured from two collections of Lymnaea proxima 

 taken from the chara sloughs at Fort Missoula, Montana, in October 1916. 

 The infection was in the Hver interstices. The parthenita (Figs. 87-89) is a 

 well-developed sporocyst, with an attachment disc, but without any indica- 

 tion of a digestive tract. It seems to approach a redia more nearly in its 

 structure than any other described sporocyst. The sporocyst is muscular and 

 heavily covered with integument. The parthenogenetic eggs develop from 

 a germinal epithelium situated at the antipodal end from the disc. The 

 stages of cleavage are clearly made out from the study of the germ cells pro- 

 liferating from the germinal epithelium (Fig. 89). This layer is closely 

 pressed against the epidermis. As the cells mature they increase in size. 

 The increase continues thruout the cleavage, so that a three-cell stage is 

 larger than a one-cell stage, and a morula is larger than a five-cell stage. This 

 continued growth of the embr^'o is accounted for by the nourishing medium 

 which bathes the sporocyst. This is a special case of nurture, where the 

 growth stimulus is reacted to immediately. The germ ball attains a con- 

 siderable size before it begins to differentiate, altho epiboly has occurred soon 

 after the morula stage has been reached. The animal is mature before it 

 breaks thru the wall of the sporocyst and swims out into the inter-cecal spaces. 



The larva has an interesting excretory system (Fig. 81). An immense 

 spheroid bladder, somewhat crenate, opens dorso-posteriad thru a small pore. 

 Anterior it receives the contents of two large muscular cornua thru a common 

 opening. These cornua extend laterad to the extreme margin of the animal. 

 At the outside of each, at the margin of the worm, arise three tubes, one 

 directed posteriad and two directed anteriad. The capillaries are dendritic. 

 The caudal tube is a median canal without any prominent tubules. 



The alimentary system consists of a pharynx with muscular fibers developed 

 early. It has a width of 30fj. and a length in section of 36/x. A short, attenuate 

 esophagus opens posteriad into two vestigial furcae. Anterior and lateral 

 to the acetabulum are the salivary-mucin glands, eight to each group. They 

 are moderately large (Fig. 85), and empty thru common duct systems into 

 the oral pocket. 



The conspicuous features of the genital system (Fig. 86) are the large swollen 

 vagina, and the prominent Laurer's canal, the latter extending out on the 

 left side of the acetabulum under its posterior margin. In the mid-area, 

 just behind the acetabulum, are two small pyriform testes. The vitelline 

 glands extend from the extreme anterior margin of the worm to the extreme 

 posterior end; they are attenuate, sparsely branching serpentine chords, 

 composed of a long anterior and a short posterior portion. The vitelline 

 ducts run in from the postero-lateral regions to the ootype, which is just 

 anterior to the testes. This tj-pe is suggestive of Plagiorchid relationships. 



