63] LIFE HISTOR Y OF TREMA TODES—FA UST 63 



Lymnaea catascopium and Physa heterostropha (1890). Rettger (1897) has 

 mentioned a larval tetracotyle in connection with a life-history study, but he 

 has failed to identify the species. 



Cercaria flabelliformis was found in three collections of Physa gyrina Say, 

 taken from the Bitter Root River in the vicinity of Corvallis, Montana, in 

 October 1916. Practically every snail from these collections bore evidence 

 of infection with the parthenita of this species, altho only 14.7 per cent of the 

 snails examined contained the tetracotyle. The mature cercaria has a length 

 of 0.48 mm. to 0.56 mm., and a width of 0.44 mm. It is about 0.2 mm. thick. 

 While the anterior end is not clearly set off from the posterior end as is usual 

 in holostomids, it does have the suctorial cup which includes all the ventral 

 suctorial apparatus, including among the rest the lateral suctorial grooves. 

 In the young larva these lateral organs are discoidal (Fig. 41); in the mature 

 tetracotyle they have become modified into lateral lappets (Fig. 40). 



The larva was found maturing within the redia, free in the liver interstices, 

 and encysted in the liver tissues. It was seldom found free in the tissues. 



The parthenita (Fig. 42) is a redia which measures 0.5 mm. in length by 

 0.052 mm. in transverse section. The head is set off from the trunk by a collar 

 prominence, while in the posterior third of the body are found the "walking 

 feet," which protrude ventrolaterad to support the redia. The posterior end 

 of the body is produced into a large knob, in which are parenchyma and 

 germinal epithehum cells. At the oral end is a wide muscular organ. It is 

 not clear whether it is a pharynx or an oral sucker. It is about 40/x in trans- 

 section and surrounds the fore-end of a gut 0.18 mm. long. On the ventral 

 side are two groups of salivary glands, six cells to each group, opening into the 

 anterior region of the gut thru a common duct for each group (Fig. 43). 

 Around the anterior end of the gut, just behind the muscular organ, is a nerve 

 complex (Fig. 42), differentiated into two ganghon masses on the dorsal side, 

 four nerve trunks, and a circumintestinal commissure. A birth-pore is 

 here ventral and shghtly sinistral. The wall of the parthenita is heavily 

 covered with an integument of non-cellular material, beneath which are 

 muscle and parenchyma elements. Running thru the parenchj^ma is a com- 

 plex diamond-pattern excretory system. 



The germinal epithelium is localized at the posterior end of the redia. 

 It offers an unusually fine opportunity for study of the maturation of the ova. 

 The detailed description of this maturation is found in the section on morpho- 

 logy (p. 16). 



The germ balls may differentiate into a second generation of rediae and cer- 

 cariae at the same time. These larvae are about equal in size as they develop, 

 but the cercariae differentiate much more rapidly than do the rediae so that 

 the two are readily distinguished. Usually only three or four cercariae are 

 found developing at one time in the redia, along with many daughter redia. 

 This fact seems to indicate that the animals have come to depend largely on 

 parthenogenetic propagation. The cercariae escape thru the birth-pore and 



