1919] Cort: The Cercaria of the Japanese Blood Fluke 491 



with an oil immersion lens and low oculars. This method of study 

 is tedious, hard on the eyes, and requires long practice to use effectively, 

 but gives extremely satisfactory results. Miyagawa (1916, p. 64) 

 noted the difficulty of observing the living cercariae on account of their 

 great activity. He advocates the method of intravitam staining as 

 employed by Narabayashi. I have, however, found no advantage in 

 intravitam staining in the study of a cercaria such as that of S. 

 japoniciim in which the structures are clearly differentiated. To 

 check my observations on the living cercaria, serial sections were 

 made of a piece of a snail's liver containing the sporocysts and 

 cercariae. These sections were cut five micra in thickness and stained 

 with Delafield 's haematoxylin with a counterstain of erythrosin. The 

 study of the sections served to corroborate the observations made from 

 the living animals and added certain important details. 



CHARACTERS OF THE CERCARIAE OF THE HUMAN 

 SCHISTOSOMES 



The cercaria of S. japonic urn agrees with the cercariae of the two 

 other species of human schistosomes in a number of definite particulars. 

 Certain specific differences will be noted, but in the present state of 

 our knowledge it is difficult to discriminate clearly in morphological 

 characters between these three species of cercariae. This is undoubt- 

 edly due more to the limitations of our knowledge than to lack of 

 specific differences. The cercariae of the human schistosomes are very 

 small and develop in elongate motile sporocysts. They are fork- 

 tailed forms in which the divided lobes of the tail are less than half 

 the length of the main stem and definitely constricted from it. They 

 are without eyespots and pharynx and have a very rudimentary 

 digestive system. The whole surface of the body is covered with fine, 

 backward pointing spines. The ventral sucker is very small and in 

 the posterior third of the body length. The posterior half of the body 

 is filled with large unicellular glands, the ducts of which open at the 

 anterior tip after passing through a highly modified oral sucker con- 

 taining a central glandular reservoir. The reproductive organs of the 

 adult are represented by a small group of nuclei back of the ventral 

 sucker. The excretory system contains a very small number of flame 

 cells and a tubular bilateral type of bladder extending into the tail. 

 To sum up: the adult structures of the cercariae of the human 

 schistosomes are only slightly developed, and adaptive larval char- 

 acters for locomotion and penetration predominate. 



