467] LARVAL TREMATODES—CORT 21 



In Cercaria inhabilis the anlage of the reproductive organs begins 

 to take definite shape. It is composed of dense masses of small heavily 

 staining nuclei. In the largest cercariae four areas connected by lines 

 are marked out. At the middle line of the body just back of the bifurca- 

 tion of the intestinal ceca, is an elongate mass which reaches up very 

 close to the ventral surface. This is probably the anlage of the ends 

 of the ducts of the reproductive system, leading up to the genital pore. 

 Slightly back of this and close together are two masses, the primordia 

 of the testes, and further back is the largest densest mass which repre- 

 sents the ovary and its surrounding structures (Fig. 16, ra). 



Along with the cercariae in the livers of the infected snails were 

 numbers of active radiae all in about the same stage of development 

 (Fig. 15). When they were freed from the snail they were very mobile 

 extending and contracting and making some progress even on the smooth 

 surface of the watch glass. There were two pairs of locomotor append- 

 ages and the posterior extremity was attenuated and pointed. The 

 anterior pair of locomotor appendages was just back of the posterior 

 limit of the intestine and at normal extension the second pair was 

 about the same distance back. The tail region was shorter and more 

 slender. These proportions varied greatly with the contraction of the 

 animal. In alcoholic material the locomotor appendages of the redia 

 are often obliterated by the contraction of the muscles. 



One of the largest of these rediae (Fig. 15) measures 1.36 mm. 

 in length and 0.31 mm. in width, and the posterior limit of the volumin- 

 ous intestine is 0.56 mm. from the anterior tip. The pharynx is very 

 small in proportion to the size of the body, measuring 0.086 mm. in 

 length and 0.065 mm. in width. The intestine contains dark brown 

 materal evidently from the liver of the host. Bunched around the 

 anterior tip of the redia just back of the oral sucker are elongate uni- 

 cellular glands of the type often found in rediae, which send forward 

 ducts to open near the tip. 



The cuticula and muscle layers of the rediae of Cercaria inhabilis 

 are quite thin. The circular muscles are the strongest but do not show 

 externally. Inside of the muscles except in the region of the anterior 

 part of the intestine and the germ gland, the wall is made up of a layer 

 of cells which is thin in the older forms. In none of the rediae are 

 embryos much differentiated, the largest showing mere stumps of tails, 

 the beginning of suckers and traces of eye-spots. Since immature cer- 

 cariae are found outside of the redia it is evident that they make 

 their way out at a very early stage, and complete their development 

 free in the snail's liver. 



