492 University of California Puhlications in Zoology [Vol. 18 



STRUCTURE OF THE CERCARIA 



Measurements of the cercaria of S. japonic um are very unsatis- 

 factory data for comparison, since the length of the living animal 

 varies greatly with extension and contraction. Preserved specimens 

 are much contracted and vary in length with the method of killing 

 and the contraction state when killed. Further, there is no evidence 

 that all cercariae of the same species will develop to the same size in 

 different sporocysts or different host individuals. Figure 1 is an enlarge- 

 ment from a camera lucida drawing of a slightly compressed cercaria 

 and gives an idea of the size and shape at an average state of extension. 

 The body of the cercaria has the power of extension to almost twice its 

 contracted length and the stem of the tail is capable of even greater 

 changes in length. The body of the cercaria and the stem of the tail 

 are cylindrical in cross-section. When the body is fully extended its 

 diameter may be reduced to that of the tail. IMiyagawa (1916, p. 64) 

 gives the length of the body in well extended specimens as 0.09 to 

 0.21 mm. Leiper (1915, p. 202) gives the body length of the specimens 

 of the cercaria of 8. japonicum measured by him as about 0.1 mm. 

 This measurement agrees with Miyagawa's measurement on preserved 

 material. Leiper, however, does not state the condition of the speci- 

 mens which he measured. 



The whole surface of the body and tail of the cercaria of 8. japon- 

 icum is covered with backward pointing, cuticular spines. Beneath 

 the delicate cuticula are found two layers of muscles, an outer circular 

 and an inner longitudinal (figs. 1, 2). The individual strands of the 

 longitudinal layer have about twice the thickness of those of the 

 circular layer. The body of the cercaria is so completely filled with 

 the various organs that the parenchymatous tissue is limited to a very 

 thin superficial layer (figs. 1, 2). Since the flame cells and tubules 

 of the excretory system develop in the parenchymatous tissue they are 

 also limited to this superficial region of the body. 



The tail of the cercaria of 8. japonicum has well developed mus- 

 cular layers, especially in the stem region (fig. 1, st). The strength 

 of these muscles is shown clearly by the power of vibration, extension 

 and contraction which the tail possesses. The main stem of the tail 

 is round in cross-section, while the divided lobes which are definitely 

 constricted off from the stem are somewhat flattened. The parenchy- 

 matous tissue of the tail is dense, containing numerous nuclei obscur- 



