80 



FISHERY BULLETIN OF THE FISH AND "WILDLIFE SERVICE 



Redia. — Specimens of this stage agree closely 

 with jiublished descriptions. 



Cewana.— Comparison with published descrip- 

 tions (tables 2, 3) reveals slight diflferences which 

 could easily result from development in a differ- 

 ent snail host or from the fixative employed. 



Most measurements of cercariae which were 

 killed with gentle heat and measured in sea water 

 tended to be greater than those preserved by the 

 classic method of relaxing them by rapidly swirl- 

 ing in a small amount of sea water before flooding 

 by hot fixative. The difference was due either to 

 incomplete relaxation before fixation occurred or 

 to agonal contraction caused by contact with 

 fixative. 



The cercariae encyst readily on any available 

 object. Sizes of living cy.sts from several sources, 

 measui'ed in situ, are comjiared in table 2. The 

 range in length and width was considerably 

 greater in my material than in that of others. 

 The differences which exist are most likely due to 

 the parasites having developed in three different 

 snail hosts. 



Comparison of cyst measurements made before 

 and after fixation revealed generally insignificant 

 fixation-induced changes: Gilson's and Carnoy's 

 fixatives caused slight shrinkage, while Bouin's 

 fixative caused slight swelling. 



Table 2. — Comparison of living P. acanthus cercariae and 

 cysts from several sources 



[All measurements in millimeters] 



Table 3. — Comparison of P. acanthus cercariae prepared 

 by different methods 



[All measurements are in millimeters] 



' = Thais lapillus. 



' Average of 46 cysts: 0.270 mm. AT 0.231 mm. 



' Unstained, measured in sea water. 

 2 Stained and mounted in Permount. 



Adult. — Table 4 compares permanent prepara- 

 tions of sexually mature adults fi'om the present 

 study with published descriptions. No major 

 differences exist. These data illustrate the range 

 of variation wliich can result from development in 

 different hosts. 



Life Cycle 



The adult was described by Nicoll (1906, 1907), 

 Linton (1914, 1928), and Stunkard and Cable 

 (1932). 



The cercaria, described by Lebour (1907), was 

 first found in rediae from Purpura {^Thais) 

 lapilhis. 1\\ 1914, after comparing it with young 

 P. acantlnus adults from herring gulls, she cor- 

 rectly inferred that C. purpurae is a larval stage 

 of P. acanthus. Later, with Elmhirst (1922), she 

 reported a life cycle which erroneously included 

 a molluscan second intermediate host (either Car- 

 dium edule or Mytilus edulis) . 



The first correct life-history description is that 

 of Stunkard and Cable (1932). By feeding cysts 

 derived in vitro from cercariae naturally emitted 

 from the oyster drill, Urosalp'mx cinerea, to com-  

 mon {Sterna hirundo) and roseate terns {S.^ 

 dougalH), they proved conclusively that only two 

 hosts are necessary : a marine snail and a marine 

 bird. 



Details of miracidial structure, cercarial anat- 

 omy and encystment, and germ cell cycle in both 

 larval and adult stages were reported by Eees 

 (1937,1939, 1940). 



