EXAMINATION OF HELMINTHES 



143 



rediae are slightly more complex. They are characterized by the 

 possession of an alimentary canal and, in the greater number of cases, 

 it is possible to make out the pharynx, together with a portion of the 

 single straight intestinal tube (fig. 65). 



The pressure exerted by the cover-glass, and by manipulating and 

 spreading the material upon the slide, will set free many cercariae in 

 various stages of development. These should be carefully studied. 

 It is more expedient, however, to remove the sporocysts without 

 subjecting them to pressure, fix them in hot sublimate and, after 

 staining, cut them into longitudinal sections. 



The sporocysts of certain varieties are branched ; 

 such are those which occur in the snail (Succinea 

 amphibia), and in certain mussels (Anodonta, Unio). 

 In both these instances, the cercariae show consider- 

 able divergence from the type described above. 

 Those which develop from the sporocysts found in 

 Succinea are not provided with a tail and do not 

 emerge from the sporocyst, but encyst in its blind 

 branches. 1 The cercariae which are found in the 

 mussel varieties, on the other hand, emerge from 

 the sporocyst and are provided with a tail which 

 is split along its entire length. 2 Cercariae with par- 

 tially split tails are occasionally found in Limnaeidae, 

 while cercariae with blunt tails are found in land- 

 snails. 



Instructive material is also furnished by en- 

 cysted Trematodes, some of which, while in this 

 stage, become sexually mature and form eggs. They 

 are found in widely different organs in hosts of 

 many species, both vertebrate and invertebrate. 3 



FIG. 65. Redia 

 of Echinostomum 

 sp. (from a fresh- 

 water snail). Mag- 

 nified. ( After Leuck- 

 art.) 



(2) CESTODES (TAPEWORMS). 



Of the tapeworms parasitic in man a small number only have any 

 very wide distribution, and these are principally met with in Central 



1 Infected Succinea may be recognized by the tentacles, which become very 

 swollen and through the thin walls of which a brightly coloured cylindrical structure, 

 which is actively motile, may be seen. This structure is the mature blind end of the 

 sporocyst. See Zeller, Z. f. wiss. ZooL, xxiv, 1874 ; and G. A. Heckert, Bibl. zool., 

 A'ol. iv, 1889; also Bronn's " Kl. u. Ordn. d. Thierr." 



- This cercaria is known as Bucephalus and is. a developmental stage of Gastero- 

 stomum, which inhabits the intestine of predatory fish. See Ziegler, Z. /. wiss. ZooL, 

 vol. xxxix, 1883 ; and Bronn's " Kl. u. Ordn. d. Thierr." vol. iv, 1 a. 



3 O. v. Linstow, " Compendium der Helmmthologie," Hanover, 1878 ; Nachtrag, 

 Hanover, 1889. 



