418 
Marine Cercariae 
alimentary canal however is not always formed so soon, especially in 
those forms in which extremely large unicellular glands occupy the 
greater part of the body. There are two kinds of glands which are 
sometimes very conspicuous in the cercaria, the cystogenous glands 
which secrete the material for the cyst and those glands which always 
are present if a boring spine or stylet occurs in the oral sucker. These 
latter glands are the “ Stacheldrtisen ” or stylet-glands which help the 
worm to bore into its host, the stylet piercing and the glands pouring 
out their fluid at the same time. Both cystogenous and stylet glands 
may be present or only one kind. The cystogenous glands sometimes 
occupy almost the whole of the body as in some of the Echinostomes, 
others occur in small masses; all open by ducts near the oral sucker. 
The stylet glands disappear after the cercaria has successfully entered 
its second host. The cystogenous glands disappear after the cyst is 
formed, as does also the tail. The intermediate host may be a Vertebrate 
or an Invertebrate. In marine Trematodes fish serve as Vertebrate 
intermediate hosts, the cercariae often encysting under the skin, in the 
muscles, nerves and various organs. Crustacea and Mollusca are the 
usual Invertebrate hosts; crabs and Amphipods are very commonly 
infected, the cercariae occurring in the liver, gonads or muscles. In 
Mollusca the foot, mantle, digestive gland or gonad serve the purpose. 
Nearly always the intermediate host, if a Mollusk, is a Pelecypod. In 
some cases the encysted stage takes place within the sporocyst. Here 
the life-history is much abbreviated and often the cercaria is tailed but 
loses its tail before it encysts. In other cases the resting stage is 
omitted because the final host swallows the redia with cercariae; here 
again we find the cercaria loses its tail before it is full grown. 
In one group, the Gyninophallus group, the cercaria is always 
without a tail, and apparently has to reach its host by creeping 
movements or depend upon currents to sweep it along. This was 
noticed by Jameson (1902, p. 160), who found that because of the 
cercariae of the “ Pearl Trematode ” not having tails only those mussels 
fixed on the ground were much infected with pearls; those clinging to 
wood above were almost free from them. 
Glassification of Cercariae. 
Ltihe (1909, p. 173) has classified the various cercariae according to 
their tails but there appears to me to be something to be said against 
this method, excellent though it be in many ways. The chief objection 
seems to be that certain forms, which by their organisation are clearly 
