682 DAVID HILT TENNENT. 
evaporation, the salinity of the pond was greatly raised the 
growth of the cercariz was inhibited. 
SUMMARY. 
The observations detailed in this paper may be summarised 
as follows: 
1. Germ-cells which arise within the wall of the sporocyst 
of G. gracilescens give off small cells, which may be con- 
sidered of the nature of polar bodies, segment, and develop to 
the cercaria B. haimeanus. 
2. The cercaria B. haimeanus, Lacaze-Duthiers (1854) 
= B. cuculus, McCrady (1868), parasitic in the oyster, 
differs structurally from B. polymorphus, von Baer, para- 
sitic in fresh water mussels, only in the shape of the tails. 
3. G. gracilescens, Wagener (1852) parasitic in Lophius 
piscatorius = Gasterostomum sp., Linton, 1899, parasitic 
in Tylosurus marinus, differs from G. fimbriatum, von 
Siebold, only in habitat (one is a marine, the other a fresh- 
water, form), and slightly dissimilar larva. 
4, B, haimeanus, as shown by experimental evidence, is 
a larval stage of G. gracilescens. 
5. Experimental evidence shows that supposedly different 
species are physiological varieties of the same species. 
6. he presence of the sporocysts and cercaria of G. 
gracilescens in the oyster prevents the formation of repro- 
ductive elements, and also renders the host incapable of with- 
standing adverse conditions. 
7. B. haimeauus thrives best in oysters growing in 
brackish water. Its growth is inhibited by increased salinity. 
It will be noticed that there is one gap in my account of 
the life-history of Gasterostomum. 1 have not yet proved 
the infection of oysters by embryos developed from the eggs 
of the adult. I hope to make this the object of experimental 
work within the near future. 
