396 Zoological N. Y. Zoological Society. [I;20 



are interesting, where they may be studied, it is not often that 

 an opportunity is aiforded to see them alive and pursuing their 

 usual mode of life. This, however, is quite possible in some 

 cases, for instance, where Microcotyle infest the gills of certain 

 fish. By exposing the gills they may be seen literally covered 

 with the worms hanging on the gill filaments while they suck 

 the blood from under the delicate mucous membrane. On the 

 gills, too, may be seen the eggs deposited in hundreds and held 

 in position by the tangled filaments which are attached to either 

 end of the egg. As soon as they are hatched they are prepared 

 to fasten themselves on the gills and to take up the reproduction 

 of their kind and to continue the work of their ancestors. The 

 great irritation caused by all this action induces an outflow of 

 thick mucus, covering the gills and preventing the access of the 

 water to their surfaces, so that the fish even may be suffocated. 

 In addition to this they cause great depletion of blood which 

 soon renders the fish exsanguine and causes its death. 



Another strange habit with most of these parasitic worms 

 is, that they have a particular locality or habitat where they con- 

 fine themselves in or on the body. Thus the Microcotylidae, Di- 

 plectana and Octocotylidae are found no where else than on the 

 gills, and the former worm is at least the cause of death of nine- 

 ty per cent of the angel and butterfly flshes in the tanks of the 

 Aquarium. They are probably not so plentiful, and consequently 

 not so fatal to these fish in their natural habitat, the open sea. 



DIPLECTANUM 

 Sub-Family Gyrodactylidae 

 Genus Tetraonchus 

 Sub-genus Diplectanum 



Among the ectoparasitic trematodes there are few which 

 while being as small, are yet more interesting than these forms. 



They have been classed in the order Heterocotylea and in- 

 cluded by Van Beneden and Hesse in the family of Gyrodactyli- 

 dae. As far as known now, they are confined to the gills of ma- 

 rine fishes and we have no record of them until 1857, when Wag- 

 ener was fortunate enough to discover three members of the 



