178 S. GOTO. 



consists of numerous concentric layers and is provided at one end 

 with a hollow filament, by means of which the egg is attached to the 

 host. In view of these facts, it can scarcely be doubted that we have 

 here a case which can not well be brought under the' category of ' 

 either commensalism or parasitism ; for the Trematode seems to 

 derive its food, like other species, from the gill of the fish, deriving 

 only some advantage by attaching itself and its eggs to the body of 

 the -copepod. It is not, however, clear to me what advantage the 

 parasite derives by attaching its eggs to the copepod. 



Another interesting fact may be here mentioned. It is this, that 

 Tristomum sinuatum and Tristomum ovale, which inhabit the same host 

 {Histio-phorus sp.), keep strictly separate from each other, and appear 

 never to wander one into the habitat of the other. T. ovale, namely, 

 is found in the mouth-cavity — on its wall or on the branchial arches 

 — while T. simtatum is confined to the inner surface of the branchial 

 plates — the branchial filaments of the same series coalescing in the above 

 mentioned fish with each other and forming a single plate. I have never 

 found any specimen of T. ovale attached to the gUl ; nor have I any of 

 T. sinuatum on the outer surface of the gUl plate. Perhaps this depends 

 on the nature of food of the two species, T. sinuatum living on blood, 

 while T. ovale seems to live on the slime of the mouth-cavity ; but 

 then it is not easy to see why T. sinuatum never wanders out to the 

 outer surface of the gill-plate. 



Locomotion — The locomotory movements of some ectoparasitic 

 Trematodes have been described by HaswelP^ and Monticelli^^ ; 

 and I have observed similar movements in Tristomum sinuatum, 

 Monocotyle Ijimae, and some species of Micvocotijle. When on one 



1). Haswell — On Temnocephala, an Aberrant Monogenetio Trematode. Quart. Joum. of 

 Mic. Sc, vol. XXVIII, 1888. p. 282 et infra. 



2). Monticelli— Saggio. pp. 19-20. 



