178 



B. GOTO. 



consists of mimerons concentric layers and is provided at one end 

 with a liollow 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 

 tlie 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 

 Tristoimim slnuatum and Tristomum orale, which inhabit the same host 

 (^Histiophorus sp.), keep strictly separate from each other, and appear 

 never to wander one into the habitat of the ()ther. T. ovale, namely, 

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

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

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

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

 found any s])ecimen of T. ovale attached to the gill ; nor have I any of 

 T. siHuatuin on the outer surftice of the gill plate. Perhaps this depends 

 on the nature of food (jf the two species, T. sinualnm living on bl(3od, 

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

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

 outer surf ice <^f the gill-plate. 



Loco:motion — The locomotory movements of some ectoparasitic 

 Trematodes have been described by liasweir^ and Monticelli"^ ; 

 and I have observed similar movements in Trintomum ."iuniatiini, 

 Monocotijle Ijlinae, and some species of Mievocotijle. When on one 



1). Haswell — On Temnocephala, an Aljerrant Monogenetic Treuiatode. Quart. Journ. of 

 Mic. Sc, vol. XXVII r, 1888. p. 282 ct infra. 

 2). Monticelli— Saggio. pp. 19-20. 



