STUDIES OX THE ECTOPARASITIC TREMATODES OF JAPAN. !«;>[ 



quence looked much redder than they really were — thi« species 

 possessing a light flesh-red colour of its own. 



CoLOURATiox — Most of the monogenetic Treni;itodes that I have 

 studied may he described as having a colourless and transparent body; 

 but since the internal organs are more or less visible from the exterior 

 the body appears coloured. The only real exceptions in this respect 

 that I have observed are Tristomum sinuatum and T. rotumlauu which 

 possess, as just mentioned, a light flesh-red colour of their own; but 

 in which tissue of the worm this colour is lodged I liave not been able 

 to make out ; possibly it may be in the investing membrane. The 

 specimens of Hexacotijlc acuta that I have myself collected had all a 

 dusky colour like that of the gill of its host, but those that were 

 collected by my friend Mr. Xozawa were perfectly colourless except 

 where the vitellarium was :seen through, which is in this species more 

 or less dusky coloured. I therefore believe that the generally dusky 

 colour of my specimens was due to a greater development of the 

 vitellarium. 



The only internal organs that possess any colour of tlieir own are 

 the vitellarium and the pigment cells of the intestine already describ- 

 ed. In fresh specimens the former looks dark and granular l)y 

 incident li^dit, and slio:htlv vellowish by transmitted liü'ht : while 

 the pigment cells of the intestine look dark brown (jr }^erfectly black 

 by both incident and transmitted light. The body of most ecto- 

 parasitic Trematodes being thus transparent and allowiijg the substra- 

 tum to be seen through — for the vitellarium is in fresh specimens 

 not conspicuously dark, and the intestinal pigment-cells are not close 

 enough to each other to constitute a single visible object — they are some- 

 what difficult to detect at first sight. This, however, must not be 

 regarded as a case of protective colouration ; for in the first place, the 

 nature of the habitat already protects the parasites from being at- 



