182 S. GOTO. 



tacked by their enemies, and in the second place, they are but very 

 imperfectly exposed to light, and thus the conditions of their exist- 

 ence prevents any effective play of natural selection. 



Injury to the Host — On this head I am only able to add 

 an observation or two to that of v. Baer on NitzscUia elongata men- 

 tioned by Braun.'^ It has already been said that Tristomum simiatum, 

 like Diplozoon Nipponicum, can extract blood from the gill of the 

 host, and thus may cause some injury to it. In one case I observed 

 that a specimen of Diclidojjhora elongata had caused an abnormal 

 excrescence to form around its caudal disc, which, together with the 

 posterior part of the body proper, was thus completely immersed 

 in the excrescence and hidden from view. 



C. Systematic. 



In this part of my paper I shall confine myself to the cha- 

 racterisation and description of the genera and species studied by 

 myself, leaving their systematic classification to those who may have 

 occasion to study personally a larger number of forms and especially to 

 reexamine the species previously described. Owing to incompleteness 

 in the literature I could gain access to, I have not been able to make the 

 historical notices as exhaustive as I could wish ; they are, however, 

 published with the hope of lightening, in ever so slight a degree, the 

 labours of future workers in this field. For the reason above stated I 

 have also been compelled to satisfy myself in many places with second 

 hand quotations and references. In such cases the authorities are 

 invariably mentioned. 



Before proceeding to the systematic descriptions of the different 

 genera and species, I wisb to remark once for all with special em- 



1). Braun — Wiirmer. p. 512. 



