[ 209 ] 



IX. On a New Genus of Trematoda, and some new or little-known Parasitic Hirudinei. 

 By John Denis Macdonald, M.B., F.B.S., Beputy Inspector- General B.N., 

 Brofessor of Naval Hygiene Army Medical School. (Communicated by G. E. 

 Dobson, M.A., M.B., F.L.S.) 



(Plate XXXIV.) 



Read April 6, 1876. 



MANY striking points of resemblance are traceable between the Trematoda and the 

 Hirudinei; but this is merely indicative of a representative relationship, or one of 

 analogy. The numerous errors into which our forefathers fell in grouping together 

 really incongruous things may be referred to the deceptiveness of prima facie resem- 

 blance, and the failing to recognize the intrinsic difference between the kind of relation- 

 ship here alluded to and that of a genuine affinity. This, I am quite sure, is one of the 

 greatest difficulties with which modern evolutional biology will have to contend. An 

 analogous property of gemmation in certain animals, in common with plants, being the 

 innocent cause, so to speak, of the dendritic or tree-bike form, so deceived the earlier 

 observers that even their very word Zoophyte is accepted by us as a merely descriptive 

 term, in a sense which is precisely the converse of that which its etymological construc- 

 tion would indicate. The balance was so far in favour of the plants, that the doubt 

 gave rise to the additional complication of confounding this group with the Corallines. 

 Now, we know that there are also other representative relationships between the two 

 great organic kingdoms, such as conjugation, fission, contractility, and motion, in con- 

 nexion with which errors of judgment are continually creeping into the dicta of science. 

 I might further say that the light available to Linnaeus and his immediate followers with 

 regard to these matters was scarcely less than that which we have at present to guide us 

 in the more difficult study of the primary modes of evolution observable in the ovum, 

 with the view of establishing the affinity or antipathy of the resultants, however different 

 or identical in type these latter may seem to be. Indeed, even up to the present, facts 

 are not wanting to show that very similar beings originate in a very dissimilar way, and 

 vice versa. Nevertheless we look forward with pleasure to the results that appear to be 

 promised to the labourers in this line of research. 



If we look upon the Trematoda as representing the Hirudinei, coupling also the 

 Oligocholia with the Bolychceta, and the Botifera with the Articulata, Prof. Huxley's 

 interesting classification, embodying a large proportion of the results just alluded to, 

 would seem to sanction the idea. Thus the corresponding part of his table may be 

 arranged, without material change, in the following manner : — 



1. Akch^ostomata. 2. Deuterostomata. 



Trematoda. Annelida. 



Oligochceta. Bolychceta. 



Botifera. Arthropoda. 



