348 ON SOMK TREMATODK PARASITES OF AUSTRALIAN FKOGS, 



passing out, and it now appears that I was formerly led to an 

 erroneous conclusion Ijy a wrong inference from the presence of 

 the muscular tissue in the walls and the size of the canal. In 

 anv case, in most of the worms studied in this paper, the metra- 

 term is of such a form and structure that, taken in conjunction 

 with the form of the penis in the same specimen, and with what 

 I have seen of the function of Laurers canal in Dolichosaccus, I 

 think there can be no doubt that this metraterm is the real 

 vagina. 



Notwithstanding this, Cohn(l:], H) seems to iiavc definitely 

 observed one or more cases of actual copulation through Laui'er's 

 canal. I think, however, the instances mentioned by liim must 

 be looked upon as abnormal cases. 



Literature— 2,4, 1 2, 14, 22, 26, 27, 29, 30, 35, 37, 44, 4o, 47, 48, 92. 



iSECTION vi. 



General Conclusions. — A consideration of the trematodes de- 

 scribed above, from Australian frogs, affords a striking illustra- 

 tion of the tendency of Helminths to occur in fauna) groups. 

 That is to say, that the Helminths found parasitic in any par- 

 ticular class of host, in a definite zoogeographical region, find 

 their nearest relatives, not in that region in which they them- 

 selves occur, but in the same class of host living in other zoogeo- 

 graphical regions. Four such groups from Amphibia are now 

 more or less extensively known, viz., European, North American, 

 Asiatic, and Australian. In each region, we find, in hosts of this 

 class, trematodes pretty closely corresponding with representatives 

 in the other regions. The group in the European region, as the 

 longest known and most extensively investigated, shows the 

 greatest immber of genera : in point of numbers, the American 

 group, to which a good deal of attenti(jn has been given of late 

 years by Stafford and othei's, follows closely on the European. 

 The Australian and Asiatic groups show smaller numbers, partly, 

 perhaps, because less completely worked up. The facts present 

 seem to indicate that, in Asia and Australia, at least, the number 

 of representatives may be further more or less extensively added 

 to by subseijuent investigations. Besides the frogs, other classes 



