Art. XXVII.—On the Geographical Distribution of Atax.
[Read before the Otago Institute, 12th July, 1892.]
Engaged as I have been upon the study of the Unionidœ, I have endeavoured to obtain Australian and New Zealand species, and in particular to secure examples containing the animals. Up to the present time I have only been able (thanks to the kindness of Professor Hutton) to obtain specimens of one species—Unio menziesii, Gray—containing the
animal, and this I find to be somewhat similar to some forms of Unio from the Brazils. The branchiœ are fastened throughout their whole length to the mantle and to the abdomen, and it is in the internal branchiœ that the eggs and the embryos are contained. Among nine specimens examined by me I have not found any parasites. What is wanted is that observations should be directed to other species, and also that collections of young individuals should be made with apices intact. Nothing is as yet known of the configuration of the beaks (umbones) of the shells of the New Zealand and Australian Unios, nothing at all (as far as I know) of their anatomy and embryology, and nothing about their parasites.
In my studies on the Unionidœ of the Brazils I was struck by the fact that certain parasites on them are somewhat nearly related to European species, while others known to occur in Europe are wanting. For example, I have never met with Aspidogaster in the pericardium, but I have come across several sporocysts, or rediæ, in examining the cercaria, especially those of Bucephalus, scarcely different from European species.
I do not know whether any one has also noticed this in New Zealand or in Australia. I have frequently also observed species of Atax, a genus of (parasitic) Hydrachnids commonly found in the Unios and Anodontas of Europe. The three or four species which I have observed were studied and described by Dr. Koenike, of Bremen, as they were all new. Dr. Koenike has verified the fact recorded by Leidy, that in the Unios of the United States there exists, among other species, one — Atax ypsilophorus—identical with a form found in Europe. With regard to the other species, it will be necessary to continue observations in order to learn which are common to the two regions and which are peculiar to each.
I was therefore curious to see if Atax also occurred in Unio menziesii; but hitherto I have not met with it. I am not acquainted with the literature and the geographical distribution of the Hydrachnids, especially with regard to those of Australia and New Zealand, and the object of this note is to draw attention to the subject as one which merits attention and study. A complete knowledge of the distribution of animals will enable us in time to reconstruct the ancient geography of the globe in the different geological epochs, so that the presence or absence of certain genera in the different countries or islands [will] enable us to form definite conclusions on the time of appearance of the genus. On this ground the geographical distribution of parasites gives us a method of arriving at their geological age which palæontology does not furnish us with.