58 M. E. Bessels on Species of Ata,x. 



but nevertheless cannot be regarded as blood-corpuscles. I 

 see in them appurtenances of the embryonal envelope which 

 Clajoar^de denominates the deutovum. Whilst at the com- 

 mencement of embryonal development of many insects a cel- 

 lular envelope separates from the blastoderm, and in some 

 Crustacea a larval skin, which is usually struct ureless^ in Atax 

 a larviform structure first separates from the blastoderm, and 

 shortly afterwards the contractile cells. This state of things, 

 when regarded in this manner, furnishes an additional reason 

 for regarding the embryonal envelope of Atax as the homo- 



logue of the protoderm of insects. 



***** 



In the course of his memoir Clapar^de suggests the ques- 

 tion whether Van Beneden has not perhaps fallen into an error 

 in representing the parasites of Aiiodonta as derived from 

 Unio, or whether the same animal is parasitic upon Anodonta 

 in Belgium that lives in Unio at Geneva, 



In an appendix to a letter sent by me to Van Beneden, 

 which will be printed in the next number of the ' Bulletins 

 de I'Academie de Belgique,' Van Beneden remarks that he 

 actually took the Atax figured in his work above cited from 

 the branchiae of Anodontce. 



I will here briefly communicate a case of migration from 

 one kind of mollusk to the other. 



When I was making my investigations of the embryology 

 of Atax^ I wished not to have to procure fresh material con- 

 stantly, and therefore placed some hundred specimens of Ano- 

 donta cygnea^ obtained from Esslingen, in a large well-trough 

 with water running through it. As I also desired to study 

 the natural history of the parasites of Unio, in about three 

 months afterwards I procured a number of JJniones from the 

 Enz, near Pforzheim ; and these I kept in a tub. But as my 

 stock gradually increased, I placed them, in about a fortnight, 

 in the same trough with the Anodontce. About four weeks 

 afterwards I perceived in an Anodonta the same species of 

 Atax which I had previously detected only in TJniones ; and 

 from this time forward I frequently found Anodontce, which 

 contained from three to four mites of the other species. 



By the great number of individuals which passed through 

 my hands, I was enabled to discover a beautiful but rare di- 

 morphism. Whilst the mites which live chiefly in Unio pos- 

 sess five suckers on each side of the sexual orifice, those from 

 Anodonta have from thirty to forty on each side. Moreover 

 the two species are distinguished by their form and size, even on 

 a superficial examination, so that any confusion between them 

 is hardly to be suspected. But I found mites which, as far as 



