HOUGHTON, ON THE FRY OF ANODONTA CYGNEA. 167 



has become uncoiled, there may be observed, on each side, 

 three, or perhaps in some cases four, curious tentacular- 

 looking organs, which spring from the middle of the rudi- 

 mentary mantle-lobes. The two of these organs, placed 

 nearest the opening of the shell, are larger than the third, 

 which is situated very n°ar the angle (fig. 6). Carus 

 notices these curious appendages, but was unable to make 

 out their nature satisfactorily. He supposed the two larger 

 pair to represent the future branchise (external and internal), 

 and the smaller pair to be the rudiments of the labial 

 tentacles. Their structure, however, would hardly seem 

 to justify this supposition, though it is not easy to replace 

 it by one at all more probable, and I shall not, therefore, 

 attempt to do so. M. de Quatrefages makes no allusion to 

 these organs, and they are not to be seen in all specimens. 

 I have observed them in one or two instances, but without 

 paying any minute attention to their structure. Mr. Busk, 

 to whom I sent living specimens, thus speaks of these 

 organs: — " They are not simple prolongations of the substance, 

 nor are they soft filamentous tags, but organs of a very 

 peculiar kind. Each appears to consist of a very fine pencil of 

 hairs, or rather to present the striped aspect of a camel's hair 

 pencil, wdiich arises from a granular mass in the interior of a 

 strong capsule, which is partly imbedded, as it seems, in the 

 granular matter of the \dsceral mass.^' (Fig- 7.) This is a 

 subject worthy of extended observation, but all is over for 

 this season. I have, since the receipt of Mr. Busk's letter, 

 opened three or four dozen Anodonta, but all had parted with 

 their branchial contents. I tliiuk the Unios are later in de- 

 positing their fry, but I have only been able to obtain a few 

 living specimens, as they are not common in the canal, whence 

 I can obtain any number of Anodontas. 



I must just allude to the fact first mentioned, I believe, 

 by MM. Baer and Pfeiffer, of the occurrence of a curious 

 species of Hydrachna {Hydr. concJiarum, Baer) that I in- 

 variably find in the palleal ca\'ity of these molluscs. These 

 little acari, with heads something like a pig, " to compare 

 great things with small," are not simply occasional visitors ; 

 for I have discovered both ova and the larval form of the 

 animal. See on this subject a paper by C. Vogt {" On some 

 Inhabitants of the Fresh-water Mussels," 'Ann. and Mag. of 

 N. H.,' 2nd series, v., p. 450), who has also recorded the 

 cccurrence as parasites in the branchise of these molluscs of a 

 number offish with the vitellary sac concealed in the abdomen, 

 which this author is satisfied were hatched from ova that had 

 been introduced into the gills by the respiratoiy current of the 



