HOUGUTON, ON THE FRY OF ANODONTA CYGNEA. 163 



embryos, the true relations of these creatures to their parent 

 are clearly made out. It also contains observations upon 

 many points of considerable general interest with respect to 

 the structure and early stages of development of the molluscan 

 ovum j and it is in this paper that Cams for the first time 

 clearly describes the curious phenomenon of the rotation of 

 the embryo within the egg-membrane, although it had 

 doubtless been observed in the ovum of some species of 

 Unionidse by Leeuwenhoek, and was also noticecl by M. 

 Bauer ; who absurdly attributes the movement to the inces- 

 sant attacks of an invisible worm, which he imagined was 

 devouring the vitellus. 



The views of Carus respecting the nature of the Glochi- 

 c?/Mm-larv8e were too well founded not to receive general 

 adoption ; but since his time nothing has been really added 

 to our knowledge of their structure or habits. M. de Qua- 

 trefages,^ it is true, took up the subject in 1836, but, strange 

 to say, apparently in total ignorance of what had been done 

 by Carus, although he cites Rathke and Jacobson. This 

 paper contains nothing of the least importance, and is filled 

 with the most curious statements, and the figures accompany- 

 ing it, so far as relates to the internal structure of the young 

 animal, are equally fanciful. M. de Quatrefages, although he 

 enters pretty fully into the development of the ovum, does not 

 appear to have noticed the rotation of the embryo ; and among 

 other strange propositions, he propounds the extraordinary 

 notion that the byssus-filaments of the young Anadontas are 

 umbilical vessels, and actually figures two umbilical cords, 

 each consisting, as it would seem, of an artery and vein going 

 to each embryo, and keeping up a communication with the 

 mother.t He comes, however, to the same conclusion as the 

 German naturalist as to the true nature of the Glochidia, 

 although he allows that " les differences de forme et d'organ- 

 isation sont certes assez grandes pour justifier ceux qui, 

 comme MM. Rathke et Jacobson ont considere ces petits 

 bivalves comme tout a fait etrangers a Fanimal qui les nour- 

 rissait.^* 



This remark is very just ; and, indeed, although there is 

 not a shadow of doubt that the Glochidia are really the 

 young of the Anodonta, the peculiar serrated tooth at the 

 point of each valve may be deemed suflScient to authorise the 



* " Memoire sur la vie intrabranchiale des petites Anadontes." ' Anu. 

 d. Sc. Nat.,' 2ud ser., torn, v., p. 332. 1836. 



t J. T. Kohl reuter appears to have entertained the same singular idea 

 with regard to the byssus-filaments (Observationes Anatom.-physiol. Mytili 

 cygnei.Y,., Nov. Act. Pet., vi, p. 23fi, 1790). 



VOL. TI. NEW SKR. M 



