ANATOMY OF THE LAMELLIBRANCHIATA. 441 



fluid the ova are small, they soon, however, from its influence, 

 enlarge, and the vitellus becomes coloured by the secretion. 

 The ova are of different figures in different species. In the 

 Unto they are round, about the seventieth of an inch in dia- 

 meter, consisting of a firm shell, containing a colourless fluid, 

 in which swims a globular yolk. A more transparent spot is 

 seen on the yolk ; this appears to become prominent, to en- 

 large, and to be developed into the young muscle. 



A remarkable difference exists in bivalves as to the dispo- 

 sal of the ova when ejected from the oviducts. In the gene- 

 rality they are immediately discharged from all further con- 

 nection with the parent, along with the respired water &c. — 

 In a few species the ova, escaping from the oviducts, remain 

 for a time in the spaces left in the interior of the branchice 

 (oviducts of Home) by the want of apposition in the mem- 

 branes which form them. This is the case in the TJnio and 

 Anodonta, in which the ova may be found for several months 

 in the external branchice after leaving the ovaria, distending 

 these organs in a remarkable manner. It is curious that they 

 are never found in the internal branchice, 1 along the edge of 

 which they may be seen to be conveyed, enveloped in mucus, 

 from the oviducts to the openings which are the posterior ter- 

 minations of the internal interbranchial spaces. They are 

 not found in these organs after the approach of spring, being 

 discharged by the anal orifice. In the ova, taken from the 

 branchice at different periods, we find different appearances. 

 In some we see merely the cicatricula described above, up- 

 on the yolk ; in others we see the yolk disappearing between 

 the valves of the animal, and becoming smaller and of a reti- 

 culated appearance on its superficies. It is in this stage that 

 the rotation of the embryo within its ovum, which has often 

 been noticed, may be witnessed. The rotation varies in quick- 

 ness ; perhaps, when most lively, there are seven or eight 

 volutions in a minute. The valves are developed, and the 

 animal has the power of opening and shutting them before it 

 leaves the shell. At length the ova crack, and the young 

 muscles are found free in the branchice adhering by a byssus. 2 

 These have been considered to be merely parasitical animals 

 by Rathke, who makes of them a new genus under the name 



1 Bojanus says lie has found them in the internal branchice. In a valu- 

 able and learned paper "by Blainville (Ann. Sci. Nat. vol. xiv.), the author 

 gives an account of all that has been done as to the anatomy of the gene- 

 rative organs of bivalves ; and of that paper, as well as of the Bibliography 

 in the Malacologie, the author has availed himself. 



2 Called umbilical vessels by Koelreuter and Mangili. 



