72 The Swan-Mussel and its Anatomy. 



stated : — The Anodonta, like all other Conchifers, is andro- 

 gynous j that is to say, each individual is both male and 

 female. Numerous authors of considerable authority have 

 maintained the existence of two distinct sexes in the Lamelli- 

 brancliiata; Leuwenhoek, Milne Edwards, Quatrefages, Owen, 

 Van der Hoven, and others, have been of this opinion. " The 

 latest and best observations of naturalists and physiologists," 

 says Professor Owen, " on the sexual character and generation 

 of the Lamellibranchiata, have established the correctness of 

 Leuwenhoek's conclusion that these mollusca are of distinct 

 sexes, some individuals being male and others female."* There 

 is no doubt, however, that the Conchifera are androgynous. 



The secretory organ occupies a large portion of the animal's 

 body; it is situated behind the liver, and is readily recognized 

 by its yellow colour in Anodonta, and by its milky whiteness 

 in JJnio. If a portion of the ovary be examined under the 

 microscope, it will be seen to consist of a number of small 

 sacs with branching vessels ; on the rupture of these sacs a 

 quantity of ova are observed, and surrounding and imbedded 

 in the substance of the ovary, as it appears, there are myriads 

 of oblong bodies of extreme minuteness, which from their 

 motions are commonly regarded as the fertilizing fluid. Neu- 

 mann has described two oviducts which have their orifice near 

 the orifices of the organ of Bojanus; both which apertures are 

 situated on the upper part of the body, covered by the lamina 

 of the inner gill, at the juncture of the two pairs of branchiaa.f 

 After the expulsion of the eggs from this genital orifice " they 

 are conducted along the base of the interior branchise as far as 

 the pallial cloaca, and from thence they reascend by the canal 

 which the mantle forms in this place, and arrive at the compart- 

 ments of the exterior branchiae. A part of these eggs is some- 

 times drawn in during the inspiration of the water and expelled 

 as if it issued from the anus — these are soon, however, taken 

 up by the respiratory current and directed towards the branchial 

 pockets." In these compartments the eggs are gradually de- 

 veloped. If an Anodon be opened at the latter end of August 

 the external pair of branchiae will be found full of eggs in an 

 early stage of development. Now may be seen the curious 

 phenomenon of the rotation of the embryo within the chorion 

 or membrane of the egg, first noticed by Carus. After tho 

 segmentation of the vitellus, the embryo begins to assume, 

 in the space of about a month, the characteristic appearance 

 which the young animal subsequently presents. The embryos 

 remain in the pockets of the exterior branchiae about eight 



* Lectures on Comparative Anatomy, vol. i. p. 287. 



t Tho orifices of the organ of Bojanus in the Anodonta are extremely diffi- 

 cult to detect. They are, however, readily seen in Unio maryaritifera. 



