LAMELLIBRANCHIATE CONCHIFERA. 97 
found in the foot, sometimes they are ramified in the mantle, which last disposition 
is present in Modiola, Anomia, Lithodomus, Hiatella, and the like. The oviducts open 
variously in different species, as described above ; the ratio of the difference appearing 
to be the situation of the ovaries. At an uncertain time before the discharge of the 
ova from the ovaria, a milky fluid, often pinkish in colour, makes its appearance in the 
latter, and is itself discharged from the oviducts. This appears to be the male fluid ; 
when examined with a powerful lens it is seen to contain minute oval bodies, not more, 
perhaps, than the four-thousandth part of an inch in length, swimming in the thinner 
fluid, and having a very vivid motion. They do not appear to have caudal appendages, 
though minute vibriones, as well as vibratile particles of the branchia, may often 
give rise to the appearance of them. The ova seem to enlarge from the influence of 
this fluid, and the vitellus becomes coloured by it. The ova are found to present a 
different shape in different genera; in the Unio they are globular and transparent, 
about the seventeenth part of an inch in diameter, consisting of a firm shell, in which 
is contained a clear fluid, with the yolk floating therein. The ova are generally dis- 
charged from the ducts immediately into the water; but in Unio and Anodonta they 
are conveyed, enveloped in stringy mucus, from the excretory organs into the inter- 
branchial spaces (oviducts of Home), where they are further developed, the shell 
breaking, and the young bivalves being attached by a byssus. It is curious that they 
are never found in the internal pair of branchia, along the edge of which they are 
conveyed to the external ones. In summer the ova leave the oviducts, and at the 
approach of the following spring, the young animals leave the branchie. A curious 
rotation may at one time be observed of the embryo within the ovum, from the action of 
the cilia, the rotation taking place seven or eight times in the minute. The young have 
the power of opening and shutting their valves before leaving the parent shell. Rathke 
considers them as parasites, under the name of Glochidiwm, in which opinion he is fol- 
lowed by Jacobson, who considers their appearance to preclude the possibility of their 
being the young of the animals on which they are found. The valves are triangular, 
with the ligament at the short straight side, the other two sides terminating in a point, 
at which we see a process of membrane to each valve, dentated on its exterior surface. 
Two pointed processes also appear projecting from the inner surface of the valves. 
There is no foot, and the muscle seems undivided, and allows the valves to be com- 
pletely opened. But on inspecting a very young Unio, we find that the valves are really 
commenced by triangular nuclei. ‘The membranes may be the branche, and the other 
processes appear to be the nuclei of the teeth of the valves. Home does not describe 
the true oviducts ; Bojanus calls the branchia, uteri, or oval receptacles ; Joerg calls the 
external ones ovaria, and the internal ones testes. The Anodonta anatina and cygnea are 
both viviparous ; though Drapernaud, on the authority of Poiret, denies that the former 
is so. In the Cyclades we always find from ten to twenty of the young fry in the internal 
branchie ; they are of different size, and are discharged one by one, when they attain 
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