and Oviducal System in the Lamellibranchiate Mollusks. 455 



point the views previously enunciated by Langer, adding to 

 them a description of the histological characters of the vessels 

 intervening between the arterial and venous systems in the Unio. 

 It may be considered as beyond a doubt that a system of tubes 

 all but entirely non-lacunar exists in these Lamellibranchiata, 

 carrying their blood from the heart through a systemic, a renal, 

 and a branchial system. No pressure that can in fairness be 

 applied will cause any extravasation of fluid thus injected. Such 

 pressure we have repeatedly applied to Anodons very fully 

 distended by injection ; and though it be not rare for fluid 

 thrown in by the oviducal outlets to find its way out, as already 

 described, by orifices along the foot, we have never found this 

 to take place with the blood- vascular system. 



In making use of the method of injections as a means for 

 showing the independence of the several vascular trees in the 

 Lamellibranchiate mollusks, we have sometimes injected the 

 animal from the oviducal orifice alone, sometimes we have 

 injected the same animal with a differently coloured fluid from 

 its venous or from its arterial system, or from both ; in a word, 

 our injections have been either single, double, or triple. 



There is no difficulty in causing an injection to enter the body 

 of any large individual of the family Unionidae from its oviducal 

 orifice ; it is especially easy, however, to effect this in the Unio 

 margaritifera, as the orifice is not in them, as in most species of 

 the family, covered by the inner lamina of the inner gill, but, 

 together with the orifice of the organ of Bojanus, lies exposed 

 and uncovered in the gill-cavity, and, besides this, is prolonged 

 out in such a manner as to render the introduction of the 

 syringe-pipe a very easy matter. 



Experiment 6. — An injection thrown in by this orifice will 

 spread itself over the whole of the viscera contained within the 

 foot, not confining itself by any means to the ovary, but passing 

 on beyond the area occupied by it or the male generative gland, 

 into the exclusively muscular part of the foot, and distributing 

 itself with especial richness along its free edge. That an injec- 

 tion thrown in by this orifice should thus spread itself would go 

 some way towards showing that in the Lamellibranchiate, as in 

 the Brachiopod mollusk, the ducts through which the generative 

 products are extruded lead elsewhere as well as to the generative 

 gland, were it possible to be sure that no transference of the 

 injected fluid had taken place from tubes confessedly in con- 

 nexion with the generative gland to another system of vessels — 

 that, namely, which carries the blood. That such a transference 

 does not rarely take place in one part of the blood-vascular 

 system, we have already mentioned*; and hence arose the 

 * Note, p. 454. 



