36 
BE. G. EOLLESTON Al^D ]ME. C. EOBEETSON ON THE AQTJIFEEOIJS 
systemic-venous system, from that into what may be called the renal-portal system of 
the organ of Bojanus, and from that into the branchiae, without any extravasation, or 
the formation of any lacuna anywhere. The pericardial space is, in the strictest sense 
of the phrase, a blood-lacuna; but, as already detailed, fluid cannot be made to pass 
into it from the blood-vessels, though such communication must take place to a certain 
extent diu'ing the life of the animal, and though the reverse direction of current is one 
easily demonstrable by artificial means, and is doubtless the ordinary one under normal 
conditions. There are two venous sinuses, however, in the Unionidae, receiving, one 
after the other, the systemic-venous blood, and transmitting it into the organ of Boja- 
nus. The first of these* lies just within the muscular foot, along its superior and 
posterior edge ; it subtends the second, the only one mentioned by authors, and opens 
into it by an orifice more or less perfectly guarded in different species of Unionidae. 
This second sinus lies between the two opposed organs of Bojanus ; and from it the 
systemic-venous blood passes into the capillaries of the renal-portal system contained in 
those organs. But neither of these sinuses at all answers the character intended to be 
expressed by the term lacuna ; they are homologous rather with the dilated great veins 
of certain vertebrata than with the lacunae which do exist in certain molluscan families. 
There is the less occasion, however, to labour further at demonstrating the non-lacunar 
character of the blood-vascular system of the Unionidse, as Von HusSLiNof, in his recent 
book on the Pearl Mussel, confirms in this point the views previously enunciated by 
Langek, adding to them a description of the histological characters of the vessels inter- 
vening between the arterial and venous systems in the Tlnio. It may be considered as 
beyond a doubt that a system of tubes all but entirely non-lacunar exists in these Lamel- 
libranchiata, 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. 
* Into this sinus the casca of the generative gland project somewhat freely from amongst the trabeculae 
which run across what we call the roof of the muscular foot, from one side to the other ; and it is here, we 
believe, that in injections from the oviducal outlets extravasation so often takes place into the blood- 
vessels. 
t Loc. cit. p. 219. Gegexbatje, in his ‘ Grundziige,’ p. 344, note, hints at some doubt stiU remaining in 
his mind as to the distinctness of these capillaries from the tissues they lie amongst. His work bears the 
same date (1859) as Vox Hesslixg’s and we suppose both to have been published subsequently to the 
reading of our paper, February 3, 1859. 
