454 Dr. G. Eolleston and Mr. C. "Robertson on the Aquiferous 



lowing words : — '^ The existence in these animals of a double 

 system of lacunae having this interpretation is attended with 

 many difficulties. For then it must be admitted that one of 

 these systems contains only water and the other blood, and it is 

 difficult to understand how two kinds of wall-less canals can 

 traverse the body without passing into each other/' It is, how- 

 ever, demonstrable that in the Unionidse, at all events, an all 

 but perfectly closed system of blood-vessels exists. We have 

 again and again, with various injecting fluids, found that they 

 will pass from the aorta through a capillary system into a 

 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 

 during 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. Tliere are 

 two venous sinuses, however, in the Unionidse, receiving, one 

 after the other, the systemic-venous blood, and transmitting it 

 into the organ of Bojanus. The first of these* lies just within 

 the muscular foot, along its superior and posterior edge ; it sub- 

 tends the second, the only one mentioned by authors, and opens 

 into it by an orifice more or less perfectly guarded in different 

 species of Unionidse. 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 con- 

 tained 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 famihes. There is the less occasion, how- 

 ever, to labour further at demonstrating the non-lacunar charac- 

 ter of the blood-vascular system of the Unionidae, as Von Hes- 

 slingt, in his recent book on the Pearl Mussel, confirms in this 



* Into this sinus the cseca of the generative gland project somewhat 

 freely from amongst the trabeculse 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. 



f Loc. cit. p. 219. Gegenbaur, in his * GrundzUge,' p. 344, note, hints 

 at some doubt still 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 Von Hessling's; and we suppose both to have been pub- 

 lished subsequently to the reading of our paper, February 3, 1859, 



