448 Dr. G. Rolleston and Mr. C. Robertson on the Aquifet'ous 



himself pointed out the close correspondences of the two systems, 

 we will but remark upon one point of discrepancy between them. 

 In the Brachiopods the genitalia are packed into the main stems 

 of the arborescent perivisceral system, in the direct course of the 

 stream, if we may speak of it as a water- vascular system ; in the 

 Lamellibranchiata, or, at all events, in the family Unionidse, the 

 caeca of the generative gland are appended laterally to the 

 divergent twigs of the principal branches of the water-vascular 

 tree ; they do not lie in the direct course of the current of the 

 aquiferous canals, and these canals, beyond and outside of them, 

 break up into a very delicate minutely divided system of capil- 

 lary tubes. What we shall attempt to prove is, that the orifices 

 on either side of the foot in the Unionidse lead not only to the 

 generative gland, the products of which may be seen to issue 

 forth from them at the spawning-season, but also to a system of 

 tubes widely spread through the entire foot. We do not believe 

 that any direct communication subsists either between the blood- 

 vascular system and this system of tubes, or between either of 

 these systems and the punctated depressions and inlets along the 

 foot-edge. The blood-vessels seem to us to constitute a system 

 of tubes closed, save at one point and at one lacuna. That point 

 and that lacuna is the pericardial space — a cavity into which, 

 besides the blood of the animal, the water in which it lives also 

 finds its way. As the bivalve shell opens, it necessarily dilates this 

 lacuna, and water is thus drawn into it through the compound sac 

 known in the Acephala as the organ of Bojanus. The water then 

 gains access to the interior of the blood-vessels, as we shall proceed 

 to show, and is carried onward within them. From the blood- 

 vessels we suppose it to transude into the system of water-tubes 

 everywhere in apposition with them, and, under normal conditions, 

 to find its exit by these tubes, whilst under such abnormal cir- 

 cumstances as the sudden removal of the creature from the water, 

 the sudden contraction of the muscular foot, causing jets of 

 water to pour forth from the dilated semitransparent mass, may 

 unload the infiltrated organ in a yet more expeditious manner. 

 As to the way by which the water used by the mollusk for distend- 

 ing its foot comes into the body, we are at one with many other 

 writers upon this subject ; but we are not aware that our views 

 as to the method by which the animal disencumbers itself of the 

 ingested fiuid are shared in by other authors. 



Our arguments will be principally based upon the results of 

 experiments made in the way of injection. The animals we 

 operated upon were almost exclusively of the family Unionidse ; 

 and, on account of the size of the specimens, as well as for oth^r 

 reasons, we employed chiefly the species Anodon cygneus and 

 Unio margarilifera. In all our experiments we strove to repro- 

 duce, as nearly as possible, the conditions of the animaPs na- 



