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tween a strong membrane on the lower or ventral aspect, and a layer of trans- 

 verse muscular fibres, which decussate each other on the upper or dorsal aspect; 

 both the membrane and the muscle pass across from the inferior margin of one 

 of the shell-muscles to the other, they consequently increase in breadth as those 

 muscles diverge, and complete the parietes of the abdomen on the ventral 

 aspect : the vein, however, maintains an uniform calibre by its proper internal 

 coat, and leaves a space on either side of it between the membrane and the 

 muscle prior to its termination. The adhesion of the proper membrane of the 

 vein to the muscular fibi-es is very strong, and these fibres form, in consequence, 

 part of the parietes of the vein on their side throughout its whole course. But 

 there are several small intervals left between the muscular fibres and correspond- 

 ing round apertures (1'. pi. 6 ; n. pi. 7.) in the membrane of the vein and in the 

 peritoneum ; so that the latter membrane is continuous with the lining membrane 

 of the vein. Thus the blood may pass into the general abdominal cavity, and 

 the fluid contents of that cavity be reciprocally received or absorbed into the 

 vein. I counted as many as fifteen of these apertures ; they were largest and 

 most numerous at the commencement of the vein ; and most of them would admit 

 the head of an eye-probe. They are too numerous and regular in their shape 

 to allow for a moment the supposition of their being accidental, even if an ana- 

 logous formation had never been met with, and this had been the soUtary in- 

 stance of so anomalous a structure ; but the discovery of a similar one in the 

 genus Aplysia prepares the mind to contemplate it with less surprise, and even 

 leads to a suspicion that it may be more generally found on a further and more 

 diligent investigation of the venous system in this remarkable class of animals. 



The great anatomist to whom we owe the discovery, concludes his account of 

 this structure in the Aplysia with the following observations : 



" This communication is so little in accordance with our knowledge (of organic 

 structure) in the vertebrate animals, that I was inclined for a long time to doubt 

 it ; and even after having communicated it to the Institute some years ago, I 

 dared not then send my memoir to the press, so much I feared that I might be 

 deceived : at length I have been compelled to yield to evidence, having, from the 

 moment that I had at my disposal as many Aplysiee as I wished, assured myself 

 in every possible way, — 



" 1st, That there is no other vessel to convey the blood to the branchiae, but 

 the two great muscular and perforated canals that I have just described : 



