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cavity can be " correctly described as a closed one, consisting at the 

 half next the ventricle, of a beautifully plicated muscular coat in 

 addition to the membranous one, but at the other half next the 

 venous sinus of venous membrane only ; the latter might be termed 

 the auricular sinus, the former the auricle proper." 



I presume that ' this delicate membrane of the venous sinuses ' 

 is what I have called the ilio-parietal band, in which the base of the 

 auricle is as it were set, like a landing-net in its hoop, but this 

 does not close the base of the auricle, the latter opening widely into 

 the visceral chamber. 



I have equally failed in detecting any arteries continued from the 

 apices of the ventricles ; and I have the less hesitation in supposing 

 I have not overlooked them, as Mr. Albany Hancock, whose works 

 are sufficient evidence of the value of his testimony, permits me 

 to say that he long since arrived at the conclusion that no such arte- 

 ries exist. 



What has given rise to the notion of the existence of these arteries 

 appears to me to be this. A narrow band resembling those I have 

 already described, is attached in Waldheimia along the base of the 

 ' ventricle ' and the contiguous outer parietes of the auricle : inferiorly 

 it passes outwards to the sinuses, and running along their inner 

 wall, forms a sort of ridge or axis* from which the genitalia, whether 

 ovaria or testes, are developed, stretching through their whole length 

 and following the ramifications of the sinuses. It is the base of these 

 ridges seen through the walls of the sinuses, where they extend 

 beyond the genitalia, which have been described as arteries. 



The upper end of the band passes into the sinuses of the upper lobe 

 of the mantle, and comes into the same relation with the genitalia 

 which they enclose. 



The walls of the auricle in Waldheimia are curiously plaited, but 

 I have been unable, in either auricle or ventricle, to detect any such 

 arrangement of muscular fibres as that which has been described 

 The epithelial investment of the auricle, on the other hand, is well 

 developed, and in the ventricle the corresponding inner coat is raised 

 up into rounded villous eminences. 



The ventricle lies in the thickness of the parietes, while the auricle 

 floats in the visceral cavity, supported only by the ilio-parietal band. 

 * This arrangement is, I find, particularly described by M. Gratiolet. 



