ANATOMY OF THE LAMELLIBRANCHIATA. 297 



by the mouth. In the Helix, Buccinum, and all the Gaster- 

 opoda in which the shell is developed, the excretory organ 

 (the mucous sac of authors) is large, and its circulation is as 

 described above. It opens in these by a canal near the anus, 

 or directly by a wide opening into the respiratory sac. In 

 the last case, which is general in the branchiated spiral Gas- 

 teropoda, it is not unreasonable to suppose that the animal, 

 by means of it, can occasionally respire air, as well as water ; 

 which last is the ordinary medium of the aeration of its blood. 



In the Cephalopoda two papilla open on each side below 

 the rectum, leading into two cavities communicating toge- 

 ther, through which the veins of the animal traverse. These 

 veins, particularly those from the viscera, are covered with 

 glandular processes or appendages, which secrete matter, 

 sometimes accumulating into considerable concretions, evi- 

 dently formed of carbonate of lime. They also give out a 

 mucous fluid. As has been shown by Cuvier, they commu- 

 nicate internally with the veins, and air blown into the latter 

 escapes from their secreting pores into the cavity in which 

 they are found. The hepatic vein separately enters the cava 

 before its bifurcation, and does not join the other visceral 

 veins ; and Cuvier notices that the latter enter the cava in a 

 direction opposite to the flow of blood, and the orifices of the 

 visceral and hepatic veins being near, the blood of one might 

 be directed into the other. Is this an intermediate state of 

 circulation between that in which the intestinal, ovarian, &c. 

 blood goes to the liver, and that in which the hepatic, intesti- 

 nal and ovarian blood all goes to the excretory organ ? The 

 bile-ducts likewise pass through this cavity, and are also fur- 

 nished with appendages, which probably secrete a similar 

 matter to that formed by those of the veins. * 



The excretory organs are more or less circumscribed. We 

 see on their internal surface depressions, which might be mis- 

 taken for the open orifices of vessels, and in some species air 

 blown into these openings gains access into canals extending 

 in the manner of vessels into the contiguous parts of the bo- 

 dy, and which, from their connection with the water without, 

 are perhaps analogous to the hydroferous canals in the Radi- 

 ata. No molluscous animal has been shown to possess ab- 

 sorbents; hence the necessity of the hard parts being external, 

 and out of the circulation, as these animals have no means of 



1 Cuvier supposes these processes to be for the purpose of absorbing fluids 

 from the cavities in which they are found. But these cavities are not shut 

 sacs. Prof. Grant considers the processes on the ducts to be analogous to 

 the pancreas. 



2 H 



