Mr. W. Clark on the Animal of Kellia rubra. 453 



pure fluid is expelled by the aperture of ingress, a small portion, 

 as before stated, passing out by the posterior siphonal apparatus. 

 In this case the apertui-es of ingress and egress are not " kept 

 distinct." 



In those niollusca with nearly closed mantles only a small por- 

 tion of fluid can enter by the restricted pedal orifices ; the far 

 greater part must be inhaled by the posterior siphons, and is 

 often expelled simultaneou.sly at both orifices, as I have observed 

 in Pholadidea papijracea, the most closed of all the bivalves ; and 

 whether the water be received through the anal or branchial 

 tube, or both, the fact of the simultaneous expulsion of the fluid 

 in almost equal streams proves that the known branchial CECO- 

 nomy of the bivalves does not require the apertm-es of inhalation 

 and exhalation " to be kept distinct," as it is clear that water is 

 expelled by one at least of the tubes of ingress. 



The doctrine of the known ceconomy of the bivalves, requiring 

 the apertures of ingress and egress to be kept distinct, cannot, I 

 think, be admitted; it has not been verified by competent 

 authority. I do not believe in it, as it is disproved by indis- 

 putable facts. 



I have little doubt that the water required for buccal and 

 branchial uses, in the mollusca with closed mantles, is received 

 through both the posterior apertures, anal and branchial as they 

 are called ; and probably at their bases there is an internal com- 

 munication, thus allowing the water from both to pass into the 

 great cavity of the branchiae, to bathe them, and for sustentation 

 of the animal ; and after these functions are fulfilled, it is in like 

 manner expelled from both orifices, and often simultaneously, as 

 may be seen in any of the Pholades, LutraincB, or MycB. 



The water, I believe, never makes a circuit, or enters the legi- 

 timate anal tube, or issues therefrom, and which, to prevent con- 

 fusion, ought to be denominated the rectum, or that portion of 

 the viscera proceeding from the stomach to its termination at 

 the posterior end of the body, where it empties itself into a con- 

 duit termed the anal tube. The legitimate anal cylinder, or 

 rectum, is only for the discharge of the rejectamenta of the ali- 

 ment entering the buccal orifice, and from thence passing to the 

 stomach ; for if the water entered this duct, either posteally or 

 anteally, in the one case it would force back the faeces into the 

 stomach, and in the other none would ever be found in the rec- 

 tum ; but the scalpel shows it is always filled from its point of 

 junction with the stomach to its terminus with a cylindrical com- 

 pact mass of faecal matters, which, as the animal requires, is dis- 

 charged by minute portions into the anal conduit ; the water 

 therefore for the branchiae and sustentation must pass into the 

 great branchial cavity, and issue therefrom by both the ducts at 



