Mr. W. Clark on the Pholadidse. 323 



In all these attempts either regurgitation ensued, or the con- 

 tinual pressure of the mercurial column caused lesions. 



This impossibility to pass anything into the rectum probably 

 arises from its sphincter, or one in the anal tube, as the stoppage 

 always occurred at that point where it empties into it. 



4thly, On applying the mercury through the mouth and oeso- 

 phagus the stomach was readily filled ; but as soon as the pylorus 

 was passed, a stoppage not to be removed occurred, from the 

 duodenum being, like the colon in the last case, filled with sand ; 

 we may therefore conclude that water can never enter the stomach 

 of bivalves from the anal tube. 



The spaces lining the roof of the anal siphon consist of four 

 longitudinal rows ; the two middle ones are the largest, and form 

 transverse parallelogramic figures, whilst the other two, one on 

 each side, are smaller subquadrangular areas. I can conceive 

 no other use for these crypts, in such families as have them, than 

 as depositaries for the ova; if so, the o\d ducts of course commu- 

 nicate with them, and the ova probably remain there some time 

 after fecundation, and the final ejection, in bivalves of this struc- 

 ture, can only take place from the anal tube. It is probable that 

 the principal use of the anal conduit in the bivalves, in which the 

 branchial cavity is completely cut off from the anal one, is to 

 receive the rejectamenta, supply water to the ova during their 

 maturation, and ultimately to eject them. 



It is necessary to state that these experiments require much 

 patience and attention, and some delicacy of manipulation, to 

 arrive at sound results ; it is very material not to use specimens 

 with accidental lesions, or those made in removing the animal 

 from the shell, which operation, from the obstruction of the 

 crotchets, cannot be effected without some practice and dex- 

 terity. 



The result of the 1st and 3rd experiments would appear to 

 demonstrate the non-communication between the branchial and 

 anal siphons in the Pholades ; this fact being established, they 

 will not be the exceptions ; but it is probable that the Myada, Sole- 

 nidce, Lutraria, &c. have a similar configuration of the branchial 

 apparatus ; and though the Veneres, Cardia, and other open- 

 mantle bivalves, have the character of their branchial sacs differ- 

 ent from those of the Pholades, in not having the siphons com- 

 pletely separated, but more or less confluent, the possibility of 

 branchial currents must be admitted. Still, as it has been shown 

 that in the Pholades the water cannot be received and discharged 

 otherwise than through the branchial aperture, or from the pedal 

 orifice, it is clear that regular separate currents by cilia cannot 

 exist between the two siphons, so as to make one the inhalant. 



