PHOLAS. 187 



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

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

 form transverse parallelogrammic 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 oviducts of 

 course communicate with them, and the ova probably remain 

 there some time after fecundation ; and the final ejection, in 

 bivalves of this structure, 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 ulti- 

 mately 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, Solenidce, 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 different from those of the Pholades, in not 

 having the siphons completely separated, but more or less 

 confluent, the possibility of branchial currents must be ad- 

 mitted. 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 



* Since this was written, doubts have arisen whether the ova after 

 exclusion are deposited for a time in the crypts of the anal tube, or in the 

 branchiae ; we have never discovered any out of the ovarium. Exmouth, 

 1853. 



