PHOLAS. 203 



appearance, but no fluid escaped from them into the branchial 

 sac." 



It is proper to state, that the Pholas crispata is the species 

 that has furnished my controversialists with their remarks on 

 my branchial theory, which is illustrated chiefly by the P. dac- 

 tylus. I am not aware that this circumstance is of much 

 moment, as we may safely conclude that the gills of all the 

 Pholades have in essentials the same character. But I ought 

 to mention, that the framework of the respiratory apparatus 

 in some tribes of the Bivalves presents a very different ar- 

 rangement. For example, there are several British families, 

 whose species I have seen alive, and which fortunately can be 

 obtained, that have a peculiar branchial construction, which 

 appears as to general configuration closely analogous to that 

 lately described in the ' Annals' to exist in the Chamostrea 

 albida and Myochama anomioides of authors, but the particular 

 parts of the mechanism in my species do not accord ; I think 

 the narrow reticulated ribands on the external surface are 

 not permeable, and do not lie on apertures that communi- 

 cate with the interbranchial tubes. I refrain, at present, 

 from extending these remarks ; but I shall be prepared with 

 some comparative notes on certain species that have only a 

 single complete gill-lamina and a rudimentary one on each 

 side of the body, which seem to me to differ essentially in 

 structure from the descriptions that have been promulgated 

 on the composition of the branchial mechanism of the species 

 that have been alluded to. 



I now enter on the counter-statement to the last quotation, 

 and beg to observe, that Messrs. Alder and Hancock, in the 

 explanatory sketch of their Pholas crispata, ' Annals/ pi. 15, 

 vol. viii. N.S., give a very intelligible outline of their theory. 

 Though entirely dissenting from it, I cannot but admire the 

 ingenious delineation, particularly fig. 3, of the gill-laminse, 

 showing the aspect of the meshes ; it has, however, one fault 

 it exhibits them all with symmetrical longitudinal fissures 

 called "orifices," which I think are ruptures of the mem- 

 brane of each mesh, not one | of which exists naturally in the 

 three species I have examined. 



