ACEPHALA LAMELLIBKANCHIATA. 25 



venous blood enter by branches into the principal branchial 

 vein and auricles, and is sent into the circulation without 

 being transmitted to the branchiae for aeration. Part of these 

 views I can confirm, as far as relates to the spongy excretory 

 glands at the anterior end of the sinus above alluded to ; they 

 are visible in Pholas dactyhis, and conspicuously in P. parva ; 

 their uses are fully described under the title of excretory 

 organs of P. dactylus. As to a portion of the blood being 

 returned to the heart without receiving the dose of oxygen, I 

 have not had it in my power to verify that fact. 



From this statement it would appear, that in some of the 

 bivalves, if not in all, the circulation is not without interrup- 

 tion, and that there is a particular one for the special purpose 

 of providing the necessary secretions. With regard to these 

 points I am inclined to think, that the veins of each organ 

 form and supply the secreting glands for itself, in its own 

 immediate vicinity ; for instance, the liver, the bile for the 

 stomach; the ovarium, the mucus for the ova; the byssal 

 gland, the filamentous fluid; the mantle its own pigments; 

 also that thin membrane which is the external duplicature on 

 the shell, springing from its margins, and spreading more or 

 less on all shells, as well as on some of the exposed soft parts 

 of the animal, producing what is termed the pilose skin, epi- 

 dermis, and exterior ligamental covering. There is a difficulty 

 in supposing that all the secretions are elaborated and distilled 

 from a mass of veins collected in a particular cavity, and pro- 

 duce the peculiar secretion for each organ, conveying it thereto 

 by a separate sac or distinct duct. I can scarcely believe that 

 such a concentrated olla podrida can act harmoniously, and 

 produce simultaneously, from a fasciculus of veins deposited in 

 the same reservoir, mucus, pigments, carbonate of lime, &c. 



"With respect to the digestive organs, the mouth there 

 being no head, eyes, nor tentacula is a large, plain, trans- 

 versely oval aperture, covered by the mantle, and is placed 

 towards the upper part of the anterior range, having on each 

 side of it a pair of palpi, which appear to be both of a tenta- 

 cular and respiratory nature, and perhaps assist the cilia that 

 clothe the branchial laminte to beat and divide the water in 



