326 Mr. W. Clark on the Pholadidae. 



tube ; nature will supply its wants by other channels if deprived 

 of the accustomed ones. I only insist on the position that the 

 usual canal for the entry of the main body of water for the use 

 of the branchiae in all bivalves is by the pedal and ventral aper- 

 tures, and that the exit is by both the branchial and pedal 

 fissures of the mantle, and that these actions are accomplished 

 agreeably to the wants and will of the animal at uncertain inter- 

 vals by the simple opening and closure of the valves, and that 

 what are called currents by cilia do not exist. I cannot help 

 again observing on this simple solution of a disputed point ; — I 

 may call it as simple a one as that of Columbus, when he showed 

 how the egg may be made to stand on its apices ; and yet it is 

 quite decisive of the desired point, how the water is admitted to 

 the branchiae. 



The Secretory Organs. 



Under this head, as I propose to offer at a future time some 

 observations on the anatomy of the Lamellibranchiata, I will 

 only at present mention the veins and glands which I think 

 produce the cartilage and ligament. On carefully opening the 

 lateral cavities on each side the anterior dorsal range, a fasciculus 

 of veins may be seen deposited therein, some of which I have 

 traced to the liver ; and it is probable that the inspissated fluid 

 thrown off by them is specially applicable to the formation of the 

 cartilage and internal portion of the ligament, as I have found 

 their excretory ducts to be spongy masses that send forth the 

 viscous humour distilled from the veins on the internal convex 

 circular areas in which the valves articulate ; and a proof of one 

 at least of the true uses of the excretory apparatus is, that in 

 Pholas dadylus the fluid is of a light brown or drab, and it pro- 

 duces two thin cartilaginous plates of those colours, whilst in 

 P. parva the colour is as dark as tar, and the plates of the carti- 

 lage correspond with it. It is probable the coarser parts of this 

 secretion are separated from the concentrated cartilaginous mat- 

 ter, and form the interior layers of the ligament, the mantle only 

 producing the external skin. It is reasonable to suppose that all 

 bivalves have these secreting organs ; and it will be desirable to 

 ascertain if such shells as the Madra, Myce, Lutraria, &c., which 

 have internal cartilages of considerable volume, have the excre- 

 tory and secretory organs correspondently developed as in the 

 Pholades ; and if in those genera that have external cartilages, 

 there is any variation of structure. 



The Reproductive Organs. 



The Acephala are hermaphrodites without congression, and 

 as it is termed, they suffice for themselves ; but this fact must 



