Page:Natural History, Mollusca.djvu/316

through and through, with small holes, the hardest limestone of our coast, these tentacular appendages are found fringing both the tubes. The tentacles in this species are simple, and appear as if cut off transversely; and some are not more than half as long as the others, with which they irregularly alternate. The object of this diversity in length will be manifested presently. In Pholas parva, the processes are few and short, and are confined to the receiving tube, from the interior margin of which they project, towards the centre. But it is in Pholas dactylus, a noble species of large size, that excavates the softer rocks on our shores, that this apparatus is developed with peculiar beauty, and its use is made most clearly manifest. The tentacular filaments are in this case also confined to the oral tube. They are numerous, each forming a little tree, with pinnate branches, bearing no small resemblance to the flower of feathery branchiæ, that expands around the mouth of a Holothuria. These branched tentacula are ordinarily bent down across the mouth of the tube, the longest of them just meeting in the centre; alternating with these are placed others of similar structure, but inferior size: and the interspaces are occupied by others smaller still, and simply pinnate; so that when the whole occupy their ordinary transverse position, the small ones fill up the angles of the larger, and the branches of all form a net-work of exquisite tracery, spread across the orifice, through the interstices or meshes of which the current of entering water freely percolates, while they exclude all except the most minute floating atoms of extraneous matter.