304 DIMYARIA.—PHOLADIDA. 
through and through, with small holes, the hard- 
est limestone of our coast, these tentacular append- 
ages 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 pecu- 
liar 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 branchie, that expands around 
the mouth of a Holothuria. ‘These branched ten- 
tacula 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 in- 
terstices or meshes of which the current of enter- 
ing water freely percolates, while they exclude all 
except the most minute floating atoms of extraneous 
matter. 
