152 THE HARMONIES OF NATURE. 



in a jet at its extremity, by a similar machinery. As the 

 burrowing bivalve usually, if not always, dwells in the 

 interior of the passage it has excavated, it is needful that there 

 should be a communication with the external water, and hence 

 a hole is always found extending to the surface of the material 

 bored. The entering and departing currents keep this passage 

 clear, a process which in mud or sand might seem at first not 

 very easy of accomplishment. It is facilitated however by 

 the faculty which the boring bivalves have of lengthening the . 

 siphonal tubes at will, and the degree to which this may be 

 accomplished depends on the depth of the cavity which the 

 species is accustomed to make. Yet since many particles of 

 matter float, even in -clear water, which from their form or 

 other qualities might be hurtful to the delicate tissue of the 

 viscera to be traversed, how is the entrance of these to be 

 guarded against in an indiscriminating current ? A beautiful 

 contrivance is provided for this necessity. The margin of the 

 entering siphon, and sometimes, though more rarely, of the 

 ejecting one, is set round with a number of short tentacular 

 processes, expanding like feathery leaves and varying indeed in 

 their length, but the longest scarcely more than equalling half 

 the diameter of the mouth of the tube. 



In Pholas dactylus, this apparatus, which is here confined to 

 the oral tube, is of peculiar beauty, forming a network of 

 exquisite tracery, spread across the orifice through the inter- 

 stices or meshes of which the current of entering water freely 

 percolates, while they exclude all except the most minute float- 

 ing atoms of extraneous matter. Thus admirably has the health 

 and comfort of the lowly shell-fish been provided for, that spend 

 their whole life buried in sepulchres of stone or sand ! 



The siphons, which are frequently connected or blended into 

 one tube, are shorter, or even reduced to simple perforations, 

 in those species which do not bury themselves so deeply; and 

 in the oysters, mussels, and other genera that are superficially 

 attached to submarine objects, the lobes of the mouth being 

 completely distinct, the water is admitted at once to the bran- 

 chiae as soon as the animal opens its valves. The chief use of 

 the shell is as a means of defence, and, to answer this purpose, 

 it must naturally increase in solidity the more its owner is 

 exposed to injury. The pholades and teredines which scoop 



