A BEAUTIFUL CONTIUVANCE. 65 



distance of many inches. It is by the expulsive force 

 of this anal current, chiefly, that the passage is kept 

 free from the deposit of mud and other substances, 

 which would otherwise' soon choke it up. 



A fi'esh supply of water for respiration, and its 

 dismissal when no longer fit for use, are efficiently 

 provided for by this contrivance, But since many 

 particles of matter float in the water, which from 

 their form or other qualities, might be hurtful to the 

 delicate tissues of the viscera to be traversed, how is 

 the entrance of these to be guarded against in an in- 

 discriminating current ? A beautiful contrivance is 

 provided for this necessity. The margin of the enter- 

 ing siphon, and sometimes, though more rarely, of 

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

 tentacular processes, varying indeed in their length, 

 but the longest scarcely more than equalling half the 

 diameter of the mouth of the tube. In Saxicava 

 rugosa, which bores through and through, with small 

 holes, the hardest limestone of our coast, these ten- 

 tacular 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 irre- 

 gularly alternate. The object of this diversity in 

 length, will be manifested presently. In Pholaa 

 jparva, the processes are few and short, and are confin- 

 ed to the receiving tube, from the interior margin of 

 which they project, towards the centre. But it is in 

 Pholas dacti/liis, a noble species of large size that 

 excavates the softer rocks on our shores, that this 

 apparatus is developed with peculiar beauty, and its 



