2.')8 FUNCTIONS AND INSTINCTS. 



that fix themselves to the rocks, stones, and other 

 substances, by means of a Byssus, which they 

 have the faculty of spinning from their foot or 

 other part, or by a tendinous ligament which they 

 protrude through an orifice in their shell. 



The general habit of ihejirst family, including 

 a vast variety of forms, seems to be that of boring 

 and burrowing, many piercing wood, and even 

 rock, and others burrowing in the sand, some- 

 times to a great depth. Thus they are instructed 

 by their instinct to form a convenient cell or 

 other habitation, either constantly submerged, 

 or only when the tide visits them, in which 

 they are enabled to procure their destined food, 

 of what nature does not appear to have been 

 clearly ascertained, although probably animal- 

 cules, introduced when they inspire the water for 

 respiration, may form a principal portion of 

 it, as the majority having no teeth for masti- 

 cation, require a kind of nutriment for which 

 it is not necessary : comparing this tribe of 

 aquatic animals with those of the antecedent 

 classes, we see the same object effected by 

 different means. The sheathed polype 1 builds 

 a house of matter elaborated in its own stomach, 

 while the ship-borer 2 pierces wood, and the 

 stone-borer the rocks, and the razor-shell 3 

 burrows deep in the sand with the same view ; 



1 See above, p. 166. n. 2. 2 Teredo. 3 Solen. 



