TESTACEA. 63 



received from the same fluid becoming stale or otherwise unsuitable for 

 them. But, such conditions being too long protracted, the animals are 

 at length exhausted : they can preserve the closeness of their shells no 

 longer, the tenant, now languid, pines and perishes. 



Seasonable supplies, however, avert the evil. Invigorated by their 

 renovation, the animals close then' shells again, and pertinaciously resist 

 all forcible attempts to open them, until of themselves the edges begin to 

 sunder, when the body protrudes as far as possible, and those endowed 

 with locomotive faculties begin to exercise their power by crawling over 

 surfaces below and ascending the sides of their vessels. 



The bivalves are always evidently cherished and refreshed by the 

 renovated element. How they are capable of sustaining protracted pri- 

 vation of this important quality, the freshness of the element, it should 

 not escape the notice of naturalists, who would profit by domestic con- 

 venience, for inspecting interesting specimens from distant regions, as a 

 single humectation on the way might prove an eflectual preservative. 



Besides demonstrating the gratification derived from the renovated 

 medium wherem they dwell, the bivalves certainly show some sense of 

 security in the formation of the byssiis afibrding them protection. Not 

 only is some solid spot selected for its original fixture, but confidence in 

 the augmented number of threads, as a reinforcement to the place of ad- 

 hesion, is indicated. All the other senses seem very obtuse and im- 

 perfect. 



Notwithstanding the dissections of some skilful anatomists, it is 

 doubtful whether the real structure of the bivalves is well understood. 



1. Mttilus— r/tc il/«.we/.— Plate XII. 



Did we rate the bivalve shells according to the real utility derived 

 from them by mankind, probably the mussel tribes should have the 

 priority. 



The numbers consumed for food, and employed for bait in the cap- 

 ture of large and valuable fishes at the various fisheries, exceeds all 

 calculation. 



