134 HABITS AND ECONOMY OF THE MOLLUSCA. 



HABITS AND ECONOMY OF THE MOLLUSCA. 



" Every living creature has a history of its own ; each has 

 characteristics by which it ma}'- be known from its relatives ; 

 each has its own territory, its appropriate food, and its duties 

 to perform in the economy of nature. Our present purpose, 

 however, is to point out those circumstances, and trace the 

 progress of those changes which are not peculiar to individuals 

 or to species, but have a wider application, and form the history 

 of a great class. 



" In their infancy the molluscous animals are more alike, both 

 in appearance and habits, than in after-life ; and the fry of some 

 of the aquatic races are almost as different from their parents 

 as the caterpillar from the butterfly. The analogy, however, is 

 reversed in one respect ; for whereas the adult shell-fish are often 

 sedentary or ambulatory, the young are all swimmers ; so that 

 by means of their fins and the ocean-currents, they travel to 

 long distances, and thus diffuse their race as far as a suitable 

 climate and other conditions are found. M3a'iads of these little 

 voj^agers drift from the shores into the open sea and there 

 perish ; their tiny and fragile shells becoming part of a deposit 

 constantly accumulating, even in the deepest pa,rts of the sea. 



" Some of these little creatures shelter themselves beneath 

 the shell of their parent for a time, and many can spin silken 

 threads with which to moor themselves, and avoid being drifted 

 awaj^ They all have a protecting shell, and even the young- 

 bivalves have e^^es at this period of their lives, to aid them in 

 choosing an appropriate locality. 



"After a few da_ys, or even less, of this sportive existence, the 

 sedentary tribes settle in the place thej' intend to occupy during 

 the remainder of their lives. The tunicar^^ cements itself to 

 rock or sea-M^eed ; the ship-worm adheres to timber, and the 

 Pholas and Lithodomus to limestone rocks, in which they soon 

 excavate a chamber which renders their first means of anchorage 

 unnecessar}'-. The Mya and razor-fish burrow in sand or mud ; 

 the mussel and Pinna spin a byssus ; the oyster and Spondylus 

 attach . themselves by spines or leafy expansions of their shell ; 

 the Brachiojaoda are all fixed by similar means, and even some 

 of the gastropods become voluntary prisoners, as the Hipponj^x 

 and Yermetus, the Rhizochilus and Magilus. 



" Other tribes retain the power of traveling at will, and shift 

 their quarters periodically, or in search of food ; the river- 

 mussel drags itself slowly along by protruding and contracting 

 its flexible foot ; the cockle and Trigonia have the foot bent, 

 enabling them to make short leaps ; the scallop (Pecten opercu- 

 laris) bounds rapidly by opening and shutting quickly its tinted 



