488 



MOLLUSC A. 



the ccononiv of ii.-iiiirc An aivouiit of these is tlieir natural history, and this is likely to prove 

 interi'stin<;, somewhat in proportion to the peculiarities of the creatures themselves, and the 

 tioveltv of the ri'velations whidi their ))o\vers, instincts, and habits unfold. 



Throuirhout all tiie classes of animals \vc Iiavc described, we have found an internal bony skel- 

 eton, formini; the foundation for the whole structure of the body — limbs, flesh, muscles, and 

 nerves. In the Mollusca the bodies arc soft, and instead of having an internal bony support, 

 thov are mostlv i)n>tecteil by a hard external shell. These soft bodies arc enveloped in a 

 muscular skin, which naturalists call the mantle, and it is this wdiich by slow degrees secretes and 

 supplies the shell. In some species the shell is of one piece, and is called univalve; in others the 

 shell is double, the two parts being united by a hinge; this is called bivalve. The snail is a 

 uiuvalve, the oyster or dam a bivalve. Other shells, on account of their structure, are called 

 mullirah'c. Manv shells, as that of the oyster, are deposited in layers, a fine membrane inter- 

 posing between each layer; they arc ihci'dova qsWqA membraneous shells. Most membraneous 

 shells are lined with a brilliant enameled substance, called nacre; mother of -pearl is the nacre 

 of the ]H>arl oyster. That of the fresh-water mussel is a beautiful azure. The other structure of 

 sliells is called porcellaneous., because they look like porcelain or china. The common cowry is 

 a well-known instance of a porcellaneous shell. Some shells are so transparent as to resemble 

 iiflass, and are therefore called vitreous. 



Starting with these simple definitions, let us take a general survey of the field of inquiry vi'hich 

 lies before us. AVe begin with the infancy of these animals, and we may remark that at this 

 point in their lives, the various kinds of Mollusca are more alike both in appearance and habits 

 than in after-life; the young fry of the aquatic races are, indeed, almost as diftcrent from their 

 parents as the caterpillar from the butterfly. The analogy, however, is reversed in one respect ; for 

 whei-eas the adult shell-fish are often sedentary, or walk with becoming gravity, the young are all 

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

 thus difluse their races as far as suitable climate and conditions are found. Myriads of these little 

 voyagers drift from the shores into the open sea and there perish ; their tiny and fraoilc shells 

 become part of a deposit that is forever increasing over the bed of 'the deep sea, at depths too 

 great for any living thing to inhabit. 



Some of these little creatures shelter themselves beneath the shell of their parent for a time; 

 many can spin silken threads with which they moor themselves, and avoid being drifted away. 



They all have a protecting shell, and even the 



young bivalves have eyes at this period of 

 their lives to aid them in choosing an appro- 

 priate locality. After a few days, or even less, 

 of this sportive existence, the sedentary tribes 

 settle down in the place they intend to occupy 

 during the remainder of their lives. The tuni- 

 cary cements itself to rock or sea-weed ; 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 unnecessary. 

 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 brachiopoda are all fixed by similar means, 

 and even some of the gasteropods become 

 voluntary prisoners, as the hipponyx and vcr- 

 metus. 



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 



MUSSKL ATTACHED BY A UTSStS TO A ROCK. 



