232 



NATURAL HISTOKY. 



ANATOMY OF CYTHEKEA. 

 u, is the umbo of the valve ; V, the hinge cartilage 

 th 



other shell of that group, we shall observe two of these scars, one on either side (a ,). This has 

 led to the former being called Monomyaria * (having only one shell-muscle) ; the others Dimyariat 

 (having two shell-muscles). 



These muscles are also called adductors, their functions being to close the valves of the 



shell and hold them tightly together. We may also notice in 

 the valve of the Monomyarian a simple unbroken line (;>) just 

 within the margin of the shell. This is the line of attachment 

 of the animal's mantle to its shell, and is called the pallialj 

 border, because the mantle is the membrane which envelops all 

 the Mollusca, and is that structure which secretes the shell. 

 This line is simple in the Oyster and its allies, which have no 

 siphonal tubes ; and indented in shells like Cytherea (s), which 

 are provided with siphons which could be drawn within the 

 valves of the shell by muscles specially provided for that 

 duty. The soft parts of an ordinary bivalve shell (see figure 



the umbo of the valve; &', tho hinge cartilage ; o^n\ i j.- c J.L i_ t' 



the " tunuie "; 6, tna posterior of the border of given on page j4:J) are very simple, consisting ot the brancnuie, 



e valve:/, the front or anterior border ; a. a', the -n/\ i / \ 



SSu^Sffl;ra5dtoS^?SS?t^ r S llls (ffff)'> the mantle (m); the mcurrent and excurrent 



siphons (whose position is marked in the figure by two arrows) ; 



the foot (f), which enables many of the bivalves to burrow, and which in. some species secretes 

 the threads of the byssus, by which they can moor themselves to rocks or floating objects. Then 

 there are the shell-muscles, and the muscles of the foot and of the siphons, and the muscular fibres 

 of the mantle, the stomach, liver, heart, and intestine. The water passing in by the incurrent 

 siphon or tube is conveyed to the gills, through the folds of which it passes, the pai-ticles of food, 

 whether living or dead, being guided by the labial tentacles, or lip-feelers, to the mouth, the excurrent 

 orifice serving, as in the Tunicata, to carry away what is done with. 



Division a. ASIPHOXIDA. In this section the lobes of the mantle are free, or only united at 

 the point v/hich separates the branchial chamber from the excurrent chamber. The animal has no 

 respiratory siphons. 



FAMILY I. OSTREID^E. 



The animals of this family are marine ; the mantle is free, only slightly adhering to the edges of 

 the shell. The shell is closed by a single muscle ; valves unequal, adhering by one valve. 



Genus Ostrea. The shell of the Oyster is irregular ; the upper valve flat, the lower convex, 

 often plaited or lamellated ; pearly within ; ligament triangular ; margin of mantle finely fringed ; 

 gills almost equal, united behind to one another and to the mantle, and completely enclosing the 

 branchial sac. There is an intimate connection between the mantle of the mollusc and its shell. 

 The Oyster, as we see it on the supper-table, is much smaller than its shell, and adheres only by its 

 glistening shell-muscle ; but when alive, its mantle extends to the very edge of the valves, lining 

 the whole interior, and having a slight adhesion, especially at the edge of the valves, which is speedily 

 ruptured, however, when the poor animal is forcibly invaded by the fishmonger's knife. Oysters are 

 found in the temperate and tropical seas all over the world. Nearly 100 species have been described. 



No shell-fish has, probably, endured more severe havoc from mankind than the common Oyster, 

 for it is only in comparatively late years that it received the protection of Mr. Frank Buckland, and 

 became a subject for Parliamentary Committees to discuss and Government to legislate for. 



The shores of Denmark and her islands are marked by vast shell-mounds (kjb'kken-muddings), 

 indicating the primitive taste for Ostrc,a edulis. No doubt vast strata of oyster-shells must exist 

 beneath London, when we consider that from 20,000 to 30,000 bushels of " Natives," and 100,000 

 bushels of " Sea Oysters " were (thirty years ago) annually supplied to the London market. And 

 although, owing to the increased price of this mollusc, a considerable falling off has occurred of late 

 in the supply of "Natives," Oysters have been imported in large quantities into England from 

 Holland and the United States. 



"Sea Oysters " (i.e., Oysters naturally grown; obtain their majority in four years, but " Natives '' 



. * Greek, mpnos, one ; and mus, a muscle. f dis, two ; and mutt. 



J From Latin, pallium, a cloak or mantle. Greek, a, without ; siphon, a tube. 



