BIVALVES. 



231 



Most bivalves are sedentary in their habits, living in an erect position, either exposed or 

 buried in the sea-bottom, resting on the edges of their shells, which are usually of equal size. 



The burrowing forms have a stout muscular foot, with which they dig for themselves a cavity in 

 the sand or mud, leaving only one end of their shell exposed to admit a current of water to the respira- 

 tory tube or opening, which also conveys to the animal within a constant supply of particles of food. 



There is a tendency observable in bivalve shells, as well as in univalves, to grow in a 

 spiral direction. This is especially well seen in Isocardia,, whose separate valves resemble two 

 spiral univalves one right-handed, the other left-handed, with small spires and large apertures. 

 In this shell, as in the common Cockle, one valve is placed on each side of the shell-fish, which 

 is usually symmetrical, and lives in a vertical position as regards the plane of its valves. 



To this there are exceptions : as, for example, the Oyster and Scallop, which (like Turbots 

 and Soles among fishes) lie, the former on its left side and the latter on its right side, and 

 fatten at their ease. The shells too are unequal, the deep valve in both cases being the lower 

 shell, and the flat valve the upper. 



The valves of the Cockle are united by an elastic ligament, and articulated by projecting 

 teeth, which form a very complete hinge. It is obvious that the valve of a shell cannot grow 

 so freely along the hinge as on the rest of the margin, but it may shoot out to great length, 

 as in the " Razor shell " (Solen), or in three directions, as in the "Hammer-oyster" (Afalleiis), 

 while in the " Heart-cockle " each valve takes a spiral. There are some fossil shells, called 

 Diceras, in which the two valves resemble horns, and others called JKequienia, with one valve 

 produced into a horn. In Cluimi the umboiies are also spiral. 



In many bivalves the beaks are turned forwards towards the head of the animal. But 

 the Oyster is again an exception in this respect, and if among the exotic or fossil species you 

 find some with spiral growth, the spiral is turned backwards ; indeed, the number of 

 exceptions is so great that one fears to make any general asser- 

 tion. Anomia, when it grows inside shells, may have its umbo 

 a little removed from the margin, and the fossil genera Hippurites 

 exhibit every condition between a marginal ligament and a spiral 

 beak, like Chama, and a patelliform valve, with a ligament 

 wholly internal, and a central umbo. 



The shell, considered as a defence, is most complete in those 

 bivalves like trie Oyster, which shut up close, and in those uni- 

 valves which have an operculum, or door, to their houses. 



Many bivalves gape a little at the sides (or ends), where 

 the foot and the respiratory tubes are accustomed to be pushed 

 out ; whilst Anoniia has a hole or notch in the right valve, 

 through which a byssal plug passes, by which the animal is 

 attached to foreign bodies. 



Others bore into more or less hard substances, as wood, clay, 

 chalk, coral, limestone, and sandstone, and many of these boring 

 Mollusca become so remarkably changed and modified in appear- 

 ance, owing to the life they lead, that, like Magiltts, already 

 described, we can hardly recognise them as bivalve shells at all. 



If we examine the inside of the empty valve of an Oyster, a Pecten, or a Spondylus shell 

 we shall readily detect a single circular scar a little to one side of the valve. This is the 

 point of attachment for the great shell-muscle. If we look at the valve of a Cytherea, or any 



* e, e, the edge of the valve along which (when living) the mantle is attached to the shell ; b and c, adductor muscle ; 

 I, a portion which separates easily from the shell, r, part firmly attached, and which has been cut through ; d, a convex broad 

 arc on mantle, indicating the part where the gills are attached to it, and forming the outer wall of the dorsal water-chamber 

 into which the gills open ; e, the true outline of the mantle, the mantle being represented in its shrunk condition, as seen 

 upon opening the oyster ; / ', silvery muscular lines on the mantle, much branched ; /, fringed edge the edge of the mantle 

 is split in its own planes about T V of an inch, and the space between the two edges is fringed with several rows of minute 

 short tentacles, between which is a dusky pigment ; g, the gills, or branchiw ; /, the liver ; t, the labial tentacles ; m, the 

 mouth ; a, the hinge-line ; r, the vent. 



ANATOMY OF A COMMON OT6TEB. 



