. Juvenile Instructor. hology. Accordingto their bodily structure, mollusks arenow generallj classified as follows: 1. Those possessing no well definedhead parts: such are the oyster, clam,and scallop, all such belong to the (from Greek a without andcephaloii, head). These are also calledlamellibranchs, or animals with foldedgills; and the class includes all thatare ordinarily spoken of as bivalves. 2. Mollusks such as univalves andslugs, having prominent head develop-ment, but with no locomotive organsattached to the head; these form theclass (head bearing). 3. T
. Juvenile Instructor. hology. Accordingto their bodily structure, mollusks arenow generallj classified as follows: 1. Those possessing no well definedhead parts: such are the oyster, clam,and scallop, all such belong to the (from Greek a without andcephaloii, head). These are also calledlamellibranchs, or animals with foldedgills; and the class includes all thatare ordinarily spoken of as bivalves. 2. Mollusks such as univalves andslugs, having prominent head develop-ment, but with no locomotive organsattached to the head; these form theclass (head bearing). 3. Those with organs of locomotionor feet, attached to the head; such asthe squid; these belong to the classCephalopoda (head footed). BIVALVES OR HEADLESS MOLLUSKS (CLASSA(JEPHALA). The division of the shell of thesemollusks into two parts has already THE JUVENILE INSTRUCTOR. 435 been illustrated in the case of the rivermussel, {Unio), figure 1, let us nowexamine in greater detail a single valveof such a shell (figure 4).. Figure 4. Parts of a bivalve shell; u,umbo; h, hinge li^auient; t, teeth; 1,lunule; a, murk of the anterior ad-ductor muscle; p, attachment of theposterior adductor muscle; r, regionof siphon retractors; P, pallial im-pression. The point at which the valves wereconnected is called the i/iiibci or beak (u);the half of the shell toward whichthis is directed is known as the anteriorvahe, the other as the posterior plates of the shell are so arrangedthat on the outer side their edges formwell defined lines of growth which areconcentric about the umbo. The pro-jections called teeth (t) fit into corres-ponding depressions on the oppositevalve; the central ones are the cardinalteetli, the side ones the lateral teeth. The/;/«,!,•(? lii^aincnt (h) tends by its elasticityto open the valves; in some species theligament is situated within the shell, andacts by expansion, in others it is outsideand operates by contraction. This ten-dency to open the shell is con
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Keywords: ., bookauthorgeorgequaylecannon182, bookcentury1800, bookdecade1890
