. Text-book of zoology for schools and colleges. Zoology. MOLLUSCA PROPER. Ill lastly, the shell is more developed on one side than the other, so as to become "inequilateral" (Fig. 81, 2). The lobes of the mantle are sometimes quite free ; but, at other times, they are more or less united to each other, and leave only two openings. Through one of these openings (the ante- rior) the " foot" is protruded (Fig. 80, /); and through the other pass the respiratory tubes or "siphons" (s). The foot in the bivalves is a mus- cular organ developed upon the lower surface of
. Text-book of zoology for schools and colleges. Zoology. MOLLUSCA PROPER. Ill lastly, the shell is more developed on one side than the other, so as to become "inequilateral" (Fig. 81, 2). The lobes of the mantle are sometimes quite free ; but, at other times, they are more or less united to each other, and leave only two openings. Through one of these openings (the ante- rior) the " foot" is protruded (Fig. 80, /); and through the other pass the respiratory tubes or "siphons" (s). The foot in the bivalves is a mus- cular organ developed upon the lower surface of the body, but not forming a creeping flattened disk, as in the ordi- nary univalves. In many cases, it is quite'rudimentary; and even when it is employed in locomotion it is usually small. Most generally, it is hatchet-shaped or pointed (Fig. 80, /), and serves to enable the animal to make short leaps. In many cases, as in the common mussels, the foot is subsidiary to a special gland, which secretes a viscous fluid, which hardens rapidly on exposure to the air. This fluid is moulded by the foot into silky threads (the so-called "byssus"), by means of which the shell is firmly fixed to some solid object. Besides the muscular foot, other muscles are pres- ent as well in the JLamelK- hranchiata. Of these, the most important are the muscles which close the shell, and are called the " adduc- tor" muscles. In one group of the bivalves (Fig. 81, 3), there. Pig. 80.âAnatomy ot aBiTalve MolluBk. Mya arenaria (after Woodward). The left valve and mantle-lobe, and half the siphons are removed. 8 s Eespiratory siphons, th4 arrows indicating the direction of the cur- rents ; a a/ Adductor muscles; & Gills; h Heart; o Mouth, surrounded by (^) labial palpi;/Foot: â uAnus; m Cut edge of the Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustration
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectzoology, bookyear1884
