. Elements of zoology, to accompany the field and laboratory study of animals. Zoology. ANATOMY AXD PHYSIOLOGY OF MOLLVSOA l'J9 hardens in contact with the air or water. Increase of tlie shell takes place chieflj' at the margin. Here the three layers of which the shell is comjiosed ar(> secreted — the prismatic layer (Fig. 192, prs) made up of prisms whose axes are per- pendicular to the sur- face, the thin outer varnish which serves to pr<jtect the shell from chemical action and bears most of the char- acteristic coloration of the shell {pre), and, in- side the prismatic laj'^er, thin s
. Elements of zoology, to accompany the field and laboratory study of animals. Zoology. ANATOMY AXD PHYSIOLOGY OF MOLLVSOA l'J9 hardens in contact with the air or water. Increase of tlie shell takes place chieflj' at the margin. Here the three layers of which the shell is comjiosed ar(> secreted — the prismatic layer (Fig. 192, prs) made up of prisms whose axes are per- pendicular to the sur- face, the thin outer varnish which serves to pr<jtect the shell from chemical action and bears most of the char- acteristic coloration of the shell {pre), and, in- side the prismatic laj'^er, thin sheets of lime («) which are secreted not only at the margin, but farther back on the Fi« mantle. The shell of a bivalve has essentially the same structure. The skin of moUusks is relatively soft, but in land species like the garden snails it gains a considerable thickness and is, moreover, abun- dantly provided with glands which pour out a mucus over the skin when it I^ecomes too dry, thus preventing further loss of water. The outer wall of the body has alwaj's an organ characteristic of mollusks; namely, the mantle. In lamellibranchs this is a great pair of folds arisiirg from near the mid-dorsal line of the body and reaching clown on each side to the mid-ventral line. This is the organ that secretes the two valves of lamellibranchs. In gasteropods (Fig. 193) the - pre prs e-p 1 192.—Cross-section of the shell and mantle of Anodonta. ct, connective tissue la^'er of the mantle ; ep. i, its outer cell-layer ; cp. 3, its inner cell-layer ; n, "mother of pearl" layer of shell ; prs, prismatic layer; pre, " periostracum " or outer varnish, also called " ; From Parker and Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Davenport, Charles Benedict, 186
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectzoology, bookyear1911
