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 



ep.z. 



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 

 " epidermis." From Parker and Ha.swell. 



