8 THE SKIN. 



by the shell. Frequently the external skin is colored by a gran- 

 ular pigment, which either is contained in cells, lying between or 

 enveloping the glands, or else sometimes appears to lie free under 

 the epithelium. 



The mantle border is the principal agent in the secretion of 

 the shell : it is thrown out by the epithelial layer as a sort of 

 cuticular development. With the organic basis of this secretion 

 is mingled carbonate of lime, originating in the epithelial cells, 

 where it may be separated from the blood : in hardening, the 

 exuded material becomes half crystalline or laminated. Usually 

 the external layer of the shell is a transparent or translucent skin, 

 the epidermis ; having no lime in its composition. It is often 

 colored by pigments lying in the outermost border of the mantle. 



Whilst the growth of the shell is thus provided for by addi- 

 tions of the aperture margin from the mantle border, the whole 

 mantle is equally capable of producing shelly substance ; and not 

 only are shells thus thickened from within by the mantle surface, 

 but breaks are repaired with new material by a similar provision : 

 only such repaired and interior portions are devoid of epidermis 

 and of color, the pigments being found only in the free border of 

 the mantle. Mollusks are even able to secrete shelly matter to 

 provide against threatening- dangers from the boring of other 

 animals into their shell.* In certain genera, as Haliotis, Turbo, 

 etc., the inner shelly layer is nacreous that is, it forms mother- 

 of-pearl. 



The mantle border by means of its sphincter muscles embraces 

 the body closely, thus closing the mantle cavity except at one 

 point, where a small opening allows the ingress and egress of 

 water for respiration. This respiratory opening is a semi-circular 

 notch, formed by muscles, and is sometimes prolonged on its 

 dorsal wall into a half-closed tube or respiratory siphon; which, 

 when present, assists by the phases of its development in the 

 classification of the mollusca. This siphon usually forms an an- 

 terior notch in the shell near the margin of the columella and the 



* A curious example of shell secretion by the visceral mantle occurs in 

 a cone belonging to the cabinet of the late Dr. Gray. A section of this 

 shell has been made, showing holes bored into the spire end by lithodomi 

 and the repeated walls erected by the animal across the ends of the 

 whorls to protect itself against the ravages of its insidious enemies, 

 (plate 2, fig. 1.) 



