12 THE SKIN. 



The cutis consists essentially of fine interlacing muscular fibres 

 with interposed cells ; often attaining a perceptible thickness. 

 The subcutaneous muscular layers of the body are immediately 

 continuous with those of the cutis : the fibres of which they are 

 formed may be clearl}^ distinguished as an outer longitudinal 

 and inner circular layer. These fibres are (as in mollusks gen- 

 erally) compressed and band-like, with pointed ends and central 

 oval nucleus. 



In Doris (iii, 39) and some other opisthobranchiates the skin 

 is consolidated and rendered rigid by a number of calcareous 

 spiculse, which have a somewhat definite arrangement and form 

 a sort of internal skeleton. In Paludina these spicules are 

 replaced by globulous calcareous concretions, in Pholas by sil- 

 iceous granules. Chalkj^ scales from the cephalopod Scseurgus 

 tifanops are represented, greatly macrnified, by fig. 59, PI. iv. 



The skin forms a fold above and surrounding the foot, and 

 this portion is technically termed the mantle; it surrounds the 

 body, behind the cephalic portion like a collar^ and thence spreads 

 dorsally over the posterior part of the animal. In the terminal, 

 thickened border of the mantle, the cutis becomes of greater 

 thickness ; its upper stratum containing very numerous glands, 

 furnishing the mucus and colors which are here mingled with the 

 secreted shell-material. Similar glands, furnishing a copious 

 supply of mucus, are found also in the cephalic portion of the' 

 body as well as in the foot, and especially in the sole ; but no 

 cells are found in that part of the animal permanently covered 

 by the shell. Frequently, the external skin is colored by a 

 granular 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 laj^er 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 to 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 : 

 such repaired and interior portions are devoid of epidermis and 

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



