^203. 



THE CEPHALOPnORA. 229 



the aquatic species, is extended over nearly the whole body, but with 

 those which are terrestrial, is confined to certain spots/^* With the Gas- 

 teropoda, its external surface is striated or tuberculated ; on the whole, the 

 skin of these animals closely resembles a mucous membrane and secretes 

 constantly a large quantity of mucus. 



It has an extraordinary contractility, due to a muscular layer intimately 

 blended in its texture. 



With many species, the skin forms around the neck or back a fold, which 

 is usually circular ; the posterior or the upper part of this fold is dilated 

 into a hernial sack containing a portion of the viscera. This portion of 

 the skin is called the mantle}'-^ With many, it can be wholly withdrawn 

 into the body, and then the orifice of the fold acts as a sphiucter.^^' 



§ 203. 



Very many of the Ccphalophora carry upon their back a univalve shell''' 

 which is formed by the border and external surface of the mantle, and, in a 

 few cases only, in its interior.'-' 



The border of the mantle is the tissue most concerned in the formation 

 of the shell. The shell's increase depends upon it, and for this purpose 

 it is always in contact with the orifice. 



With the majority of the terrestrial Gasteropoda '•^' the completed shell has 

 a lip at its orifice, which, in some aquatic species,'*' is repeated several times 

 at regular intervals during the development. In many Pectinibranchia, 

 the border of the mantle has prolongations, which also secrete lime and pro- 

 duce around the orifice of the shell wing-like or spinous processes.'^' With 

 many species of this sub-order, one of these processes has a kind of canal, 

 called the Siphon, which conducts the water into the respiratory cavity. 

 With some, this siphon is contained in an appendix to the orifice of the 

 shell,'®' while, with others, it is protruded through a fissural opening of this 

 last.'^' 



With some Gasteropoda, the mantle is folded over a large portion of the 

 external surface of the shell, which it covers with a calcareous substance.'^ 



The borders of the mantle have numerous, short, glandular follicles, whose 

 walls are composed of large cells, some of which contain a finely-granular 



1 Ciliated epithelium covers the entire surface of 1 With Chiton, exceptionally, the shell is 

 the body with Lymnaeus, Planorbis, fhysa, formed of several imbricated pieces so united as to 

 Paliidina, Vulvata, Tcrgipes, Flabellina, and be movable. In some species the organic so much 

 Polycera. With the terrestrial Gasteropoda, I exceeds the calcareous substance, that the shell 

 have found it only on the surface of the foot, and has a horny aspect, as with Aplysia, Hyalea, 

 with Avion, upon also the borders of this organ, and Cleodora. With Cymbulia, the shell is even 

 which is separated from the rest of the body by a cartilaginous ; that of Cypraea, on the contrary, is 

 longituduial furrow. I cannot, therefore, support composed almost exclusively of lime, — the quan- 

 the assertion of Valentin (fVagner'x Handwcirt tity of organic substance being very small, 

 erbuch der Physiol. I. p. 429) that with Helix and '^ With Bullaea, Limax, and Testacella, the 

 Z/Jmao;, the whole surface of the body and even the shell is wholly concealed in the mantle. With 

 tentiicles are covered with this epithelium. ^rj'on, the lime secreted in the interior does not 



2 With Limax, Arion, and some allied genera, unite with the shell, but forms a mass of loosely 

 this mantle is quite rudimentary, — covering like a juxtaposited granulations. 



shield only a small portion of the back. 3 With the Auriculacea, and many of the Heli- 



3 The genus Sagitta differs, in this respect, from cina. 



aU the other Cephalophora. Its skin is without * Murex, Harpa, Scalaria. 



folds, forms a kind of cylinder, and consists of a 5 Strombus, Pterocera, Mnrex. 



dense dermis perfectly smooth and non-contractile. C Cerit/iiu7n, Murex, Rostellaria, Turbinella, 



At first, its structure appears to l>e homogeneous, Fasciolaria. 



but a more careful examination shows extremely 7 Harpa, Oliva, Voluta, Buccinujn, Dolium, 



delicate parallel fibres running from before back- Conns. 



wards in unint<:rrupted and apjjarently varicose 8 Ovula, Cypraea. 



rows ; they resemble nuclear fibres [of HenWl. 



20 



