$ 203. 229 
THE CEPHALCPHORA. 
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 tubereulated; 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 With many, it can be wholly withdrawn 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 sphincter. 
§ 208. 
Very many of the Cephalophora 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 
the body with Lymnaeus, Planorbis, Physa, 
Paludina, Valvata, Tergipes, Flabellina, and 
Polycera, With the terrestrial Gasteropoda, I 
have found it only on the surface of the foot, and 
with Arion, upon also the borders of this organ, 
which is separated from the rest of the body by a 
longitudinal furrow. I cannot, therefore, support 
the assertion of Valentin (Wagner’s Handwért 
erbuch der Physiol. I. p. 429) that with Hediz and 
Limaz, the whole surface of the body and even the 
tentacles are covered with this epithelium. 
2 With Limaz, Arion, and some allied genera, 
this mantle is quite rudimentary, — covering like a 
shield only a small portion of the back. 
8 The genus Sagitta differs, in this respect, from 
all the other Cephalophora. Its skin is without 
folds, forms a kind of cylinder, and consists of a 
dense dermis perfectly smooth and non-contractile. 
At first, its structure appears to be homogeneous, 
but a more careful examination shows extremely 
delicate parallel fibres running from. before back- 
wards in uninterrupted and apparently varicose 
rows; they resemble nuclear fibres [of Hende]. 
20 
1 With Chiton, exceptionally, the shell is 
formed of several imbricated pieces so united as to 
be movable. In some species the organic so much 
exceeds the calcareous substance, that the shell 
has a horny aspect, as with Aplysia, Hyalea, 
and Cleodora. With Cymbulia, the shell is even 
cartilaginous ; that of Cypraea, on the contrary, is 
composed almost exclusively of lime, — the quan- 
tity of organic substance being very small. 
2 With Bullaea, Limaz, and Testacella, the 
shell is wholly concealed in the mantle. With 
Arion, the lime secreted in the interior does not 
unite with the shell, but forms a mass of loosely 
juxtaposited granulations. 
_3 With the Auriculacea, and many of the Heli- 
cina. 
4 Murex, Harpa, Scalaria. 
5 Strombus, Pterocera, Murez. 
6 Cerithium, Murex, Rostellaria, Turbinella, 
Fasciolaria. 
7 Harpa, Oliva, Voluta, Buccinum, Dolium, 
Conus. 
8 Ovula, Cypraea. 
