440 MOLLUSCA SUB-KINGDOM VI 
the shell. The porcellanous material is composed of three distinct layers, 
each of which is made up of thin laminae, and the laminae in turn of very 
small oblique prisms. The laminae of the middle layer are disposed at right 
angles to those of the adjacent layers. 
Many Gastropods have a calcareous or horny plate, called the operculum, 
attached to the posterior part of the foot, and serving to close the aperture 
more or less completely when the animal withdraws into its shell. Being 
most commonly of corneous nature, it is seldom preserved fossil; sometimes, 
however, it is calcareous, and may attain considerable size and thickness. On 
the outer surface it may be smooth, furrowed, granulated, or covered with 
excrescences. The nucleus cr initial point of growth is sometimes central, and 
again eccentric or even marginal in position; it may be surrounded by con- 
centric markings, or form the origin of a spiral consisting of few (paucispiral) 
or many (multispiral) whorls. Certain Solariidae have a conical operculum, 
which is covered externally with numerous spiral lamellae. 
The embryonic stages of Gastropods are completed in the egg. Very early 
in its development the embryo forms a small shell, called the protoconch or 
nucleus, which consists sometimes of several whorls, and not infrequently 
differs in form from the shell of the adult. The protoconch remains attached 
to the apex for a time, in the form of a small glistening knob, or a short 
smooth spire, which occasionally stands at an angle to the rest of the shell, or 
is even twisted in a contrary direction (heterostrophic). Should the protoconch 
become decollated, a small calcareous plate closes over the apex of the spire. 
All branchiate Gastropods are aqueous in habitat, but there are some 
forms having a lung cavity which live permanently in fresh water (Limnaecdae), 
and others which are exclusively marine (Siphonariidae). 'The greater number 
of Gastropods, especially the large and solid forms, frequent the coast-line, 
and inhabit comparatively shallow water. Some become attached to shoals 
and plants, others burrow in sand or mud. A great reduction in the Gastropod 
fauna is noticed at a depth of between 70 and 100 meters, but many genera 
(Pleurotoma, Fusus, Natica, Odostomia, Eulima, Scisswrella, Turbo, Cylichna, 
Tornatina, Actaeon, etc.) persist into the greatest depths yet explored. Most 
marine Gastropods are killed by removal into fresh water; a few genera, 
however, are able to maintain their existence in brackish or in fresh water 
(Cerithium, Littorina, Rissoa, Trochus, Purpura, ete.). On the other hand, many 
fresh-water forms (Melania, Melanopsis, Neritina, Ampullaria, Limnaea, Planorbis) 
can survive in brackish or even strong salt water. 
Most Gastropods are herbivorous, but a few subsist upon living or decom- 
posed animal food. Many genera (Natica, Buccinwm, Murex) perforate the 
shells of other Mollusks with their radula, and extract the contents. 
Classification.—Ordinal divisions have been based since the time of Cuvier 
and Milne-Edwards upon the respiratory organs, and the structure of the foot 
(whether adapted for swimming or crawling). The reproductive or gans, and 
the structure of the heart and nervous system, are also of prime importance. 
For separating smaller groups, shell characters and the radula are largely 
employed. Gastropods may be divided into two sub-classes: Streptoneura, 
with the orders Ctenobranchiata and Aspidobranchia ; and Luthyneura, with 
the orders Opisthobranchia and Pulmonata. 
