CLASS IV GASTROPODA 437 
large cerebral ganglion. The ventral aspect of the creature is commonly 
formed by a broad foot; but in the Heferopoda this is modified into a vertical, 
laterally compressed fin ; and in the Pteropoda it is represented by two wing- 
like swimming membranes near the head. The base of the foot is sometimes 
of considerable size, and in some forms (Strombidae) the animal is enabled to 
spring quite a distance by contracting the foot. The mantle lobe is elevated 
along the back like a hood, extending as far as the head, and usually secretes 
a shell from its outer surface. The shell covers the intestinal sac and lung 
cavity, and usually permits of retraction into it of the entire body of the 
animal. Body and shell are united by muscular attachment ; in spiral shells 
the muscle is fastened to the columella, but in bowl-shaped forms to the inner 
surface of the shell. 
The nervous system consists of two cerebral ganglia, the paired pedal and 
visceral ganglia, and two or three additional pairs, all of which are united by 
commissures. A complete crossing of the commissures of the visceral ganglia 
sometimes takes place (Chiastonewra), but in other forms they run parallel 
(Orthoneura). 
The peculiar armature of the mouth, although developed in all classes of 
Mollusks except Pelecypods, is especially characteristic of Gastropods. This 
consists of two jaw-like horny plates on the upper wall of the oesophagus, 
opposed to which is a chitinous grating, strap, or radula, resting upon the 
tongue or odontophore. The tongue itself is merely a swelling at the bottom 
of the buccal cavity. The radula is usually quite long, and is beset with 
innumerable small teeth or hooks, placed in transverse and longitudinal rows. 
The exceeding diversity of the radula amongst the different groups was 
advantageously employed by Lovén and Troschel as a basis of their 
classification. 
The oesophagus conducts into a long, coiled, intestinal canal, which is 
surrounded by a large liver, the kidneys, and numerous glands. The intestine 
ends in an anal opening placed anteriorly. The heart, as a rule, has one 
auricle (Monotocardia), more rarely two (Diotocardia), and serves as a central 
organ for the supply of a much branched system of blood-vessels.. When the 
gills or lungs are placed in front of the heart (Prosobranchia, Pulmonata), the 
auricles are anterior to the ventricle; but when placed behind the heart 
(Opisthobranchia, Pteropoda), the auricle is posterior. 
Only a few Gastropods breathe through the general surface of the body, 
and are without distinct organs of respiration; the vast majority possess gills 
or lungs. The gills are lamellar or tuft-like, sometimes branched or feathered 
lobes of the integument, and are usually placed in the gill-cavity below the 
mantle ; more rarely they project freely on the back or at the sides. Only 
exceptionally are they present in large numbers and symmetrically developed ; 
and when so disposed they are always secondary structures not homologous 
with the normal ctenidia. Typically there are two gills, but the left usually 
becomes completely atrophied, and the right takes up a median position, con- 
sequent upon the torsion of the body, or even migrates over to the left side. 
Air-breathing snails have the gills replaced by a sac-like cavity, the lung 
occupying the place of the gill-cavity. The walls of this respiratory cavity 
are covered with a finely branched network of blood-vessels. The Ampul- 
larvidae and Siphonariidae possess both gills and lungs. The opening of the 
respiratory cavity is reduced to a round or crescentic aperture, called the 
