CLASS IV GASTROPODA 515 



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 (which may be wanting or obsoles- 

 cent in the adult) 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 (Chiastoneura), 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 one or more horny plates on the anterior upper margin 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, and exceedingly constant in form throughout the several 

 groups. The characters of the radula among the different groups were 

 therefore advantageously employed by Loven and Troschel as a basis of 

 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 (Montocardia), more rarely two (Diofocardia), 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, Pidmonata), the 

 auricles are antex'ior to the ventricle ; but when placed behind the heart 

 {Opisfhobranchia, Pteropoda) the aiu-icle 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- 

 lariidae and Siphonariidae possess both gills and lungs. The opening of the 

 respiratory cavity is reduced to a round or crescentic aperture, called the 

 breathing pore. The sides of this pore, in operculated snails, are often pro- 

 duced outwards, so as to form a closed or cleft tube, corresponding with which 

 there is frequently a canal-like process of the shell. 



