386 



GASTEROPODA. 



part being comparatively soft ; it would seem, 

 therefore, that in proportion as the anterior part 

 is worn away, the parts behind it assume 

 gradually the necessary firmness and advance 

 to supply its place. The action of this curious 

 instrument is as follows : in the upper part 

 of the circumference of the mouth we find a 

 semicircular horny plate, resembling an upper 

 jaw, and the tongue, by triturating the food 

 against this, gradually reduces substances how- 

 ever hard. On opening the Patella, the tongue 

 is found doubled upon itself, and folded in a 

 spiral manner beneath the viscera. 



The tongue of Oscabrio resembles that of 

 the Patella, except in its armature, being fur- 

 nished on each side with a series of hooked and 

 three-pointed scales, and another set of long, 

 sharp, and recurved spines, whilst its centre is 

 simply studded with tubercles. In Turbo pica 

 the scales, which are cutting and denticulated, 

 are arranged transversely along its surface. 



The tongue of Buccinum (Jig. 182, f), is 

 placed at the extremity of the proboscis, form- 

 ing a most extraordinary apparatus, capable of 

 destroying by its constant action the hardest 

 shells ; externally it resembles rather a mouth 

 than a tongue, being divided into two lips, 

 each of which is studded with sharp horny 

 teeth. These lips are supported upon two 

 cartilages which occupy the anterior half of the 

 proboscis, and are moved upon each other by 

 strong muscular fasciculi (A) in such a manner 

 that the spines which arm the surface of the 

 organ are alternately erected and depressed by 

 their action, a movement the constant repe- 

 tition of which soon wears away the substances 

 upon which it is made to act. This spiny 

 tongue is situated just within the entrance to 

 the oesophagus (cT), and besides acting upon 

 foreign bodies will materially assist in pro- 

 pelling the food into that tube. 



In other Gasteropods the tongue is short 

 and merely an organ of deglutition : thus, in 

 Aplysia it is broad, heart-shaped, and studded 

 with sharp points. In Onchidium and Doris, 

 the surface is marked with transverse grooves, 

 which are crossed at right angles by others of 

 great fineness. And in the Snail and Slug, in 

 which the surface of the tongue is similarly 

 marked, the striae are so delicate that they can 

 only be seen with a microscope. 



Alimentary canal. We shall commence 

 our description of the intestinal canal of the 

 Gasteropod Mollusca by the examination of 

 the simpler forms which it presents. In the 

 Snail (jig. 190), the whole alimentary tube 

 (e, /, g, k) is thin and membranous. The 

 stomach, which is merely a dilatation of the 

 oesophagus, is semitransparent, but studded 

 with opaque points and internally folded into 

 delicate longitudinal plicae. From this arises 

 an intestine, of considerable length, without 

 coeca, valves, or remarkable appearance inter- 

 nally, except near its termination, where the 

 orifices of minute follicles may be detected ; 

 the intestine having performed several con- 

 volutions enveloped in the masses of the liver, 

 with which it is connected by cellulosity 

 and numerous vessels, at last runs along the 



margin of the pulmonary cavity, close to the 

 orifice of which it terminates. In Vaginulus 

 the arrangement is nearly similar (Jig. 189, 

 g, A, z.) In Tritonia and Doris the structure 

 of the digestive tube is equally simple, and in 

 these as well as in the majority of the Gaste- 

 ropoda the only remarkable differences are 

 found in the proportional size of the stomach 

 and the length of the intestinal convolutions. 

 In Doris we find near the orifices by which 

 the bile is poured into the stomach, an aper- 

 ture communicating with a round vesicle or 

 coecum, the inner surface of which is evidently 

 glandular, and from its large supply of blood 

 derived from one of the hepatic arteries, pro- 

 bably furnishing an abundant secretion ana- 

 logous to that of the pancreas. In Phasia- 

 nella the stomach is very voluminous and 

 sacculated internally. In Buccinum the di- 

 gestive apparatus is more complicated in its 

 structure. The oasophagus commences, as we 

 have already seen, at the extremity of the pro- 

 boscis, and of course follows all the motions 

 of that organ ; when the proboscis is protruded 

 in search of prey, the gullet is straight and 

 adapted to the reception of food ; but when the 

 proboscis is retracted within the body, the 

 oesophagus is bent upon itself, so as to be 

 partially contained within the proboscis, whilst 

 the greater portion is folded beneath that organ 

 in its retroverted state. After making another 

 fold it dilates into a small crop, the lining of 

 which is plicated in the direction of its axis, 

 and to this succeeds the stomach, which is a 

 moderately sized round cavity, irregularly ru- 

 gose internally. The intestine is very short, 

 and has a small coacum appended to its side ; 

 it terminates in a capacious rectum, placed, 

 as is invariably the case, in the vicinity of the 

 respiratory cavity, and having its lining mem- 

 brane gathered into prominent longitudinal 

 rugse. Many of the Gasteropoda are provided 

 with several digestive cavities, resembling in 

 some degree the stomachs of ruminating Mam- 

 malia. In Janthina, which is furnished with 

 a proboscis like that of the Buccinum, the 

 oesophagus arising from this terminates by a 

 narrow slit in a membranous cavity or first 

 stomach, to which succeeds a second, having 

 thicker walls and plicated internally. The in- 

 testine is extremely short, terminating as usual 

 in the neighbourhood of the respiratory cavity. 

 In Pleurobranchus the resemblance of the 

 stomachs to those of a ruminating quadruped 

 is very striking. The first stomach (fig. 185, 

 a), which is membranous, receives the bile by 

 a large orifice (6) placed near its communi- 

 cation with the second digestive cavity (c), 

 which is smaller and more muscular ; to this 

 succeeds a third (d\ the sides of which are 

 gathered into broad longitudinal lamellae, pre- 

 cisely similar to those of a ruminant ; and 

 to render the analogy still more perfect, a 

 groove is found running along the walls of the 

 second cavity from one orifice to the other, 

 apparently subservient to rumination. The 

 fourth stomach (e) is thin, and its walls smooth. 

 This animal lives on Alcyonia and small Zoo- 

 phytes. 



