318 



Gasteropcda.- 



-MOLLUSCA.- 



-Gastehopoda. 



to enable it to rise tlian the full unfolding of its organs, 

 au.l tlieir protrusion from the shell. " I incline rather 

 to the conclusion," he says, " that the sole functions of 

 the air chambers is that of the balloon ; and that the 

 power whio'h the animal enjoys of altering at will its 

 specific gravity must be analogous to that possessed by 

 the fresh water testaceous Gasteropods, and that it 

 depends chiefly upon changes in the extent of the 

 surface which the soft parts expose to the water, 

 according as they may be expanded to the utmost, 

 and spread abroad beyond the aperture of the shell, or 

 be contracted into a dense mass within its cavity. The 

 Nautilus may likewise possess the additional advantage 

 of producing a slight vacuum in the posterior parts of 

 the chamber of occupation, which is bhut out by the 



horny cincture ai]d muscles of adhesion from the rest 

 of that cavity." 



The shell of Naittihis 2)ompilius appears to have been 

 known to Aristotle, and is mentioned in sufficiently 

 precise terms by several of the ancient authors. It 

 has long been used as an ornamental substance, and 

 the Dutch have been for a great length of time fond of 

 carving it with marine sketches — ships and dolphins, 

 &o. In India, too, they ornament it in a similar manner, 

 by carving and painting grotesque devices on its outer 

 surface, and converting them into elegant drinking cups. 



Four species have been described, two of which, 

 however, have been considered by some naturalists as 

 only the males of the other two. They are natives of 

 the Indian Ocean, China Seas, and Persian Gulf. 



Class II.- GASTEROPODA (Univalves). 



The class of Gasteropods, the Univalves or Snails, are ' 

 what may be called the tijpes of the Mollusca. Of all 

 the classes into which this sub-kingdom is divided, the 

 Gasteropods present in the most prominent and most 

 marked degree the leading and essential features of 

 molluscous organization. The land snail and the 

 periwinkle will give a very good general idea of the 

 animals of this class. The animals have all a distinct 

 head furnished with eyes and tentacles, and when full 

 grown have a more or less distinct foot, placed under 

 the body, by which they are enabled to creep on the 

 ground or on the surface of the water. This foot in | 

 some is a broad fleshy disc (see Murex), in others it is ' 

 narrow and formed like a furrow (see Stromhus), or it is 

 thin and presents the appearance of a slim plate placed i 

 in a vertical position. This organ is for the most part 

 of a uniform structure throughout, its muscular fibres 

 being interwoven with each other, and not collected 

 into distinct and separate bands. Some have it very 

 large, and it would ajipear that the larger and broader 

 the foot, the slower is the motion of the animal, the 

 narrower and more elongated it is, the quicker is its 

 progress. They are, however, proverbially slow of 

 pace, as every one must know who has watched the 

 march of tlie common snail ; and even it is one of the 

 quickest walkers of the class. The heart is almost 

 always composed of a ventricle and an auricle, and its 

 position is near the back of the animal, but on the side 

 opposite to that in which the reproductive organs are 

 situated. The arterial system is generally well developed, 

 but the veins are imperfectly so ; the venous blood in 

 many of the species flowing through open spaces among 

 the different organs. Some of the Gasteropods are 

 aquatic, while others are denizens of the dry land. The 

 organs of respiration, therefore, are constructed in such 

 a manner as to enable the animals either to breathe 

 atmospheric air or respire water. Those of the former 

 group are provided with lungs, which have an external 

 communication to admit air, opening under the outer 

 edge of tlie mouth. In the latter, respiration is carried 

 on by means of branchiae or gills, the exact position of 

 which varies very much in different species ; in many 



being inclosed in a cavity on the back, in some being 

 lodged in a furrow betw-een the mouth and the foot, 

 and in others, being situated upon the back of the 

 animal, and floating freely in the water. In general 

 the respiratory organs are atrophied on the left side, 

 though in some, as the Chitons, &c., they are repeated 

 on both sides. Usually, also, the Gasteropods are 

 unsymmetrical in conformation ; the body instead of 

 being developed lengthwise, in a straight line, taking 

 somewhat of a spiral direction, so that the mouth and 

 the excretory opening are brought in a slight degree 

 near to each other, instead of being placed at the two 

 extremities of the body. 



Their food is either vegetable or animal ; and the 

 mouth is provided with a denticulated tongue. " This 

 tongue is muscular, and armed with recurved spines 

 (or linijual teeth), arranged in a great variety of patterns, 

 which are eminently characteristic of the genera. Their 

 teeth are amber-coloured, glossj', and translucent; and 

 being silicious (they are insoluble in acid), they can be 

 used like a file, for the abrasion of very hard substances. 

 With them the limpet rasps the stonj' nullipore, the 

 whilk bores holes in other shells, and the cuttle-fish 

 doubtless uses its tongue in the same manner as the 

 cat. The tongue, or lingual ribbon, usually forms a 

 triple band, of which the central part is called the 

 rachis, and the lateral parts i^leurcc ; the rachidian teeth 

 sometimes form a single series, overlapping each other, 

 or there are lateral teeth on each side of a median 

 series. The teeth on the pleurm are termed uncini ; 

 they are extremely numerous in the plant- eating 

 Gasteropods. Sometimes the tongue forms a short 

 semicircular ridge, contained between the jaws ; at 

 others it is extremely elongated, and when withdrawn ; 

 its folds extend backwards to the stomach. The 

 lingual ribbon of the limpet is longer than the whole 

 animal ; the tongue of the whilk has one hundred rows 

 of teeth ; and the great slug has one hundred and sixty 

 rows, with one hundred and eighty teeth in each row. 

 The fiont of the tongue is frequently curved or bent 

 quite over ; it is the part of the uistrument in use, and its 

 teeth are often broken or blunted. The posterior part 



