180 CLASSIFICATION OP THE ANIMAL KINGDOM. 



system. In the Common Snail, (Helix aspersa,) which belongs to the order 

 Palmobranchiata, we have a very good type of the organic structure peculiar 

 to the whole class, though the different genera and species often vary con- 

 siderably in some one or more particular parts. The body of the animal "can 

 be protruded or shortened at pleasure, in consequence of the highly-developed 

 muscular system. When in motion, four tentacula are protruded from the 

 head, the upper pair of which is furnished with a perfect eye at each extremity, 

 and the lower pair more especially form the instruments of touch. The 

 mouth, which is provided with a singular cutting instrument, is placed under 

 the head, and the tongue is covered over with transverse striae, which, in 

 some species are represented by small recurved hooks. It also possesses an 

 oesophagus furnished with large salivary glands, a stomach, into which four 

 biliary ducts from a large liver pour their secretions, and a simple con- 

 voluted intestine, which terminates on the right side of the neck. The 

 respiratory system consists of a good-sized cavity, the roof of which is 

 interlaced with innumerable blood-vessels, and this chamber communicates 

 with the exterior by a wide orifice on the right side of the body, close to 

 the anal aperture. The generative system is most complex and peculiar. 

 Every Snail is hermaphrodite, but not self-impregnating. The ovary is 

 situated in the inmost recesses of the shell, and is provided with a very 

 lor:g oviduct. The Gasteropods are divided into groups, according to the 

 arrangement of their respiratory apparatus. Those that inhabit the sea 

 are provided with branchial appendages of various construction; some of the 

 branchiae are external, and are placed along the back, and the animals 

 possessing them are termed NudibrancMata ; others, as the Inferobranchiata, 

 have these appendages placed on each side of the body, under the projecting 

 edge of the mantle; others have the same organs on one side of the body 

 only; others, as the PectinibrancMata, which comprise the inhabitants of 

 spiral univalve sea-shells, have their branchiae placed in an internal capa- 

 cious cavity. In these, and other orders not enumerated, the digestive 

 system varies considerably; the mouth alone presents four different types 

 of organization, one like that of the Common Snail already described, another 

 consisting of a muscular proboscis capable of considerable distension and 

 contraction, but without any dental or cutting apparatus, a third like a 

 pair of shears, and a fourth consisting of a singular boring instrument, as 

 in Buccinum, with which those little round holes so frequently seen in 

 dead cast -up shells are drilled. Many species are armed internally with a 

 formidable apparatus of teeth; others are provided with sharp cutting 

 instruments in the gizzard to assist their digestion. The respiratory system 

 also varies considerably in the different tribes. The construction of the 

 heart in the great majority of Gasteropods is like that of the Common 

 Snail, and consists of a single auricle and ventricle, but in some of the 



