542 GASTEROPODA. 



most important modifications, it would be easy to overwhelm the most 

 patient reader with accumulated details. 



(1434.) The mouth we shall consider as exhibiting four distinct 

 types of organization ; one of which, namely that met with in the Snail 

 and the generality of pulmonated Gasteropoda, has been already de- 

 scribed ( 1380). 



(1435.) The second form of mouth that, for instance, of Pleuro- 

 brancJiiis (fig. 267, a) and of Pterotrachea (fig. 269, b) consists of a 

 simple muscular proboscis, or fleshy tube, which is capable of consider- 

 able elongation and contraction : such an oral apparatus is entirely 

 devoid of teeth or any cutting instrument, but is nevertheless fully 

 able to seize and force into the stomach such materials as are used for 

 food. 



(1436.) A third kind of mouth, by no means so frequently met with 

 as the last, is not a little extraordinary, and forms a more efficient 

 cutting instrument than even that of the Snail. We shall offer, as an 

 example of this remarkable organ, that of the Tritonia Hombergii, re- 

 presented in the annexed figure (fig. 272), 

 whereof Cuvier gives the following graphic de- Flg> ^ 



scription*. In this animal the mouth forms a 

 large oval and fleshy mass enclosing the jaws 

 and their muscles, as well as a tongue covered 

 with spines ; and its opening is guarded by two 

 fleshy lips. The jaws form the basis of all 

 this apparatus : their substance is horny ; their 

 colour a yellowish brown ; and their form (very 

 extraordinary for an organ of this kind) cannot 

 be better described than by comparing them 

 to the shears used in shearing sheep. They Mouth of Tritonia Hombergii. 

 differ, however, in the following particulars : 



instead of playing upon a common spring, the two blades are found to 

 work upon a joint, and, instead of being flat, they are slightly curved. 



(1437.) These two blades are very sharp, and there is nothing that 

 has life that they cannot cut when the animal causes the cutting edges 

 to glide over each other. For this purpose muscles of great strength 

 are provided, the fibres of which are transverse; and their office 

 is to approximate the two blades, that are again separated by the 

 natural elasticity of the articulation whereby they are united at one 

 extremity. 



(1438.) The aliment, once cut by the jaws, is immediately seized by 

 the papillae of the tongue, which, being sharp and directed backwards, 

 continually drag, by a kind of peristaltic movement, the alimentary 

 materials into the oesophagus. 



(1439.) The fourth and most complicated form of mouth is found 

 * Memoire sur le Tritonia. 



