36 ON CLASSIFICATION. 



is the chief organ of locomotion ; and three principal pairs of 

 ganglia — cerebral, pedal, and parieto-splanchnic. When they 

 are provided with a heart, which is usually the case, it is divided 

 into auricular and ventricular chambers ; but the mantle, instead 

 of beins: divided into two lateral lobes, is continuous round the 

 bodv, and when it secretes a shell from its surface, that shell is 

 either in a single piece, or the pieces are repeated from before 

 backwards, and not on each side of the median line. The shell 

 of a Branchiogasteropod may, therefore, be univalve, or com- 

 posed of a single conical piece, straight or coiled ; or it may be 

 multivalve — formed of a number of segments succeeding one 

 another antero-posteriorly ; but it is never bivalve. 



Sometimes a shelly, horny, or fibrous secretion takes place 

 from the foot, giving rise to a structure resembling the byssus 

 of some Lamellibranchs ; it is the so-called " operculum," which 

 serves to protect the animal when retracted into its shell ; but as 

 the operculum is developed from the foot and not from the mantle, 

 it can obviouslv have no homology with the valves of either a 

 Brachiopod or a Lamellibranch. The Branchiogasteropoda 

 (Fig. 14) commonly possess a distinct head, provided with a 

 pair of tentacles and a single pair of eyes, both of which are 

 supplied with nerves from the cerebral ganglia. Cephalic eyes 

 of this kind are not known in the Lamellibrancliiata. But the 

 characters which most definitely distinguish the Brancliio- 

 gasteropoda are to be found in the alimentary canal. The 

 cavity of the mouth is invariably provided with an organ which 

 is usually, though not very properly, called the tongue, and 

 which might more appropriately be denominated the " odonto- 

 phore." It consists essentially of a cartilaginous cushion, sup- 

 porting, as on a pulley, an elastic strap, which bears a long- 

 series of transversely disposed teeth. The ends of the strap are 

 connected with muscles attached to the upper and lower surface 

 of the hinder extremities of the cartilaginous cushions ; and 

 these muscles, by their alternate contractions, cause the toothed 

 strap to work, backwards and forwards, over the end of the pulley 

 formed by its anterior end. The strap consequently acts, after 

 the fashion of a chain-saw, upon any substance to which it is 

 applied, and the resulting wear and tear of its anterior teeth 



