MOLLUSCA : GASTEROPODA. 259 



is effected by means of the ' foot,' which is usually a broad 

 muscular disc, developed upon the ventral surface of the 

 body, and not exhibiting any distinct division into parts. In 

 the Heteropoda, however, the foot exhibits a division into three 

 portions an anterior, the ' propodium ;' a middle, the ' meso- 

 podium;' and a posterior lobe, or ' metapodium.' 



In others, again, the upper and lateral surfaces of the foot 

 are expanded into muscular side-lobes, which are called ' epi- 

 podia.' In many cases the metapodium, or posterior portion 

 of the foot, secretes a calcareous, horny, or fibrous plate, 

 which is called the * operculum,' and which serves to close the 

 orifice of the shell when the animal is retracted within it. 



The head in most of the Gasteropoda is very distinctly 

 marked out, and is provided with two tentacles and with two 

 eyes, which are often placed upon long stalks. The mouth is 

 sometimes furnished with horny jaws, and is always provided 

 with a singular masticatory apparatus, called the ' tongue,' or 

 ' odontophore.' ' It consists essentially of a cartilaginous 

 cushion, supporting, 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 are made good by the incessant 

 development of new teeth in the 

 secreting sac in which the hinder 

 end of the strap is lodged.' 

 (Huxley.) The teeth of the 

 odontophore (' lingual teeth') are 

 composed of silica, and are 

 usually arranged in a central 

 (' rachidian '), and two lateral F ig. 77.-p ar t of the lingual ribbon "or 



('pleural') rOWS. The mouth odontophore of Valvatapiscinalis,mag- 

 ,, , . , ,. , . nified. 



leads by a gullet into a distinct 



stomach, which is sometimes provided with calcareous plates 

 for the trituration of the food. The intestine is long, and \ts 

 first flexure is commonly 'haemal,' or towards that side of tte 

 body on which the heart is situated; though in some the 

 flexure is ' neural.' Distinct salivary glands are usually pre- 

 sent, and the liver is well developed. 



A distinct heart is usually present, composed of an auricle 



s2 



