MOLLUSCA : GASTEROPODA. 287 



In most of the Gasteropoda the body is unsymmetrical, and is 

 coiled up spirally, " the respiratory organs of the left side being 

 usually atrophied." (Woodward.) The body is enclosed in a 

 "mantle," which is not divided into two lobes as in the Lamel- 

 libranchiata, but is continuous round the body. Locomotion 

 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, and in the Wing-shells (Strombidce), the 

 foot exhibits a division into three portions an anterior, the 

 " propodium ; " a middle, the " mesopodium j" and a posterior 

 lobe, or "metapodium." 



In some, 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" (fig. 102, 0), 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 " (fig. 101). " It consists essentially of a carti- 

 laginous cushion, supporting, as on a pulley, an elastic strap, 

 which bears a long series of transversely-dis- 

 posed teeth. The ends of the strap are con- 

 nected 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 ante- 

 rior 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 ~ '!' 



,1 i _ , .-' rig. 101. r ragment 



the incessant development of new teeth in of the lingual nb- 

 the secreting sac in which the hinder end of ^ of thlcoSl 

 the strap is lodged." (Huxley.) The teeth of mon Whelk (&*# 

 the odontophore ("lingual teeth") are com- SffiX(Afti 

 posed of silica, and are usually arranged in a Woodward.) 

 central (" rachidian ") and two lateral (" pleural ") rows. The 

 mouth leads by a gullet into a distinct stomach, which is 



