756 GASTROPODA. 



is on the left, when the shell becomes "reversed" or "sinistral." 

 The left-handed spiral may be the normal condition of the shell, or 

 it may be only a variety of a normally dextral form. 



As regards their internal anatomy, the head of the Gastropods is 

 usually very distinctly marked out, and is generally provided with ten- 

 tacles and eyes. Within the pharynx is found the singular dental appar- 

 atus which is known as the " odontophore." The essential portion 

 of this is a chitinous band, which is beset with minute transversely- 

 arranged teeth, and is known as the " radula " or " lingual ribbon." 

 The radula is supported upon a cartilaginous cushion, which can be 

 made to rotate by special muscles, the ribbon thus coming to act as 

 a file, rasp, or chain-saw. The arrangement of the teeth in the 

 radula varies much in different cases, but they are usually disposed 

 in a median series, flanked by two or more lateral rows ; and their 

 form and disposition are so constant as to afford one of the most 

 valuable aids to the classification of the recent Gastropoda. As, 

 however, the characters of the radula cannot be determined in the 

 case of fossil Gastropods, a classification based upon the structure 

 of this organ is necessarily defective from a palaeontological point 

 of view. 



Respiration in the Gastropods is variously effected ; the members 

 of one great section (Branchiogastropoda) being, with few excep- 

 tions, constructed to breathe air dissolved in water, while in 

 another division (Pulmogastropoda) the respiration is aerial. In 

 the former division, respiration may be effected in several ways. 

 Firstly, there may be no specialised respiratory organ, the blood 

 being simply exposed to the water in the thin walls of the mantle- 

 cavity (as in some of the Heteropodd). Secondly, the respiratory 

 organs may be in the form of outward processes of the integument, 

 exposed in tufts on the back and sides of the animal (as in the 

 Nudibranchiatd). Thirdly, the respiratory organs are in the form 

 of pectinated or plume-like branchiae, contained in a more or less 

 complete branchial chamber formed by an inflection of the mantle. 

 In many members of this last section the water obtains access to 

 the gills by means of a tubular prolongation or folding of the 

 mantle, forming a "siphon" (fig. 639, s), the effete water being 

 expelled by another posterior siphon similarly constructed. The 

 number of gill-plumes differs in different groups. In most cases 

 there is only a single branchial plume, placed on the right side of 

 the neck ; in other cases an additional gill is present on the left 

 side ; and in other forms, again (e.g., Patella), the gills are multiple 

 and are arranged in a circle. Lastly, in the Pulmonate Gastropods 

 the breathing-organ is a pulmonary chamber, formed by an inflec- 

 tion of the mantle, the walls of which are richly supplied with blood, 

 while air is admitted to its interior by a distinct aperture. A tran- 



