356 MOLLUSCA. 



Class IV. GASTROPODA.* 



Molhtsca wit ft a distinct head generally bearing tentacles, with a 

 ventral muscular foot usually used for creeping, and typical/ y with a 

 continuous mantle-fold. The shell when present is composed of one or 

 of more than two pieces. 



The organization of the Gastropoda is Avith one exception asym- 

 metrical, but the asymmetry is confined to the organs of the visceral 

 mass, and rarely, if ever, affects the head. The head has the mouth 

 at its anterior extremity, and usually bears one (Streptonen ra, Theco- 

 somata, Pftyllirho^, Elysiids, and some Pulmonates) or two (most 

 Opisthobranchs and some Pulmonates) pairs of tentacles, and two 

 eyes, which are placed at the base, sometimes at the apex of a pair 

 of tentacles. The tentacles are contractile, and in the stylommato- 

 phorous Pulmonates they are invaginable. 



Their form varies and they are often modified, and may even be absent 

 (Olivella, Homalogyru, Pttrotntchca, etc.); in most of the Bulloids both pairs 

 are widened out and transformed into a quadrangular cephalic shield, the four 

 corners of which correspond to the apices of the four tentacles. The single pair 

 of the Aiirphibolidnr, Otinidae, f>ij>hoitari/(li/i\ and Gadiniidnc are reduced, and 

 give to the head the aspect of a flattened disc. The anterior pair of Pleuro- 

 branchidac, Tritonii'd.*"', etc., is transformed into a frontal velum. They may 

 also be bifid, flattened or branched. The labial palps, which are processes of 

 the lips, found in some forms, are not to be regarded as tentacles ; nor are the 

 small lobe-like processes (palrnettes) sometimes found between the tentacles. 

 The pseudopallium is a process of the cephalic integument which projects back 

 over the shell. 



The foot is a muscular organ, and projects from the ventral surface 

 of the body. Typically it has a flat sole, and is used for creeping. 



* Martini and Chemnitz, Conchylien Cabinet, 12 Bde., Niirnberg, 1837-1865. 

 Sowerby, "Thesaurus conchyliorum, or Figures and Descriptions of Shells," 

 London, 1832-62. Reeve, " Conchologica iconica, etc.," London, 1842-62. 

 H. and A. Adams, " The Genera of the recent Mollusca," 3 vols , London, 1858. 

 H. Troschel, "Das Gebiss der Schncckcn," Berlin, 1856-78. S. P. Woodward, 

 "Manual of the Mollusca," ed. 3, London, 1875. V. Hensen, "Ueb. das Auge 

 einiger Cephalophoren," Z. /. w. Z., 15, 1865. J. W. Spengel, "Die Geruchs- 

 orgaiie u. d. Nervensystem der Mollusken." Z. f. w. Z., 35. Souleyet, " Voyage 

 de la Bonitc" Zoologie, T. 2., 1852. Hilger, "Beitrage zur Kenntniss des 

 Gastropodenauges, " Morph. Jnhrb., 10, 1885. Willem, "Observations sur la 

 vision et les organes visuels de quelques Mollusques, etc.," Arch. Biol., 12, 

 1892. Leydig, "Ueb. d. Gehororgan der Gastropodeu," Arch. f. Mic. Anat., 

 7, 1871. Lacaze Duthiers, " Otocysts ou capsules auditives des Mollusques," 

 Arch. Zool. Exp. et Gen. (1), 1, 1872. Houssay, "Recherches sur 1'opercule 

 et les glandes du pied des Gasteropodes," Arch. Zool. Exp. (2), 2, 1884. Grobben, 

 " Die Pericardialdrii.se der Gastropoden," Arb. Zool. Inst. Wicn, 9, 1890. 

 Baudelot, "Recherches sur 1'appareil generateur des Mollusques Gast.," Ann. 

 Sci. Nat. (Zoologie) (4), 19, 1863. P. Pelseneer, "Introduction a V 'Etude des 

 Mollusques," Paris, 1897. A. H. Cooke, "Molluscs," Cambridge Natural 

 History, London, 1895. 



