286 Introduction to Animal Morphology. 



around the axis of the spire ; its lines of growth thus 

 mark its surface with a spiral which may be multi- or 

 pauci-spiral* according to the number of whorls. 

 When the generating figure is a circle, then the oper- 

 culum grows by regular additions all round, and is 

 concentric. Other forms are called imbricate, unguicu- 

 latc, articulate (when marked with processes). The 

 operculum has always more conchiolin in its composi- 

 tion than the shell whose mouth it closes. In Pul- 

 monates the side of the foot (not an opercular lobe) 

 secretes a temporary covering, the cpiphragm or 

 clausilium, which is shed after hybernation is over ; 

 this is solid, consisting of calcium phosphate 86 per 

 cent., and organic matter 6.5 per cent., but is not a 

 true operculum. Litiopa, Rissoa, and Cerithidea are 

 fixed by a byssus formed as a dried mucous secretion, 

 but not from a special gland. 



The body of a Gasteropod is divided into a pro- 

 soma and a metasoma. The former includes the head 

 and neck, and to its under surface the foot is often 

 united in front of the mantle edge. The mantle is 

 only part of the differentiated body wall of the meta- 

 soma. The head in Opisthobranchs often bears a 

 velum in the adult, and this may be lobed at its free 

 edge (in Tethys forming a funnel) : in Prosobranchs it 

 bears tentacles, and often eyes, on stalks (pmmato- 

 p/icrcs), or sessile. The mouth opens in many Pro- 

 sobranchs at the end of a proboscis, which is a re- 

 tractile part of the head, not an extensile portion of 



* The number of whorls depends on the curve of the opening, and the 

 consequent rate of rotation necessary to keep the operculum geometrically 

 constant in shape. The spiral is usually dexiotrope. In Atlantidae there 

 is a laotrope shell, and also a laotrope operculum. The spiral is usually 

 only seen on the outer surface (pagina externa). 



