^96 LECTURE xxir. 



plicated : in some shells it is solid ; in some hollow ; in the latter case 

 its aperture is termed the ' umbilicus.' 



The aperture which forms the base of the spiral univalve is bounded 

 by an ' outer lip ' and an ' inner lip,' which oflPers a smooth convex 

 surface, over which the foot of the Gastropod glides to reach the 

 ground. In many univalves the aperture of the shell is entire ; in 

 others, the margin is broken by a notch, or perforated by one or more 

 holes, or a portion of it is produced into a canal or siphon. These 

 modifications are important on account of the constancy of their re- 

 lation to certain conditions of the respiratory organs. Thus the 

 Pectinibranchiate Gastropods, in which the water is conducted to the 

 shell by a muscular tube or siphon, have the margin of the aperture 

 of the shell either notched or produced into a canal. 



In some of the Gastropods the shell consists of one piece, when it 

 is termed an * inopercular univalve ; ' but the aperture of the shell 

 is in the majority of the species closed by a plate, attached to the 

 back of the foot, and called the ' operculum.' It is sometimes cal- 

 careous, forming a second shelly plate ; but it more frequently consists 

 of albuminous membrane only, or is horny ; thus presenting the con- 

 dition which the univalve shell itself manifests in certain genera, as 

 Limax and Aplysia. Some opercula increase by the addition of 

 matter to their entire circumference, and these are either concentric, 

 as in Paliidina, or excentric, as in Ampullaria and most of the Pecti- 

 nibranchiate Mollusks. Other opercula grow by the addition of matter 

 to part of their circumference ; and these are either spiral or imbri- 

 cated ; in the latter the layers of growth succeed each other in a 

 linear series. No operculum presents an annular form. As the 

 operculum sometimes varies in structure in species of the same genus, 

 as it is present in some volutes, cones, mitres, and olives, and absent 

 in other species of those genera, and as some genera in a natural 

 family, as Harpce and Dolium among the Buccinoids, are without an 

 operculum, whilst the other genera of the same family possess that 

 appendage, it obviously affords characters of very secondary import- 

 ance. In Lithedaphus ( Calyptrced) equestris the whole base of the 

 foot secretes a calcareous plate which is cemented to the rock, and 

 the shell appears to consist of two valves. In the Chiton the shell is 

 divided into many pieces arranged like scales upon the back. 



Most univalve shells are composed of three strata, which differ in 

 the arrangement of the calcareous particles. Hunter* discovered 

 that the molluscous inhabitant of a shell had the power of absorbing 

 part of its dwelling. This property, which is now generally recog- 



* Philos. Trans, 1785, p. 343. 



