PTEROPODA AND GASTROPODA. 543 



sinistrorsus, are sinistral. The part around which the spiral cone is 

 wound is termed the " columella : " this is sometimes simple, some- 

 times grooved, sometimes plicated ; in some shells it is solid, in 

 some hollow; in the latter case its aperture is termed the "umbilicus *' 

 ( Solarium). 



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

 by an "outer lip" {d\ and an "inner lip;" the latter offers 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 left side is formed only by the " body-whorl ; " 

 or the peristome (as the margin is called) may be broken by a notch, 

 or perforated by one or more holes, or a portion of it be produced 

 into a canal or siphon. These modifications are important on 

 account of the constancy of their relation to certain conditions of 

 the respiratory organs. Thus all 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 Sometimes there is a posterior channel 

 which is anal in its function {Strombidce) : it is represented by a 

 slit in ScissureUa, a tube in Typhis, a perforation in Fissurella, and 

 a series of holes in Haliotis. 



The Gastropods which first appear in the palaeozoic strata have 

 entire mouths ; the siphonated species are not found lower than the 

 lias, and they go on increasing in numbers in and from the tertiary 

 series to the actual sea shores. 



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 " {Jig. 208, b). This is 

 sometimes calcareous, forming a second shelly plate ; but it more 

 frequently consists of albuminous membrane only, or is horny ; 

 thus presenting the condition which the 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 Paludina, or excentric, as in Ampullaria 

 and most of the Pectinibranchs. Other opercula grow by the 

 addition of matter to part of their circumference, and these are 

 either spiral or imbricated ; in the latter the layers of growth 

 succeed each other in a linear series. No operculum presents an 

 annular form. Deshayes figures the operculum of Solarium patulum 

 as composed of many distinct and spirally disposed lamellce. Mr. 

 E. Layard has discovered a similarly complex operculum in the 

 Cataulus Austeiiianus, a little univalve of Ceylon. As the oper- 



