CLASS IV 



GASTROPODA 



139 



Apex 



When the inner parts of the whorls coalesce to form a columella, the shell 

 is said to be imperf orate ; it is perfwate when they do not so coalesce, but 

 leave a central tubular cavity instead. The opening of this perforation brlnw. 

 in the centre of the base, is designated the umbilicus (Nabel). A true 

 umbilicus reaches to the apex of the shell j when confined to the last whorl 

 only, it is called a false umbilicus. An umbilical fissure is sometimes pro- 

 duced through a partial covering of the umbilicus by the reflected inner lip, 

 or by a shelly growth termed the callu*. 



The aperture is variable in form, being most commonly oval, roundrd. 

 crescentic, or half-round, but is sometimes contracted or even fissure-like. Its 

 margin is called the peristome, the outer part of which forms the outer lip. and 

 the part next the columella the inner lip. Some 

 shells have a continuous, uninterrupted peristome, 

 but as a rule the inner and outer lips are discon- 

 nected. The aperture is said to be entire when 

 rounded anteriorly (inferiorly), as in the Holo- 

 stomata ; it is channelled when a basal notch or 

 canal, caused by an inbending of the margin next 

 the base of the columella, is developed. This 

 anterior canal serves for the lodgment of the 

 siphon, as the tube is called which conducts water Sutu 

 to the gills ; it may be either straight or re- 

 curved, and in the Siphonostomata it is greatly 

 produced, sometimes even exceeding the aperture 

 in length. The outer lip may be entire or incised, 

 thin and sharp or thickened, curved outward (re- 

 flected) or inward (inflected), even or crenulated, 

 or it may be produced into alar or finger- like 

 processes. It is sometimes channelled by a canal 

 at the posterior border, in which the anal or ex- 

 current canal is placed. The upper or posterior 

 portion of the inner margin is commonly desig- 

 nated, especially in the Siphonostomata, as the true 

 inner lip, in contradistinction from the lower or 

 columellar portion. The inner lip is formed either 

 by the wall of the penultimate whorl, or by a 



calcareous callus ; like the outer lip and columella, gawed thiough longitudinallj . shljwilli! 

 it may bear spiral folds, which in some cases columella with M'U. 

 extend backward as far as the apex (Fig. 794). 



The external ornamentation usually consists of impressed lines or grooves, 

 or of elevated ridges, ribs, folds, nodes, spines, and the like. The markings 

 are called spiral when they run parallel with the suture, and frtmtm'se or 

 longitudinal when they meet the suture at right angles or obliquely. Many 

 Gastropods are brilliantly coloured, others have a velvety or hairy epidermis. 

 The fossilisation process is usually destructive not only of the epidermis, but 

 of the coloration as well. 



The essential constituent of univalve shells is aragonite, which usually 

 forms a homogeneous, porcelain-like layer. Many families have in addition 

 to this an inner nacreous layer, which is made up of alternating strata >f 

 conchiolin and calcium carbonate, running parallel with the inner surface of 



Canal---- 



Fio. 7:>4. 

 [il, I. inn. Vii'\vfif sh'-ll 



