CLASS IV GASTROPODA 439 
When the inner parts of the whorls coalesce to form a columella, the shell 
is said to be imperforate, it is perforate when they do not so coalesce, but 
leave a central tubular cavity instead. The opening of this perforation below, 
in the centre of the base, is designated the wmbilicus (Nabel). A true 
umbilicus reaches to the apex of the shell; 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 callus. 
The aperture is variable in form, being most commonly oval, rounded, 
crescentic, or half-round, but is sometimes contracted or even fesune: 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 Sutwen~ 
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 canal 
by the wall of the penultimate whorl, or by a Fic, 794. 
calcareous callus ; like the outer lip and columella, i re tin DAE ate 
it may bear spiral folds, which in some cases columella with folds. 
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 transverse 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 of 
conchiolin and calcium carbonate, running parallel with the inner surface of 
Spire 


Last, or body whorl 

