344 MARINE INVERTEBRATES 
first whorls of embryonic growth are usually referred to as the 
nepionic whorls. The interstices between the successive whorls 
are called sutures, and these lines of juncture may be shallow or 
deep according to the convexity of the whorls. The seulpturing 
of the whorls is often of great complexity, and exhibits nodules, 
varices, ribs, revolving strie, spinous processes, and lines of growth. It 
frequently happens that there is a complete absence of any sculp- 
turing whatever, the whole surface of the shell being perfectly 
smooth, save for the faint lines of growth to be detected only 
under a magnifying-glass. The outer edge of the mouth or 
aperture of the shell is called the outer lip. This may be more or 
less thickened, expanded, or simple. The inner lip is called the 
columella, and may be greatly extended anteriorly. When the 
mouth of the shell is round, and there is no notch or indentation 
either above or below, it is said to be continuous. 
The anterior canal is for the accommodation of the siphon of 
the animal, which, as noted above, may be very long or almost 
entirely absent; therefore the character of the shell generally 
indicates whether or not the animal itself possesses a long siphon. 
There is in some forms a notch at the upper extremity of the 
mouth, which is for the accommodation of the anal discharges. 
When present it is called the posterior canal. 
It will be noted that all the specimens figured as examples, 
save one, are spiral, the coils revolving about an imaginary axis. 
When the whorls are not too closely coiled there is left open to 
the view a hollow space throughout the entire length of the shell, 
which is called the wmbilicus. When the coils of the shell are 
very compact there is no umbilicus to be seen. The umbilical 
opening may be very large, giving a clear view of the inner sur- 
faces of all the whorls right through to the apex. The usual 
gasteropod shell is in reality a hollow tube wound about an 
imaginary axis. Since the tube increases in size as it progresses, 
the base of the shell is necessarily wider and larger than the apex. 
Thus all gasteropod shells may be reduced in shape to the single 
type of a cone or top, modified by the excessive length or short- 
ness of the spire, or otherwise altered by special peculiarities. In 
Polynices duplicata (page 368), it will be observed that while the 
