SHELLS. 
9 
For convenience sake, therefore, mollnsks are 
described in the following pages chiefly by their 
shells ; but the student is earnestly advised, when- 
ever it is possible, to carefully study the organs 
and habits of the living animals. 
The parts of the shell may easily be learned 
by referring to the diagrams on Plate I. The 
univalve shell, Fig. 1, consists of a single tube, 
coiled in a spiral manner round a central axis. 
This tube grows larger as it advances, and 
usually leaves marks which indicate its stages, 
which are called lines of growth. The tube 
varies much in form in different species, some- 
times being flattened, sometimes angled, and 
again, nearly cylindrical. The opening of the 
tube, which often may be closed by the opercu- 
lum , is called the aperture , and this also varies 
greatly in form, being sometimes nearly round, 
and in other species prolonged into a tube or 
trough, called the canal . Those mollusks whose 
shells have this canal, (m, Fig. 1 ), are mostly 
carnivorous, while most round-mouthed shells 
belong to vegetable feeders. Each complete 
turn of the spiral tube is called a whorl , the last 
and largest being the body whorl , and the others 
forming the spire. The point of beginning is 
called the apex , and the spiral line separating the 
whorls is the suture . The axis round which the 
tube revolves is called the columella ; sometimes 
it is hollow for a little way, then this indentation 
is known as the umbilicus . The aperture, some- 
times called the mouth, is bounded on one side 
by the outer lip ; the inner lip is commonly 
grown to the body whorl or the columella. Spi- 
