MOLLUSCA PROPER, 199 
different species, supported upon a kind of strap which can be 
made to work backwards and forwards over a cartilaginous 
cushion, thus acting like a chain-saw. In addition to the 
odontophore there are sometimes horny 
jaws as well. The mouth leads by a gullet 
into a distinct stomach, which sometimes 
is provided with calcareous plates for grind- 
ing down the food. The intestine is long, 
and always terminates in a distinct anal 
aperture. Distinct salivary glands are 
usually present, and the liver is well de- 
veloped. 
A distinct heart is almost always present, 
and consists of two chambers, an auricle 
and aventricle. Respiration is very vari- 
epee : Fig. 103.—Portion of the 
ously effected—one great division being Wesualepbuaee tte 
constructed to breathe air by means of common whelk, magni- 
water, whilst in another section the respir- fied (after Woodward). 
ation is aerial. In the former of these—often spoken of as the 
“branchiate” Gasteropods—respiration may be carried on in 
three ways. Firstly, there may be no special breathing organ, 
the blood being simply exposed to the action of the water, 
as it circulates through the thin walls of the mantle-cavity. 
Secondly, the breathing organs may be in the form of outward 
processes of the skin, exposed to view on the back or sides of 
the animal (fig. 105). Thirdly, the breathing organs are in the 
form of plume-like gills, contained in a more or less complete 
chamber, formed by a folding of the mantle (fig. 94, ¢). In many 
members of this group the water attains access to the gill-chamber 
by means of a tubular prolongation or folding of the mantle, form- 
ing a siphon (fig. 102, s), and often the effete water is expelled 
by another tube which is similarly constructed. In the second 
great section—often called the “pulmonate” Gasteropods—re- 
spiration is effected by a pulmonary chamber or lung, formed 
by a folding of a mantle, and having air admitted to it by a 
distinct aperture. 
The sexes in the Gasteropoda are mostly distinct, but they 
are sometimes united in the same individual. The young, 
when first hatched, are always provided with an embryonic 
shell, which may be entirely lost in the adult, or may simply 
become concealed by a fold of the mantle. In the water-_ 
P 
