336 MARINE INVERTEBRATES 
the ventral surface of the animal, giving to the creature, when 
active, the appearance of crawling upon its stomach. 
If a living gasteropod is placed in a jar of sea-water and left 
undisturbed it will soon crawl up the side of the glass and thus 
afford an excellent view of the extended under surface of its foot. 
The majority of the gasteropods are lethargic and slow in move- 
ment, but others will be found to be exceedingly lively and able 
to creep up the sides of the jar and make their escape in a sur- 
prisingly short time. Further modifications of the foot will be 
mentioned later. 
THE MANTLE CAVITY 
Now let us place the animal with the foot below and the head 
facing us. The spiral visceral hump will point back and away 
from the observer. The mantle covering the visceral portion 
is thin and translucent like an ordinary skin, but at the base 
of the visceral mass the mantle suddenly thiekens very greatly, 
and spreads over the dorsal portion of the foot or the body like a 
cloak. The space between the body and the loose-lying mantle 
is called the mantle cavity, and within this area are to be found 
several important external organs. To find these it is well to cut 
the mantle flap in a straight line, beginning just back of the head 
and ending at the point where the mantle is attached to the body. 
This will most easily be done with scissors. Having thus divided 
the mantle in front, throw back the two flaps. On the right-hand 
side (left of the animal), and attached to the inner side of the 
mantle thus exposed, will be seen the branchie, or gills. 
THE BRANCHI#, OR GILLS 
They consist of a row of flattened filaments bound together at 
the base like the leaves of a book. The blood is conveyed to 
these gills by a large vein, and is then forced through the thinly 
walled filaments, being thus brought into close contact with the 
water, to which it releases its carbon dioxide, and from which 
it receives the life-giving oxygen. In certain gasteropods there 
are two sets of gills, one placed upon each side of the body 
