156 SEA-SHORE LIFE 
through careful study, and the only point we should remember is 
that it is not quite correct to say that the squid “swims backward” 
but that it swims with the middle point of its back directed forward. 
Feathered gills arise from the body, and project into the cavity 
of the mantle-bag, so that they are bathed by the water that comes 
in through the slit-like opening between the mantle and the sides of 
the neck. In nautilus there are four, whereas in all other cephalo- 
pods there are only two gills. In common with other mollusks the 
heart pumps blood from the gills to other parts of the body. 
The kidneys, genital organs, and alimentary tract also open into 
the cavity of the mantle-bag and their products are discharged with 
the waste water through the siphon. 
The ink bag is a peculiar organ that is found in all forms ex- 
cepting nautilus. Its duct opens into the intestine near the vent, 
and when the animal becomes excited or alarmed the inky fluid is 
discharged through the siphon thus darkening the water and ena- 
bling the animal to bewilder its prey, or to itself escape from danger. 
Both india ink and sepia are made from the fluid of the ink bags of 
cuttle fishes. 
The nervous system is far better developed than in snails and 
other mollusks. The brain is large and is protected by a sheath of 
cartilage, while the eyes bear a close but only accidental resemblance 
to those of vertebrates. In nautilus, however, the eye is a mere cup- 
shaped cavity, the bottom and sides of the cup being lined with 
sensory cells constituting the retina; but in all other cephalopods 
the eye is much more complex. On the outside we see the glassy 
cornea or window of the eye,and close behind it les the iris with 
its circular or slit-like aperture of the pupil. The lens is large and 
spherical and lies back of the iris, almost filling the cup-lke cavity 
of the eye which is elsewhere filled with a transparent fluid. The 
retina lining the cavity of the eye is not turned inside out as in the 
case of vertebrates, but receives the light directly. There are two 
ear-like organs on the side of the head, which enable the animal 
not only to hear, but also to maintain its equilibrium in the water. 
The wonderful color changes of the Cephalopods have caused 
them to be described as ‘“‘ chameleons of the sea.” Immediately un- 
der the skin there are large numbers of little pigment cells with 
muscular walls. When the walls contract each pigment cell is re- 
