156 SEA-SHORE LIFE 



through careful study, and the only jooint 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 mautle-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 moUusks the 

 heart pumps blood from the gills to other jDarts of the body. 



The kidneys, genital organs, and alimentary tract also open into 

 the cavitj^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 cu]3- 

 shaped cavity, the bottom and sides of the cup being lined with 

 sensory cells constituting the retina; but in all other cephalopoils 

 the eye is much more complex. On the outside we see the glassy 

 cornea or window of the eye, and close behind it lies 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-like cavity 

 of the eye which is elsewhere filled with a transparent fluid. Tlie 

 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 ec^uilibrium in the water. 



The wonderful color changes of the Cephalojoods 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- 



