CEPHALOPODA. 265 



by its reaction forces the animal in the other direction. 

 This apparatus forms with many, and especially with the 

 squid, the chief organ of locomotion, and in these the tip 

 of the siphon can be bent in any direction, so that the ani- 

 mal may go forwards, backwards, etc., according as it 

 wishes. 



In the mantle-cavity are one or two (Nautilus) pairs of 

 feather-like gills, and into the same chamber empty the 

 ducts of the kidneys and reproductive organs, as well as 

 the intestine, and the ink-sac connected with it. This last 

 organ secretes a dark-colored fluid, which when discharged 

 into the water makes a cloud, and thus the animal is en- 

 abled to escape unseen. From this ink the pigment sepia 

 and some kinds of India-ink are manufactured. 



Imbedded in the skin of the mantle are pigment spots or 

 chromatophores, which are interesting from the fact that 

 they can be enlarged or contracted by the nervous system. 

 When enlarged they nearly touch each other, and thus give 

 the body their general hue (red). When contracted they 

 appear as minute black points, while the general body color 

 (translucent white) then prevails. As a result we have in 

 these animals a power of color-change far more striking 

 than that of the chameleons. 



Most living Cephalopods have no external shell. Inside 

 of the back, however, is a shell the pen which may be 

 either feather-shaped and horny, or broader, thicker, and 

 calcareous. In this last condition it furnishes the " cuttle- 

 bone" so often given to cage-birds. The paper nautilus 

 has a shell which is formed only by the female ; it is secreted, 

 not by the mantle, but by one pair of the arms, and this 

 shell is really a protection for the eggs. In the pearly nau- 

 tilus, on the other hand, there is a true shell, which is 

 coiled in a spiral and is divided by partitions into a series 



