MOLLUSCS. 209 



mantle, the water will stream from the siphon in a strong 

 jet, which 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 animal 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 dis- 

 charged into the water makes a cloud, and thus the animal 

 is enabled 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 chamseleons. 



Most living cephalopocls 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 se- 

 creted, not by the mantle, but by one pair of the arms, 

 and this shell is really a protection for the eggs. In the 

 pearly nautilus, on the other hand, there is a true shell, 



