stream-lines of its shape. Decreased frictional re- 

 sistance also results in trailing the arms behind. 

 Here, then, we have a sort of animated syringe : in 

 short, the squid has solved the locomotor problem 

 not by swimming as a fish or becoming a paddler, 

 but by making of itself virtually a hydraulic 

 apparatus. 



Another fact should be touched upon here; al- 

 though I shall have occasion later to refer to it 

 more fully. The squid, unlike certain fishes, is not 

 in hydrostatic equilibrium with its surrounding 

 medium. It can, and usually does maintain itself 

 at any level while resting, by a slight undulatory 

 movement of its broad tail fin; but if such move- 

 ments should cease the animal would sink at once 

 to the bottom. In this it is like large crustaceans 

 and other creatures generally which are without an 

 air (or gas) bladder. Lithocysts, or balancing 

 organs, are present, however, and with the aid of 

 these it holds its position relative to a horizontal 

 plane. 



The striking color changes which the squid mani- 

 fests under excitement and even at times when 

 there are no outward provocative causes, are owing 

 to the numerous pigment cells immediately under 



[337] 



