198 INVERTEBRATE ZOOLOGY 



the foot. The arms are provided with heavily rimmed sucking 

 discs arranged in rows. In some forms the effectiveness of these 

 grasping organs is increased by the presence of a strong clawhke 

 hook within the cavity of each disc. The siphon is a highly 

 muscular tube through which water is drawn into the mantle 

 cavity for respiration and the body wastes are forcibly expelled. 

 The jet of water issuing from the siphon drives the squid or 

 devilfish backward with remarkable speed and constitutes one 

 of the chief means of locomotion. The devilfish can move 

 forward only by using the arms in crawling, but movement of the 

 fins on the sides of the body of a squid enable it to swim either 

 forward or backward. A fleshy mantle encloses a cavity which 

 contains the gills and at the same time forms a protective covering 

 for most of the other viscera. At will many cephalopods may 

 eject a cloud of ink from an ink sac. The pigment sepia is 

 derived from the cuttlefish, for the ink sac which is characteristic 

 of the Decapoda stores quantities of this pigment. When 

 the animal is disturbed, clouds of this ink are shot from the 

 mantle cavity through the siphon and serve as an effective cover 

 under which the cuttlefish moves off to safety. 



Some cephalopods have unusual powers of changing color. 

 Variously colored pigment cells in the skin may expand or 

 contract, producing wonderful combinations. At one moment 

 the entire body may be of a uniform color which may be changed 

 instantaneously by a play of flashes of varied hues over the body. 

 Some species of squid can produce almost every tint of the 

 rainbow. 



A pair of eyes, on the sides of the head beneath the bases of 

 the tentacles, very closely resemble the eyes of vertebrates. 

 In finer structure and origin, the cephalopod and vertebrate 

 eyes are widely different, for despite their superficial resemblances 

 the two seem to have had entirely independent origin. The 

 cornea of the cephalopod eye is perforated and thereby allows 

 water to enter the anterior chamber, while it is not perforated in 

 the vertebrate eye. The arrangement of the sensory cells, 

 the retina, is just the reverse in the two types of eye. The 

 vertebrate retina is said to be inverted, for light passing through 

 the eye strikes the sensory cells of the retina on the same end 

 that bears attachment to the nerve endings. In contrast with 

 this, the direct retina of the cephalopod eye is so organized 

 that the light passing through the eye falls upon the free ends of 



