210 THE SCIENCE OF LIFE. 



the eyes of Vertebrates, except that there is no aqueous 

 humor, and the lens, which is double, is bathed freely 

 by the water in which the animals swim. The nervous 

 system is more concentrated than in other Invertebrates ; 

 the cerebral ganglia are even inclosed in a cartilaginous 

 cranium. All the five senses are present. The integu- 

 ment contains pigment sacs, or chromatophores, which 

 sometimes tint the animal with variegated colors. It is 

 probable that they in some way subserve the sense of 

 sight, as the animal swims with its head backward. 

 Some Cephalopods have an internal shell, secreted by a 

 fold of the mantle, called the " cuttle-bone" or " pen." 



Two or four pairs of plume-like gills are situated in 

 the pallial cavity, into which the sea-water is admitted 

 at one end and expelled through the funnel at the other 

 by muscular contraction. These contractions serve both 

 for respiration and locomotion, the pressure of the ex- 

 pelled water driving the animal in an opposite direction. 

 The systemic heart pumps the blood all over the body, 

 which then returns through capillaries into veins which 

 conduct the blood back to the gills, where it is purified, 

 and whence it is propelled to the heart by contractile 

 sacs, called branchial hearts, placed at the base of each 

 gill. In addition to other viscera, a large secreting sac, 

 the ink-bag, is often present, containing a dark fluid 

 which the animal ejects at will through a duct opening 

 at the base of the funnel. The sexes are always distinct. 

 During reproduction the spermatozoa are temporarily 

 transferred to one of the arms, which becomes curiously 

 altered and unfit for locomotion ; in this condition it i 

 said to be hectocotylized. 



