512 



ZOOLOGY, 



substance.* The tetrabranchiate cephalopoda have no such 

 organ. 



602. We have said above that the mollusca have no 

 solid articulated framework comparable to the skeleton of the 

 vertebrata. Nevertheless, in the cephalopoda we still find 

 vestiges of something analogous ; for there is in the head a 

 cartilage which not only protects the brain, but also spreads 



Fig. 536. Nautilus ; the Pearly Nautilus.f 



out in different directions, furnishing points of insertion to 

 the different muscles of the animal. It is also to be ob- 

 served that the abdomen of these animals is in general sup- 

 ported by a sort of internal shell, which in the loligo is horny, 

 but in the sepia is of a calcareous nature, and is called the 

 bone of the sepia. 



* It would seem, however, that the colouring matter employed in the 

 fabrication of China ink is nothing but carbon, minutely divided. 



t In this figure the shell is represented open : t t the tentacula ; <?, the 

 funnel j p, the foot ; m, a portion of the mantle ; o, the eye ; , the syphon. 



