XI 



PHYLUM MOLLUSCA 



303 



FIG. 176. Shell of Argonauta argo. 



In addition to the shell there is in all the Cephalopoda 

 an internal skeleton of cartilage, supporting and protecting 

 the nerve-centres and other parts. 



The Cuttle-fishes and other Dibranchiata when alive will be 

 observed to undergo frequent changes of colour, and blushes 



771 



FIG. 177. Chromatophore of Sepia, magnified, nuc. nuclei in wall of sac ; pigin. 

 pigment ; rad. mus. radiating strands of muscle. (After Vogt and Jung.) 



