PHYLUM MOLLUSCA 



3°3 



cate spiral structure, the internal cavity of which is not 

 divided into chambers. It is not secreted by the mantle 

 like the shells of other Mollusca, but by the surfaces of a 

 pair of the arms ending in expanded disc-like extremities, 

 which become applied to its outer surface ; its chief func- 

 tion is to carry the eggs. The argonaut inhabits deep water, 

 70 to 100 miles off the coast of New England, its shells 

 being in very rare cases found cast ashore on our coast 

 south of Cape Cod. 



Fig. 188. — Shell of Argonauto 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 of different hues are to be noticed passing over the 

 surface. These are due to the presence of numerous con- 

 tractile, pigment-containing cells or chromatophores , situated 

 in the deeper layers of the integument over the entire 



