PHYLUM MOLLUSCA 



269 



the order Tetrabranchia. The chambered or pearly nautilus, 

 Nautilus pompilius (Fig. 194), lives on the bottom of the sea 

 near certain islands of the South Pacific. The shell is spirally 

 coiled in one plane and is composed of compartments (1) of 

 different sizes, which were occupied by the animal in successive 

 stages in its growth. The compartments are filled with gas 

 and are connected 

 by a calcareous 

 tube in which is a 

 cylindrical growth 

 of the animal called 

 the siphuncle (Fig. 

 194, 7). The gas 

 in the compart- 

 ments counterbal- 

 ances the weight of 

 the shell. 



Octopods. — The 

 Octopoda differ 

 from the decapods, 

 like Loligo, in the 

 absence of the two 

 long tentacular 

 arms (Fig. 191, 6). 

 The paper nautilus, Argonauta argo (Fig. 195), is an octopod, 

 the female of which secretes a delicate, slightly coiled shell. 

 The octopus or devil-fish, Octopus vulgaris (Fig. 196), lives in the 

 Mediterranean Sea and West Indies. It may reach a length of 

 over ten feet and a weight of seventy- five pounds. Devil- 

 fishes have been accused of serious attacks on man, but are prob- 

 ably not so bad as generally supposed. 



7. Mollusca in General 



Morphology. — The Mollusca are unsegmented, triplo- 

 blastic animals with bilateral symmetry (except in most of the 



Fig. 196. — The octopus, Octopus vulgaris. A, at 

 rest; B, in motion. /, funnel; the arrow shows direc- 

 tion of propelling current of water. (From the 

 Cambridge Natural History, after Merculiano.) 



