Cephalopoda. 145 



ites, a spiral form very similar to the nautilus, and a perfectly straight 

 conical shell, hence named Orthoceras. 



The Cuttlefish. In the cuttlefish the lateral fins extend along 

 the whole of the length of the side, and the body is less sharply 

 conical. Though essentially like the squid, they are less swift in their 

 flight. In the sharp prow of the squid one sees the build of a racing 

 shell, while the greater width of the cuttlefish suggests the increased 

 breadth of beam in an ordinary rowboat. Cuttlefishes feed on crabs, 

 clams, and fishes. The internal shell of the cuttlefish is calcareous, 

 instead of horny as in the squid, and is well known from its use in fur- 

 nishing limy material to canary birds. The ink of the cuttlefish is the 

 basis of the pigment sepia. Cuttlefishes live near shore and are used 

 extensively as food (in the Old World) as well as for the ink and 

 cuttle bone. 



CHARACTERISTICS OF THE CEPHALOPODA. 



1. There is a distinct head with highly developed eyes. 



2. The foot has developed around the head (hence the 

 name Cephalopoda), and is divided into a number of arms. 



3. Part of the foot develops into a funnel-like siphon. 



4. The shell may be external, internal, or lacking. 



5. Chromatophores are found in the skin. 



6. There is a beak and a lingual ribbon. 



CHARACTERISTICS OF THE MOLLUSKS. 



There is such a great diversity among the mollusks that 

 it is very difficult to make any concise statement of their 

 common characteristics. Some have shells, others none ; 

 some are aquatic, others terrestrial ; some live in fresh 

 water, others in the sea ; some breathe by gills, others by 

 lungs ; some are herbivorous, some carnivorous ; some are 

 free, others sessile; the limpet, though free, practically is 

 glued to its place on a rock ; the slug is so slow that we 

 have borrowed his name to make a common adjective, 



