GENERAL ZOOLOGY 



Arms 



Fig. 13.16. The pearly nautilus, Nautilus pompilius. A, lateral view of an empty shell. B, 

 an animal from which the right half of the shell has been removed; the exposed surface of the 

 body within the large chamber is the mantle. (Reproduced from L. E. Griffin, 1900, Memoirs of 

 the National Academy of Sciences, vol. 8.) 



excurrent channel leading from the mantle cavity. The general structural 

 features of a representative scaphopod, Dentalium entale, are shown in Figure 

 13.15. 



THE CLASS CEPHALOPODA 



The cephalopods are mollusks in which the visceral mass is much elongated 

 dorsoventrally, and in which the foot is subdivided into numerous flexible, 

 muscular arms or tentacles bearing rows of adhesive cups. The class includes 

 the squids, cuttlefishes, octopi, and nautili, all of which are among the most 

 highly developed types of mollusks. The giant squids, for example, attain a 

 size unmatched by any other invertebrate animal; one recorded specimen had 

 a body 18 feet long, with 34-foot arms. The class is subdivided into the 

 Tetrabranchiata, with external shells, many arms, no ink sac, and two pairs 

 of gills; and the Dibranchiata, with internal shells or none, fewer arms, an 

 ink sac, and one pair of gills. The tetrabranchiates include a very large 

 number of extinct species, the nautiloids and ammonoids (see Fig. 20.4, 

 p. 624). The ammonoids were a conspicuous type of marine life during the 

 Mesozoic Era (ended about 60,000,000 years ago), but there are no living 

 representatives of this group. The nautiloids, once flourishing in the past, 

 have only one modern representative, the genus Nautilus (Fig. 13.16). The 

 nautilus, like the brachiopod Lingula (p. 363), is a primitive and long- 

 persisting type; it now has a very limited geographical range, being found 

 only in the tropical Pacific and Indian Oceans. 



The dibranchiates include all other living cephalopods. A familiar ex- 

 ample is the squid, Loligo pealeii (Fig. 13.17); a brief discussion of structure 

 and function in this animal will serve for cephalopods generally. The 

 tapered, cylindrical body of the squid is almost entirely covered externally 



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