220 



GENERAL ZOOLOGY 



of lamellibranchs are also the source of pearl buttons, and 

 along the Mississippi River a large number of persons 

 (3592 in 1915) are engaged in collecting mussels for this 

 industry. A recent publication of the Bureau of Fisheries 

 lists forty species which are of value. 



CLASS 3. CEPHALOPODA 



Most cephalopods (Fig. 88) are far too active to carry a 

 heavy exoskeleton about, and the shell is as a rule absent or 



ABC 



FIG. 88. Cephalopoda. A, an octopus or devil-fish; B, squids swimming with 

 fish and resting on bottom; m, mantle; s, siphon; C, squid eggs. 



small and enclosed within the mantle. Aside from the rare 

 nautaloids, there are two common types of cephalopods: 



(1) squids, are elongated, have ten arms, and swim actively; 



(2) devil-fishes, or octopods, have eight arms, climb more 

 than they swim, and have short bodies. The mantle (ra) in 

 all cephalopods forms a muscular tube which encloses the 

 whole body, except the head and arms. The method of 

 swimming is peculiar. Water is drawn into the mantle 



