MANUAL OF NATURAL HISTORY 121 



III. ORDER— Polypods (Polypoda). 



Body without fins ; head not separate from the 



body ; with a great number of cylindrical, retractile, 



annulated arms, without cups ; gills four. 



1. Family. — Nautili (Nautilidse). Siphuncle slit ; 



a distinct foot-like appendage ; body enclosed 



in the last chamber of a shell ; shell involute, 



discoidal, few-whorled; septa dividing the 



chambers simple ; inner surface pearly. 



PTEROPODS. 



The Pteropods rank among those little-known 

 beings that serve to people the expanse of the great 

 oceans, occurring both in northern latitudes and in 

 intertropical regions in vast multitudes. Being noc- 

 turnal or crepuscular in their habits, they are not 

 generally observed ; at the setting of the sun, how- 

 ever, they come from various depths towards the 

 surface, and may then be readily taken in a towing- 

 net. They swim rapidly, though in an irregular 

 manner, progressing by sudden jerks, and move in 

 every direction by means of their large lateral wing- 

 like fins. The different species make their appear- 

 ance near the surface at certain fixed times, and 

 seem to dwell in the water at a depth peculiar to 

 themselves ; some come to the surface at the setting 

 of the sun, but many of the larger ones do not 

 ascend till towards midnight. Those genera that 

 have their bodies protected by a shell are most nu- 



