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CHAPTER XVII. 
PTEROPODS, 
‘* Natura non facit saltus,”—LINN#US. 
THE position of the Pteropoda is somewhat unsatisfactory. Their 
organisation in some respects places them below the level of the 
Gasteropods ; yet the general feeling amongst naturalists has been 
to assign them a place between the Gasteropods and the most highly 
organised of the molluscs, the Cephalopods. The number of genera 
and species is very much less than that of the other great classes 
of molluscs. 
The principal characteristic of the Pteropoda is a membranous 
expansion. situated on the right and left side of their head, from 
which they take their name of Pteropoda, from wrepiv, wing, and mois, 
gen. rodds, foot, winged feet. 
The wings ‘or flappers. with which they are provided enable them 
to pass rapidly through the water, reminding us strongly of the 
movements of a butterfly, or other winged insect, and like them, 
their motions are long continued. They advance in this manner in 
a given direction, while the body or the shell remains in an oblique 
or vertical position. 
These little molluscs may be seen to ascend to the surface,very 
suddenly, turn themselves in a determinate space, or rather swim, 
without appearing to change their place, while sustaining themselves 
at the same height. If anything alarms them -they fold up their 
flappers, and descend to such a depth in. their watery world as will 
give them the security they seek. They thus pass their lives in the 
open sea far from any other-shelter, except that yielded by the gulf 
weed and other alge. In appearance and habits, these small and 
sometimes microscopic creatures resemble the fry of some other 
forms of mollusca. They literally swarm both in tropical and 
arctic seas ; and are sometimes so numerous as to colour the ocean 
for leagues. They are the. principal food of. whales and sea-birds in 
high latitudes, rarely..approaching- the coast. Only one or two 
