CHAPTER XV. 
MOLLUSCOUS PTEROPOBS. 
“ Natura non facit saltus.” Linnaeus. 
Between the Gasteropoda and Cephalopods naturalists place a small 
§ r oup of marine animals very limited, both in genus and species, the 
Principal characteristic of which is a membranous expansion situated 
°E the right and left side of their head, from which they take their 
Untile of Pteropoda, from 7row-7rrepd«, winged feet. 
Our space only permits us to notice the most remarkable of these 
nirious creatures. To omit them would he to leave a gap in our 
history of molluscous animals. 
The wings or flappers with which they are provided enable them 
*° pass rapidly through the water, in which they feel about with rapid 
""d continual movements, which can only be compared to those 
y a butterfly. Like these, the movements of the Pteropods are 
'Mcessaut, their flappers being not unlike the wings of the insect, 
hhey advance in this manner in a given direction, while the body or 
'ho shell remains in an oblique or vertical position. 
These little molluscs may be seen to ascend to the surface very 
H| iddenly ; turn themselves in a determinate space, or rather swim, 
"About appearing to change their place while sustaining themselves 
A the same height. If anything alarms them they fold up then - flappers, 
,lli d descend to such a depth in their humid world as will give them the 
Purity they seek. They thus pass their lives in the open sea far 
any other shelter, except that yielded by the gulf weed and 
°ther algae. In appearance and habits, these small and sometimes 
'Microscopic creatures resemble the fry of some other forms of mollusca. 
bey literally swarm both in Tropical and Arctic seas; sometimes so 
