HABITS OF PTEROPODS. 299 
surface during spring and winter, sparkling in the water like 
needles of glass. 
“The pteropods are the winged insects of the sea,” says M. 
Godwin-Austen, “reminding us, in their free circling move- 
ments and crepuscular habits, of the gnats and moths of the 
atmosphere ; they shun the light, and if the sun is bright, you 
may look in vain for them during the life-long day—as days 
sometimes are at sea; a passing cloud, however, suffices to bring 
some Cleodore to the surface. It is only as day declines 
that their true time begins, and thence onwards the watches of 
the night may be kept by observing the contents of the towing- 
net, as the hours of a summer day may be by the floral dial. 
The Cleodore are the earliest risers; as the sun sets, Hyalwa 
gibbosa appears, darting about as if it had not a moment to 
spare, and, indeed, its period is brief, lasting only for the Me- 
diterranean twilight. Then it is that Hyalea trispinosa and 
Cleodora subula come up; Hyalea tridentata, though it does 
not venture out till dusk, retires early, whilst some species, 
such as Cleodora pyramidata, are to be met with only during the 
midnight hours and the darkest nights. This tribe, like a 
higher one, has its few irregular spirits, who manage to keep it 
up the whole night through. All, however, are back to their 
homes below before dawn surprises them.” 
The lamellibranchiate Acephala, or headless molluscs with 
comb-like gills, are distinguished from the preceding orders of 
molluses by a more simple organisation and the peculiar forma- 
tion of their external coverings. They are all contained within 
a bivalve shell, articulated after the manner of a hinge, and to 
which some of their families are attached by one strong muscle 
(Monomyaria), others by two (Dimyaria). In this shell, which 
is secreted by two large flaps or folds of their skin or mantle, 
they generally lie concealed like a book in its binding, and bid 
defiance to many of their enemies. When danger menaces, the 
sea-snail withdraws its head and closes the entrance of its her- 
mitage with a lid, but the bivalve shuts its folding-doors when 
it wishes to avoid a disagreeable intruder. A strong elastic 
