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, yon 

 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 Cleodorse 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 Cleodorse are the earliest risers ; as the sun sets, Hyalcea 

 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 Hyalcea trispinosa and 

 Cleodora subula come up ; Hyalcea 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 

 molluscs 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, aod 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 



