THE PTEROrODS. 115 



some day, and sink, as the sun rises, into the bosom of the 

 deep, to attain that shaded gloom which suits them ; but on 

 the evening's approach they gradually again ascend to the 

 surface, and regain all their vivacity, — so that if some will 

 fancifully seek amongst them the molluscan analogues of 

 insect-butterflies in their manner of natation, they will not 

 fail to mark the correspondency between them and the moths 

 in their crepuscular and nocturnal habits. The little Hyales 

 (Fig. 19) first appear. About five in the Fi 19 



afternoon, when the garish eye of day be- 

 gins to grow dim, two or three species 

 venture upwards to the field of their occu- 

 pancy ; as evening advances several small 

 species of Cleodores rise in great number 

 with other Hyales and Atlantes, but the 

 larger kinds do not leave the abyss and 

 mingle in the crowd until night lends them 

 her friendly veil ; and some species, as the 

 Hyalaea balantium, are even so fearful of 

 the light's malign influence that they do 

 not come to the surface excepting when the night is very 

 dark. After a few hours' disport, the lesser species begin 

 to descend and disappear ; the larger follow at a little 

 later hour, so that towards midnight only a few wander- 

 ing individuals can be taken. These may possibly remain 

 even to the dawn, but the sun's rise is the signal which 

 recals them to their home. After this not a single Ptero- 

 pod is to be seen either at the surface or at any depth 

 to which the eye can penetrate. Each species has its own 

 time at which it rises up and goeth down, determined not 

 by the clock, as you will readily believe, but by the 

 degree of obscurity in the heavens, so that in an oversha- 

 dowed day they rise earlier than in a cloudless one, and sink 

 earlier also to repose. From these habits M. d'Orbigny 

 infers that each species dwells habitually in the water at a 

 depth peculiar to itself, and where it can enjoy the shade of 

 obscurity suited to its disposition, the light being of course 

 tempered in exact proportion to the thickness of the layer or 

 bed of water it has had to pass through. The species then 

 comes to the surface only at that time of day when the dusk 

 is nearly the same as that which reigns in the zone occupied 

 by the creature in the bosom of the deep. As the sun rises, 

 the Pteropod sinks lower until it has reached its maximum 

 of descent ; but when the sun has passed the meridian, the 

 snail's upward course begins, and with a gradual ascent, 

 regulated by the sun's decline, he passes up and up, until 



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