160 THE HARMONIES OF NATUEE. 



circle; and, being eminently constructed for a sea life, never 

 approach the shore. They are all, moreover, nocturnal or 

 crepuscular, voyagers agreeing that they are never to be seen 

 during a clear day, when the sun shines brightly ; but towards 

 five o'clock in the evening, when the weather is cloudy, two or 

 three species begin to make their appearance at the surface of 

 the water. Thence onwaMBs their numbers increase, but each 

 species seems to have its appropriate hour, or rather its ap- 

 propriate degrees of darkness. As the night sets in, all the 

 smaller species gradually disappear, as do the large ones a little 

 later ; and towards midnight a few stragglers only of different 

 kinds are to be met with. At sunrise not a single pteropod 

 is to be seen, either at the surface or at any depth to which the 

 eye can penetrate. They swim in a very peculiar manner. 

 Their cephalic fins are only able to support them by a constant 

 repetition of rapid movements, resembling those of the wings 

 of a butterfly. These fiffs are kept in motion continually ; and, 

 according to the direction of their stroke, the animal advances 

 horizontally, or mounts or descends, the body remaining all 

 the time either in a vertical position or slightly inclined. If, 

 while they are thus in motion, the appearance of any strange 

 body causes them alarm, their wings fold upon their bodies, or in 

 some species are entirely withdrawn into their shell, and the 

 animal sinks rapidly to a greater depth. They seize their prey 

 partly like the Pneumodermas, with tentacular arms; partly 

 like the Clios, by means of six conical appendages to the 

 head, which, when fully expanded, form a kind of star round 

 the mouth, and when strongly magnified, exhibit a truly 

 admirable structure, and indeed quite 

 unparalleled in the animal creation. For 

 each of these small cones is furnished with 

 about three thousand prehensile organs, 

 consisting of a transparent sheath (a) 

 enclosing a central body composed of a 

 stem terminated by a kind of tuft, which 

 last can be protruded at times beyond 

 the margin of the sheath. When viewed 

 laterally, it is apparent that this central 

 body consists of several filaments or 

 tubes, every one of which (6) expands at its extremity into a 



