CEPHALOPODA. 221 



quisite delicacy. This beautiful little Pteropod uses its shell as a boat, and by means 

 of its wing-like fins rows itself about on the surface of the water. 



The Hyalgea (Fig. 236) is an inhabitant of warmer regions. Its shell is somewhat 

 like that of a bivalve without a hinge, the hinder part being consolidated and armed 

 with three spines. At the sides are narrow fissures, through which membranous ex- 

 pansions are protnided, resembling those of the Clio. 



FIG. 236. GLASS SHELLS: a. Hyalcea triden'.ata ; b, C leodora pyramidata . 



The Cleodora (Fig. 236) is another exceedingly delicate and, beautiful example. 

 The shell of this little creature is of glass-like transparency, very fragile, and somewhat 

 m the form of a triangular pyramid. The animal in the dark is vividly luminous, and 

 presents a very striking appearance as it shines through its pellucid lantern. 



"The Pteropods," says Mr. Godwin Austen, "are the winged insects of the sea 

 reminding us, in their fre'e circular movements 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 throughout the livelong day. It is only as day declines that their true 

 lime begins ; and thence onwards, the watches of the night may be kept by observing 

 ilie contents of the towing-net, as the hours of a summer's day may be by the floral dial. 

 The Cleodorce are the earliest risers. As the sun sets, Hyalca gildwa appears, darting 

 r.bout as if it had not a moment to spare, and indeed its period is brief, lasting only for 

 tiie Mediterranean twilight. Then it is that Hyalua trispinosa and Cleodora subnla 

 come up. Some species retire early, while others are to be met with only during the 

 midnight hour-; and in the darkest nights. There are, however, a few of irregular 

 babits, who manage to keep it up the whole night through. All, however, are back to 

 their homes before day surprises them." 



CHAPTER XXI. 



SEVENTH CLASS OF MOLLUSKS. 



CEPHALOPODA.* 



FROM the strange combination of characters employed to 

 designate the last and highest order of the Mollusca, the 

 student will, no doubt, be prepared to anticipate something re- 



* Are$a\77, cephalc, tJic Jicad : TTOI'S, TroSos, pous, podos, afoot: head-footed. 



