468 THE OCEAN WORLD. 
has deciduous vela like the sea-snails, before the proper locomotive 
organs are developed.” _~ 
The Pteropods seem to be eminently sociable and gregarious, 
swatming together in great numbers; they present some’ analogical 
resemblances to the Cephalopoda; but permanently they represent 
the larval stage of the sea-snails. De Blainville divides the group 
into two sections, Zhecosomata and Gymnosomata, the first including 
the Hyaleide and Limacinide; the second contains one family, the 
Chide. Of these three principal families of Pteropods, the first, 
the Ciid@, contains Cymodocea, Pelagia, Pneumodermon, and Clio ; 
the second, Zimacinide, contains Macgillivrayia, Cheletropis, Spirialis, 
and Limacina ; the third, Wyaleide, contains Tiedemannia, Cymbulia, 
Figs. 313 and 314.—Hyalea gibbosa (Rang.). Figs. 3x5 and 316.—Hyalea longirostris (Lesueur). 
Eurybia, Theca, Cleodora, and Hyalea. The Hyaleide have smail 
horny shells, very thin and transparent, globular, or elongated, open 
anteriorly, cloven on the sides, and truncate .at the posterior ex- 
tremity. Their globular body is formed of two parts, the one 
including the head, bearing two very strong tentacles, and two large 
fins or flappers in the form of wings, springing from each side of the 
mouth. 
These molluscs are small, and generally of a yellowish-blue or 
violet colour. They are inhabitants of the deep sea, and rarely 
seen out of what sailors call “blue water.” They plough the waves 
with great rapidity by the aid of their powerful fins. Certain winds 
throw them sometimes in great numbers on the shores of the 
Mediterranean. ‘These little creatures, so inoffensive, and living 
together in vast numbers, seem to be an easy and ready-prepared 
prey, which the great marine animals may swallow by-thousands. 
| Twenty species of AZyalea are described as actually living in the 
Atlantic and Australian seas. Of these Hyalea gibbosa (Figs. 313, 
314) and Hyalea longirostris (Figs. 315, 316) are here represented. 
