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, 

 swarming 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, Thecosomata and Gymnosomata, the first including 

 the Hyaleidce, and Limacinida ; the second contains .one family, the 

 CliidcB. Of these three principal families of Pteropods, the first, 

 the Cliidcz, contains Cymodocea, Pelagia, Pneumodermon, and Clio ; 

 the second, Limacinidce, contains Macgillivrayia, Cheletropis, Spirialis, 

 and Limacina ; the third, Hyaleidce, contains Tiedemannia, Cymbulia, 



Figs. 313 and 314. Hyalea giboosa (Rang.). Figs. 315 and 316. Hyalea longirostris (Lesueur). 



Eurybia, Theca, Cleodora, and Hyalea. The Hyaleidse have small 

 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 Hyalea 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. 



