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CHAPTEK XY. 



MOLLUSCOUS PTEROPODS. 

 " Natura non facit saltus." 



THE position of the Pteropoda is somewhat unsatisfactory. Their 

 organization in some respects places them below the level of the 

 Gasteropods; hut yet the general feeling amongst naturalists has 

 been to assign them a place between the Gasteropods and the most 

 highly organized of the molluscs, the Cephalopods. The number of 

 genera and species is less than that of the other great classes of 

 molluscs. 



There are three principal Families of Pteropods. First, the Cliidse, 

 containing Cymodocea, Pelagia, Pneumodermon, and Clio. Second, 

 Limacinidae, containing Macgillivrayia, Cheletropis, Spirialis, and 

 Limacina. Third, Hyaleidse, containing Tiedemannia, Cymbulia, 

 Eurybia, Theca, Cleodora, and Hyalea. 



The principal characteristic of the Pteropoda is a membranous 

 expansion situated on the right and left side of their head, from 

 which they take their name of Pteropoda, from Trovs-Tnepos, winged 

 feet. 



The wings or flappers with which they are provided enable them 

 to pass rapidly through the water, reminding us strongly of the 

 movements of a butterfly, or other winged insect, and like them, their 

 motions are long continued. They advance in this manner in a 

 given direction, while the body or the shell remains in an oblique or 

 vertical position. 



These little molluscs may be seen to ascend to the surface very 

 suddenly, turn themselves in a determinate space, or rather swim, 

 without appearing to change their place while sustaining themselves 

 at the same height. If anything alarms them they fold up their 



