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grow and thrive, and often lay their eggs in the form of long,, 

 yellow-and- violet twisted strings , which hang from the sides 

 of the tanks. 



One of the most beautiful among these hind-gill snails( Op istJw- 

 Iranchia) is the large Tethys leporina. It has a delicate ,. 

 white, half-transparent body, a head distinguished by a large- 

 umbrella-shaped sail, and its back is adorned with two kinds 

 of appendages in a double row : that is , small bunch-shaped 

 transparent gills, and larger, hollow, pear-shaped fringes crossed 

 by bright red-and-black lines; the function of these fringes is 

 still unknown. They easily fall off, especially shortly before 

 the animal dies. 



This snail belongs to the periodic inhabitants of the Aqua- 

 rium, like the pelagic animals. It is caught by the fishermen 

 in glasses when floating on the surface of the sea, and when 

 f^esh and lively, is a beautiful object, so transparently white,, 

 with its coloured fringes and graceful motions in swimming. It 

 swims by alternate pulsations of its sail and quick undulations 

 of its body. In confinement it rarely lives more than a few 

 weeks. 



Among the larger specimens of the hind-gills, we must men- 

 tion the Pleurobranchus, which has a shield-shaped mantle 

 and lateral protruding gills. Also several species of the Doris, 

 with a feather-like tuft of gills on the hinder part of the back. 

 From these snails come the yellow and white tangled jelly- 

 like strings, containing thousands of eggs, that are seen glued 

 to the rocks of the tanks. 



The Mollusks also furnish a contingent to the pelagic world; 

 of crystalline* animals. Besides the transparent Medusae we find 

 the wonderful Heteropods or Keel-snails , and the Fin-snails 

 (Pteropods). Especially in spring and autumn, when the sea is 

 crowded with pelagic animals of all kinds, then these small and 

 lively creatures make their appearance either periodically or 

 at incalculable periods. But they cannot endure confinement 

 long, sometimes living only a few hours, and on this account 

 they are rarely to be found in the tanks. Among the keel- 

 snails the genus Pterotraehea deserves special notice. These- 

 animals are long and transparent as crystal, with proboscises 

 bent like a knee , axe-shaped oar-fins ( which answer to 

 the foot of a snail) , and a knot of intestines glittering like 

 silver. These creatures swim actively by rapid lateral mo- 



