MOLLUSCA I PTEROPODA. 343 



a mass below the oesophagus " (Woodward), and the eyes are 

 rudimentary. 



The sexes are united in all the Pteropods, and the young 

 pass through a metamorphosis, having at first a bilobed ciliated 

 veil attached to the sides of the head. 



The Pteropoda are divided into two orders, termed Thecoso- 

 mata and Gymnosomata; the former characterised by possess- 

 ing an external shell and an indistinct head; the latter by 

 being devoid of a shell, and by having a distinct head, with 

 fins attached to the neck. 



The Pteropoda, as already said, are found swimming near 

 the surface in the open ocean, and they are found in all seas 

 from the tropics to within the arctic circle, sometimes in such 

 numbers as to discolour the water for many miles. They are 

 nocturnal in their habits, and, minute as they are, they con- 

 stitute in high latitudes one of the staple articles of diet of 

 the whale. They themselves are, in turn, probably carnivo- 

 rous, feeding upon small Crustaceans and other diminutive 

 animals. Though all the living forms are small, Geology leads 

 us to believe that there formerly existed comparatively gigantic 

 representatives of this class of the Mollusca. 



SYNOPSIS OF THE FAMILIES OF THE PTEROPODA. 

 (AFTER WOODWARD.) 



ORDER I. THECOSOMATA. 



Animal with an external shell ; head indistinct, foot and ten- 

 tacles rudimentary ; mouth situated in a cavity formed by the union 

 of the locomotive organs. Respiratory organs contained within a 

 mantle-cavity. 

 Fam. I. Hyaleidcc. 



Shell symmetrical, straight or curved, globular or needle-shaped. 

 111. Gen. Hyalea, Cleodora, Theca, Conularia. 

 Fam. 2. Limacimikz. 



Shell minute, spiral, sometimes operculate. 111. Gen. Limacina, 

 Spirialis. 



ORDER II. GYMNOSOMATA. 



Animal naked, without mantle or shell ; head distinct ; fins at- 

 tached to the sides of the neck ; gill indistinct. 

 Fam. 3. Cliidce. 



Body fusiform, foot distinct, with a central and posterior lobe ; 

 head with tentacles. 111. Gen. Clio, Pneumodermon. 



