268 MANUAL OF ZOOLOGT. 



Fam. 4. Limn&idce. Shell thin, horn-coloured, well developed. 



Aperture simple, lip sharp. 111. Gren. Limn&a, Physa, Ancylus, 



Planorbis. 

 Fam. 5. Auriculidce. Shell spiral, with a horny epidermis ; aperture 



elongated, denticulated. 111. Gen. Auricula, Conovulus. 

 DIVISION II. OPERCTJLATA. Shell with an operculum. 



Fam. 6. Cydostomidce. Shell spiral, rarely elongated, often depressed. 



Aperture nearly circular. Operculum spiral. 111. Gren. Cyclostoma, 



Cyclophorus, Pupina. 

 Fam. 7. Aciculidce. Shell elongated, cylindrical; operculum thin, 



and sub-spiral. Gfen. Acicula, Geomelania. 



CHAPTER XLIX. 

 PTEROPODA. 



CLASS III. PTEROPODA. The Pteropoda are defined by "being 

 free and pelagic, swimming by means of two wing-like appen- 

 dages (epipodia), developed from each side of the anterior 

 extremity of the body. The flexure of the intestine is neural. 

 As to the position of the Pteropoda in the Molluscan scale, 

 they must be looked upon as inferior in organisation to any of 

 the Gasteropoda, of which class they are often regarded as the 

 lowest division. They permanently represent, in fact, the tran- 

 sient, larval, stage of the sea- snails. 



Fig. 81. Pteropoda. a. Cleodora pyramidata. b. Cuvieria columnella. 

 (After Woodward.) 



The Pteropods are all of small size, and are found swim- 

 ming at the surface of the open ocean, often in enormous 

 numbers. Locomotion is effected by two wing- like fins, deve- 

 loped from the sides of the head, and composed of the greatly 

 developed ' epipodia.' The true ' foot ' is rudimentary and 

 rarely distinct, but the ' metapodium ' is sometimes provided 

 with an operculum. There is usually a symmetrical glassy 

 shell {fig. 81), either consisting of a dorsal and ventral plate 





