SYSTEMATIC 

 ARRANGEMENT OF MOLLUSCS, 



CLASS XIII. 

 MOLLUSCA. 



ANIMALS covered by a soft moist skin, mostly forming over the 

 back a duplicature, free at the margin (mantle). Head more or less 

 distinct, furnished with tentacles and often with two eyes. Shell 

 calcareous, mostly univalve, in some multivalve, never bivalve ; in 

 a few internal, in some absent. Organs of circulation and respi- 

 ration mostly distinct; heart always aortic. A nervous ring around 

 the oesophagus ; nerves proceeding from ganglia, various in number, 

 towards the peripheral parts of the body. 



Many aquatile, some 'terrestrial, almost all swimming or 

 creeping. 



ORDER I. Pteropoda. 



Molluscs furnished at the anterior part on both sides with a 

 natatory expansion or pinna, with head often little distinct, herma- 

 phrodite, marine. 



Wing-footed. CUVIER first distinguished this division under this 

 name in 1804. He characterised it by the absence of the foot or the 

 ventral disc ; this part, however, appears to be not so entirely want- 

 ing, though formed in a different fashion, especially laterally, and to 

 be developed into the so-named wings or fins. Hence some writers, 

 and amongst them SOULEYET, unite the Pteropoda with the Gastero- 

 poda. At all events they have a closer alliance with these than 

 with the Cephalopoda, and must, on account of the simplicity of 

 structure, be placed below the Gasteropoda. 



