58 CLASS III. GASTEROPODA. ORDER VI. PULMOBRANCHIATA. 
Order VI. GASTEROPODA PULMOBRANCHIATA. 
Branchiz per aérem tantum respirantes, retem vascularem in cavitate 
dorsali formantes ; pallio supra cavitatem szpissimé perforato, ori- 
ficio valva carnea instructo. 
This order includes the whole of the pulmoniferous or air-breathing 
Mollusca. ‘Their respiratory organs form a kind of vascular network, 
somewhat analogous to that in the Mammalia, lining a cavity in the back, 
whilst the mantle is generally perforated in that part which covers it, the 
orifice being furnished with a small fleshy valve, which the animal opens 
and shuts at will. The greater part of the Pulmobranchiate Gasteropoda 
are terrestrial; some, however, are aquatic ; they live in water, but are 
compelled to rise to the surface in order to breathe air; at some seasons 
of the year they are constantly floating upon the surface of stagnant 
waters ; at others again they sink, and are supposed to have the power of 
suspending their respiration for a considerable time. Cuvier established 
two separate divisions for these animals, the Pulmones terrestres, and the 
Pulmonés aquatiques ; some of these animals, however, cannot positively 
be referred to either, for though not aquatic they are not strictly terres- 
trial, as they can only subsist in wet and marshy places ; some indeed 
are amphibious. 
We divide the Pulmobranchiata into five families, as follows : 
LIMACINEA. AURICULACEA. 
CoLIMACEA. LYMNZANA. 
CyYCLOSTOMACEA. 
