ON MOLLUSCA. 205 
invest the internal face of this cavity. This disposition takes 
place in the different species of mollusca which live habitually 
in the air; but these mollusca may really appertain to diffe- 
rent families. The greatest number, however, belong to those 
of the limacine and limnexe; but there are some also in the 
family of the cyclostomata, and that of the cyclobranches, and 
even, according to M. de Blainville, in that of the cervico- 
branches; for it would seem that the true patella respire by 
a lung, and not by gills. 
The form of the organs of respiration varies still more. In 
fact, in the aérial mollusca it is always a cavity more or less 
inclining towards oval; but in the aquatic, the organ may be 
simple or multiple: it may be formed of species of ramified 
arbuscula, as in the tritonia; of little tufts, as in scyllea; of 
lamine, or shred-like strips, asin the caroline and eolides ; of 
triangular pyramids, very large, one on each side, as in sepia, 
&c.; or very small and numerous, as in phyllidia and osca- 
brio, the latter of which, nevertheless, so much differ from the 
former ; of sorts of combs more or less elongated, as in most 
spirivalve cephala, in the dismembered genera of the patelle ; 
and of large semi-circular plates, as in most of the acephala ; 
or, finally, of a reticulated tissue, as in the ascidiew; or of a 
long fringe, as in the biphore. 
The situation of the respiratory organ presents, perhaps, 
still greater variations than its form. Thus, ina great number 
of species, it is exterior, and can then be constituted only by 
gills. ‘This is observed in all those genera which, from this 
circumstance, have been named by M. Dumeril dermo- 
branches, and by M. Cuvier nudibranches. It even occurs in 
the inferobranches. This disposition would be still more evi- 
dent in the pteropods, if it were certain that the gills formed a 
net-work at the surface of the natatory appendages. In all the 
others, the respiratory organ is more or less interior, but more 
in the pulmonary than in the other genera, where it may be 
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