67 
tory process. But in order that the access of these 
elements, or rather that of the single one, air, may 
be given to the organs of respiration, the air must 
in some mode envelope the surface of the body: nay, 
the cutaneous surface itself appears at first as the 
sole respiratory organ; and it is only in a more ad- 
vanced state of developement of the animal body 
that peculiar structures, air and water cells, (lungs, 
air vessels, and gills,) appear; which, however, may 
be considered collectively as processes of the skin, 
ramifying sometimes internally, sometimes exter- 
nally*. In zoophytes, polypes, and gorgonie, the 
whole surface appears to be a respiratory organ; as 
in plants without leaf, or where leaf and stem are 
undistinguishable, the cacti and junci. In the me- 
dusee more distinct organs appear in the form of 
air bladders, on the upper surface of the physophora 
and rhizophysa: in the echini and asterie the ten- 
tacula appear to absorb water for respiration. These 
may be considered as rudimental gills; and they are 
noticeable as so operating in the actinie. The cavi- 
ties of the medusz appear to be rudimental lungs. 
Cavities for respiration, and external respiratory or- 
gans, appear in different species of other classes 
nearly allied. The ascidie exhibit a large respira- 
tory cavity. In most other of the mollusca ace- 
phala, particularly those which have shells, the or- 
K See Outlines of Physiology, by William P. Alison, M. D. 
Edinburgh. 
Of Respiration, sect. ix. p. 173. for an interesting synopsis of 
facts relative to this subject. 
F Q 
