306 On Respiration. 
any vascular circulation of their juices, require the free introduc- 
tion of air into every part of their bodies. The necessity for air 
appears, also, to be more urgent than for food ; since animals 
may subsist a considerable time without nourishment, but all will 
speedily perish if deprived of air, The results of Spallanzani’s 
numerous experiments were stated in illustration of this prin- 
ciple. . 
Aquatic animals being precluded from the benebh of the direct 
action of the air in its gaseous state, or as it exists in the atmo- 
sphere, receive its influence through the medium of the sur- 
rounding water, by which it is absorbed in large quantities, and 
applied to the organs of respiration. In the lower Zoophytes, 
this influence appears to be exerted by the intervention of the 
surface of the body: so that in the Polypus, for example, while 
the interior surface digests the food, and performs the office of a 
stomach, the external surface probably acts as an organ of re- 
spiration. Many of the Vermes appear, in like mamer, to have 
an external respiration: this is the case with the leech and the 
earth-worm, in which a superficial net-work of vessels receives 
the influence of the surrounding fluid. In some genera of this 
class, it was stated, this. structure is confined to particular parts 
of the surface ; and in others, again, the respiratory organs shoot 
out from the body in the form of bushy fibrils. The different 
situations of these arborescent gills, which are frequently kept 
in incessant motion, were pointed out in several orders of mol- 
luseous and crustaceous animals. 
Dr. Roget then proceeded to examine the extensive series of 
animals in whom respiration takes place in the interior of the 
body: beginning with the Aolothuria, the ramified tubes of 
which exhibit the first trace of a structure adapted to this ob- 
ject; the asteria, and the echinus, in which the arrangement. is 
somewhat more complicated ; aud the larger Crustacea, as the 
lobster and crab, in which the filaments are collected into a num- 
ber of pyramidal organs on each side of the body, protected by 
the shell, and terminating with the more regular structure of 
gills proper to the ordinary Mollusca, and Fishes. The disposi- 
tion of these organs, with reference to the shell, and to the aper- 
tures in the mantle, by which the water is admitted to them ; 
and the provision of tubes, capable of being extended and re- 
tracted, in those shell-fish that burrow in the sand; were se- 
verally pointed out and described. The two auxiliary hearts of 
the cuttle-fish, at the origin of the branchial arteries, by which 
the blood of that animal is propelled with force to the respira- 
tory organs, while the principal heart carries on the aortic or 
greater circulation, were particularly noticed. 
The importance of the respiratory functions increases as we 
rise 
