AQUATIC RESPIRATION :—USE OF CILIA. 
273 
is especially desirable in tropical climates, where shallow 
lakes are often dried-np by continued drought, so that their 
inhabitants must perish, if they were not thus enabled to 
migrate. One of the most curious of these Fishes (most of 
which are inhabitants of India and China) is the Anabas or 
climbing-perch of Tranquebar; which climbs bushes and trees 
in search of its prey, a species of land-crab, by means of the 
spines on its back and gill-covers.—The gills of the Amphi¬ 
bious Reptiles, in their Tadpole state, resemble those of Fishes, 
and are connected with the mouth in the same manner. 
319. In the respiratory actions of nearly all these animals, 
a very important part is performed by the cilia (§ 45) which 
cover the surfaces of the gills. Even in such as do not 
possess any special respiratory organs (§ 311), the action of 
the cilia is very important, in causing a constant change in 
the water that is in contact with their surface. Thus in 
Zoophytes, which are for the most part fixed to one spot, the 
action of the cilia produces currents in the surrounding water. 
On the other hand, in the actively-moving Animalcules, the 
same action propels their bodies rapidly through the water ; 
though in some of them, which occasionally fix themselves 
like Polypes, the action of the cilia resembles theirs. In 
either case there is a continual change in the layer of water 
which is in immediate contact with the surface; and thus a 
constant supply of the air contained in the water is secured. 
A similar action goes-on, still more energetically, over the gill- 
tufts of the Annelida; and this action continues after the 
death of the animal, or after the tuft has been separated from 
it, producing evident currents in the water in which it is 
placed. It is by the action of the cilia alone, that the con¬ 
tinual current of water is kept-up through the respiratory 
chamber of the lower Mollusca; but this is superseded in 
Cephalopods and Fishes by the other means for sustaining 
this current which have been already noticed. Ciliary action 
may be well observed in the young Tadpole of the common 
Water-Rewt, whose gills hang freely from the neck on either 
side; the cilia are themselves so minute that they cannot be 
readily distinguished; but the motion of the water produced 
by them may be at once perceived in a tolerable microscope, 
especially when small light particles, such as those of 
powdered charcoal, are diffused through it. 
T 
