AQUATIC RESPIRATION : USE OF CILIA. 273 



is especially desirable in tropical climates, where shallow 

 lakes are often dried-up 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 Andbas 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 Pishes, 

 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 chamber of the lower Mollusca; but this is superseded in 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-Newt, 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. 



