RESPIRATION IN AMPHIBIOUS ANIMALS 105 



figures strongly support the view that insects arc tlie most 

 dominant creatures on our earth. 



The system connected with one spiracle is usually con- 

 nected with that of the neighbouring spiracles by longitudinal 

 tracheae, so that should one of these openings become 

 blocked, air can still be distributed to those parts of the body 

 which it primarily serves. In many insects, such as bees, 

 there are large swellings or bladders in the tracheae which 

 lighten the body and help it in flight. 



It has been pointed out that those animals which are 

 amphibious and are capable of living both in water and on 

 land are animals which, as a rule, have well-protected gills. 

 Certain Crustacea and gastropod molluscs and a few fishes 

 are able to breathe both in water and in the air. There is 

 a certain little fish which skips about on the rocks above 

 high water mark in the tropics, whose gills do not fill the gill 

 chamber, and this chamber contains both water and air, and 

 there are others, such as the climbing perch, which is said 

 to be able to climb palm trees, having pouches from the gill 

 chamber containing respirable air. When the water in which 

 these fishes are living is insufficiently oxygenated they will 

 come to the surface and swallow a bubble of air. Land-crabs 

 found in many tropical islands have greatly enlarged cavities 

 for their gills, and the walls of this cavity are richly supplied 

 with vessels, and these walls act as lungs when the animal 

 is out of the water. But these creatures always return to 

 their old home to breed, and when they are reproducing the 

 gills come into action again. 



Dust and Sand 



The fact that fine dust or sand in the air interferes with 

 breathing is recognized by our Government in numerous 

 Mining and Factory Acts, and fine dust and turbid water 

 are equally bad for aquatic animals. During tlie great 

 eruption of Vesuvius in April 1906 a very large number of 

 marine animals were killed in the bay of Na}:)les by the 

 fine volcanic dust which fell on the waters and slowly sank. 

 Even the sea-lilies, Antedon, were killed ])y the blocking of 

 the openings of their ambulacral j^ores, which lead from the 



