AMPHIBIANS 253 



prefer the upper and warmer strata of the water, or are fond of enjoying 

 the enlivening rays of the sun. (Progs, newts. Why does the number 

 of amphibious species increase as we approach the equator? and why 

 are none to be found in the frigid zone ?) 



(b) The skin in amphibia is perfectly naked, and is only slightly 

 protected against excessive evaporation by the slimy mucus secreted 

 from numerous glands opening on its surface. Hence, dry heat soon 

 proves injurious to these animals. If a frog or newt, for instance, is 

 confined in a dry space,- it will dry up and die in a short time, often even 

 after half a day ; if, on the other hand, such an animal, nearly parched 

 up, is wrapped in a damp cloth, leaving only the head exposed, it will 

 rapidly increase in weight, showing that the skin also takes up water. 

 No amphibian takes in water by drinking. This explains why many 

 amphibians lead an aquatic existence, or at most keep within an easy 

 distance from the water (e.g., the edible frog); while those which are 

 terrestrial are exclusively nocturnal in their habits (on account of the 

 dew, moist night air), and only in exceptional cases, in wet weather, 

 leave their damp hiding-places during the day (e.g., toad and spotted 

 salamander. (Why cannot amphibians live in deserts? and why do 

 many which inhabit the torrid zone pass the summer in sleep ?) 



(c) In addition to the above-mentioned mucus, the skin of amphibians 

 secretes regularly from special glands an acid juice of unpleasant taste, 

 or poisonous, which more (spotted salamander) or less (tree-frog) effec- 

 tually protects the animals from the pursuit of enemies. 



(d) The colour of the skin is a further and very important means of 

 protection (protective or warning coloration — examples). Many amphi- 

 bians are even capable of changing their colour in such a manner as to 

 make it correspond with their surroundings. How is this effected ? In 

 the skin are " cells " (i.e., minute protoplasmic elements, of which the 

 bodies of all animals and plants are composed) filled with brown, black, 

 yellow, red, and other coloured pigments. These coloured particles are 

 •sometimes closely packed together in the centre of the cells ; at other 

 times they are spread apart over the whole cell, and the colour of the 

 animal varies according as the one or the other of these arrangements 

 obtains in the cell. Thus, if the brown pigment is spread out over the 

 •cell (as in the tree-frog when it sits on the brown bark of a tree or a 

 dark-coloured leaf), the whole skin assumes a brown colour, etc. In 

 considering the question as to whether the animal is able to change its 

 colour voluntarily, we may be guided by what happens in ourselves. 

 Thus, if we see anything which rouses our sense of shame or our anger, 

 our face blushes with shame or rage, or, on the other hand, it may grow 

 pale from the sight of anything which instils fear or alarm. In both 



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