6 A TEXTBOOK OF PHYSIOLOGY 



stylets of the turbellarian worms, the stings of the nettle cells of hydra. 

 'jCuili l\v cells are the stings of bees and wasps, the poison hairs in the 

 larvae of some moths, the poison glands of some spiders, the stinging 

 spine of the weaver-fish and of the whip-ray, the weak poison of the 

 pectoral fin of the cat-fish, the spine of the porcupine, the claws of 

 the higher animals, and the nail of man. 



The integument may provide lubricating material either all over 

 the body surface or only in special areas. The lubricant material 

 may at the same time serve additional functions for example, as a 

 preservative from water, as a cleanser, or as a food-gatherer, and so 

 forth. The lubricating material may be slimy (mucus), oily, or watery, 

 in nature. 



The mucus provided by certain of the clams serves the purpose of 

 lubricating, and also of removing dust and aiding the collection of 

 food. The slime produced by the earthworm serves the double purpose 

 of lubricating the animal and of preparing its dwelling-place. The 

 mucus of the salivary glands of the mammal and of the mucous cells 

 lining the alimentary tract is both lubricant and protective. 



The second form of lubricating material, that of an oily nature, 

 is found in the higher animals (birds and mammals). It is a protec- 

 tion against both the drying of the skin and the wetting of the feathers, 

 hair, or skin. Such a material is formed in the oil glands, by w r hich 

 birds oil their feathers, and in the sebaceous glands, by which the 

 hairs and skin of mammals are kept greased. Sometimes these oils 

 possess a distinctive scent, either repulsive or agreeable; such is the 

 case, for example, in the musk rat, musk ox, and the skunk. 



The watery (serous) form of secretion is comparatively rare. 

 Possibly the lachrymal glands moistening the eyes of mammals, and 

 the sweat glands moistening the skin, may be grouped here, as well 

 as the secretion which lubricates surfaces of joints, the synovial fluid. 

 By the evaporation of sweat the body is cooled. 



Among the lower animals an extremely adhesive fluid is sometimes 

 produced, which enables them with the aid of a sucker or pad to stick 

 to surfaces e.g., that of the head of the leech and the legs of beetles. 

 In other cases the adhesive fluid hardens into a thread; thus the 

 cocoons are formed by the spinning glands of the larvae of moths 

 (silkworm). 



Many of the lower animals also possess odour-producing glands 

 e.g., the skunk; a well known example also is the so-called stink-pot 

 turtle. Other reptiles for example, the American toad (Bufo) 

 produce an extremely offensive fluid. The secretion of Bufo is partly 

 mucous, partly serous, and it is said to be poisonous. But it is among 

 the invertebrates that this power of producing offensive and attractive 

 odours, as judged by man, has been reduced to a fine art. Various 

 butterflies produce distinctly pleasant odours. Such odoriferous 

 glands are situated in various parts of the body or wings. 



But besides rendering themselves efficient in this direction, the 

 multicellular organisms have developed other systems of tissues, "well 

 adapted to meet the conditions under which the animals live. With 



