CHAPTER II 



DIGESTION AND METABOLISM 



Animals obtain the energy necessary for the maintenance of their 

 bodily processes, such as muscle contraction, gland secretion, 

 nervous activity and so on by the breakdown of complex organic 

 substances obtained in their diet. This diet may be taken in a 

 variety of ways : grazing, filter feeding, predation or burrowing, 

 but in all cases the food must eventually be broken down by the 

 animal ingesting it, i.e. digested, then absorbed and finally utilized 

 i.e. metabolized. The mechanism of synthesis of animal protein 

 takes place using only the simple building blocks of amino-acids, 

 carbohydrates, etc., that can be absorbed by the gut. The excep- 

 tions to these generalizations are, of course, parasitic forms in 

 which the alimentary system may have disappeared altogether. 



The majority of oligochaetes are omnivores. That is to say they 

 are non-specific eaters. Non-selective would be too widespread a 

 concept since it has been shown that particular types of leaf are 

 eaten in preference to others, at least by the earthworm (Mangold, 

 1951; Wittich, 1953), indicating that the animal evidently recog- 

 nizes differences in its diet, presumably by sensory means. Earth- 

 worms (L. terrestris) come to the surface of the ground at night, 

 searching mainly in the area surrounding the burrow entrance for 

 food and drawing suitable leaves into the mouth of the burrow. 

 When the litter layer is freshly fallen, leaves are grazed in a 

 different preference order to the regime existing when decompo- 

 sition has been proceeding for some while (see Chapter XI). This 

 is possibly explicable in terms of chemical changes occurring under 

 such conditions. 



It is obvious that during the passage of earthworms through soil, 

 and the making of burrows, much soil passes along the gut. This 

 soil contains not only plant matter, but also the decomposing 

 remains of animals large and small, hving protozoa, rotifers and 



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