CHAPTER 4 



NUTRITION 



1. In Hirudidae 



The nutrition of leeches is of interest for several reasons. One is 

 that typical leeches are blood-sucking ectoparasites and they are 

 remarkably adept at removing from the host a very considerable 

 quantity of blood without being noticed. This requires sharp, 

 precise cutting equipment and the assistance of a local anaesthetic. 

 Secondly, the blood must be prevented from clotting in the gut, 

 for during locomotion the leech becomes alternately short and 

 thick and long and thin and this would be impossible if the gut 

 contained a mass of clotted blood. Finally, a series of investigators 

 failed to identify any proteolytic enzymes in the gut of Hirudo and 

 it appears that the function of digestion has been taken over 

 entirely by symbiotic bacteria. 



The jaws of Hirudo have been described in an earlier section 

 (p. 8). As soon as they begin to saw into the tissues of the host 

 a secretion from the salivary glands is poured into the wound. 

 It has long been known that an extract of the head of Hirudo con- 

 tains a powerful anticoagulin (Haycratt, 1884) which v/as given 

 the name of hirudin and the presence of this substance was thought 

 to account for the fact that a wound made by a leech bleeds freely 

 for a very long time. However, Lindemann (1939) collected some 

 blood from such a wound and found that its clotting time was 

 normal. He found that leech head extract also contains a histamine- 

 like substance capable of causing the dilatation of capillaries so he 

 postulated that this was the substance actually injected into the 

 wound and that the free flow of blood was due to the enlargement 

 of the blood vessels rather than the inhibition of clotting. He sug- 

 gested that the act of biting and the secretion of the salivary glands 

 is divisible into two phases (i) the biting phase, when the incision 

 is being made and the histamine-like compound is being injected 



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