NUTRITION 41 



known that the young leeches have P. hirudinis in their guts before 

 they leave the cocoon. The extraordinary picture with which we 

 are presented of an animal handing over the whole of its digestive 

 processes to symbiotic bacteria has few if any parallels in the 

 animal kingdom (perhaps the nearest are those mammals and 

 insects which rely on micro-organisms for the digestion of 

 cellulose), yet it is an arrangement which enables the leech to store 

 a supply of food for a very long period and to receive a steady 

 supply of soluble nitrogenous compounds. It is capable of 

 evolving slowly from the more normal arrangement, the leech 

 producing progressively fewer enzymes and relying more and 

 more on the bacteria ; since it enables the leech to make its meals 

 last longer there is an obvious selective advantage. 



The site of absorption may be the crop, the intestine, or both, 

 but the intestine is much more vascular and it w^ould be surprising 

 if it did not play some part in absorption. During digestion the 

 haemoglobin is split into its component parts globin and haematin 

 and the latter into proto-porphyrin and inorganic iron. It is the 

 globin which forms a major source of nutriment for the leech. It 

 has been reported that during assimilation there is an accumulation 

 of glycogen and fat in the gut epithelium and since there is little 

 glycogen in the meal of blood it is presumably synthesized from 

 the products of protein digestion. 



Turning from Hirudo to Haemopis we find that the teeth of this 

 leech are blunt and food organisms such as earthworms and slugs 

 are devoured whole. Studies of the digestive enzymes present 

 (Antrum and Graetz, 1934; Graetz and Autrum_, 1935) showed 

 that enzymes for initiating protein digestion were absent, although 

 there were several enzymes for the later stages of the process. They 

 found a dipeptidase and a carboxypeptidase working optimally 

 at pH 7-8, an aminopeptidase (pH 8-05) and peptone splitting 

 factors (pH 7-6 and 8-2). Lipases were located in the intestinal 

 wall and the body tissues. They therefore postulated that the 

 initial stages of protein digestion were due to putrefaction, although 

 one difficulty about this theory is that boiled earthworms were 

 digested about as rapidly as fresh ones. All the evidence points to 

 the existence of symbiotic bacteria as yet not isolated, which have 

 taken over a part of the digestive process. 



