20 THE PHYSIOLOGY OF EARTHWORMS 



Regional localization of the enzyme secretory cells of the gut 

 occurs. Applied closely to the pharyngeal wall at the anterior end of 

 the gut (Fig. 7) is a gland termed "salivary" which secretes a 

 protease in Allolohophora and Liimbricus (Keilin, 1920). Bahl and 

 Lai (1933) have also described the production of protease by 

 glands in segments 79-83 of the intestine of Eiityphoeiis, and of 

 amylase by caecae in segments 22-26 of Pheretima. Protein is 

 digested in the stomach ( = crop) and anterior intestine of 

 Pheretima. 



The early work on enzymic digestion, mentioned above, w^as 

 expanded and elaborated by Robertson (1936) during an investiga- 

 tion of the properties and functioning of the calciferous glands in 

 segments 10-12 of L. terrestris. This author was interested in 

 whether or not the calcareous concretions produced in these 

 glands affected the pH of the intestinal contents and consequently 

 the efficiency of the digestive enzymes. He incubated watery 

 extracts of the intestine with, first, starch, and found an amylase 

 activity having an optimum pH of 6-8-7-0, producing an unidenti- 

 fied mixture of sugars as the end product. Secondly the extracts 

 showed the presence of a lipase by the destruction of methyl 

 butyrate at a pH optimum of 6-4— 6-6, with the probable occurrence 

 of a second enzyme acting on ethyl butyrate at pH 7-3-7-7. Lastly 

 the extracts were incubated with gelatin or casein for evidence of 

 a protease but little hydrolysis was noted even after incubation for 

 periods of up to 24 hours. Prolonged digestion lasting 48 hours 

 revealed slight activity with an optimum at the alkaline pH 8. 



Although Robertson (1936) decided that the proteolytic proper- 

 ties of the intestine of L. terrestris are very weak, Millott (1944) was 

 able to show the production of a protease in the anterior intestine 

 (Fig. 8, from Heran, 1956). This enzyme brings about a rennin- 

 like action, clotting milk containing calcium as a co-factor. The 

 secretion of this enzyme is considerably increased by electrical 

 stimulation of the segmental nerves, the result being a diminution 

 of the clotting time of the milk. Rennin in mammals, such as cows 

 and human infants, induces mild hydrolysis of milk but the function 

 of such an enzyme type in coping with the normal diet of an earth- 

 worm is not yet clear. It is rather surprising also that Robertson (1936) 

 found little sign of casein hydrolysis when incubated with intestinal 

 extract since casein is the major protein of milk. 



I 



