26 THE PHYSIOLOGY OF EARTHWORMS 



(4) The neutralization of the organic acids formed during 

 digestion of vegetable matter contained in the gut; and 



(5) The excretion of respiratory carbon dioxide, which we may 

 now read more widely as acid-base balance. 



In the early years of this century Combault (1907, 1909) 

 thought the anatomy of the calciferous glands of earthworms 

 closely resemble that of the gills of fish and by postulating muscu- 

 lar movements designed to draw water through the glands he 

 thought that oxygen could be taken up by the extensive blood 

 supply to these structures. Combault noted a difference in the 

 colour of the blood entering the gland and leaving it and thought 

 there was a difference in the oxygen tension of afferent and efferent 

 blood in the glands, but no figures are available to show whether 

 or not this is true. This theory has now generally been discarded. 

 So also has that of Michaelsen (1895). 



Michaelsen (1895) considered that the glands are absorptive in 

 function and serve in removing nutrients from the gut contents. 

 The excellent blood supply and lamellated structure provide 

 support for this opinion, but unfortunately the glands lie anterior 

 to the main absorption region, the intestine. As we have seen 

 previously (p. 20), in certain earthworms protein digestion begins 

 under the influence of a protease in the pharynx in some species, 

 but the majority of the digestive enzymes are not encountered in 

 front of the crop and gizzard. Although the glands are well supplied 

 with blood and have a large surface area none the less in many 

 oligochaetes the connection of the glands with the alimentary 

 canal is reduced to a narrow tube, thus rendering the access of gut 

 contents very difficult. 



In the two cases considered above experimental evidence has 

 been notably lacking, but the remaining theories have all received a 

 certain amount of support from experimental studies. The inges- 

 tion of plant remains from the soil or in some cases, soil alone, 

 very often entails the concurrent intake of organic acids or acid 

 soil particles. The secretion of calcium carbonate has been thought 

 a means of neutralizing this acid content. Robertson (1936) has 

 provided us with a review of various aspects of this problem which 

 has been but little modified in the light of later results. The pH of 

 the alimentary canal of L. terrestris lies between pH 6-2 and 6-7 and 



