116 LIFE IN THE SEA [CH. 



to produce enzymes in greater quantity when it is 

 in contact with a trace of the decomposition products 

 of a food organism ; that is a signalling substance, 

 or hormone, would be produced just as it is in the 

 mammalian intestine, and this would stimulate the 

 cells to elaborate the enzymes. But if the food 

 organisms were too big for the cells to ingest then 

 we may suppose that their contained enzymes would 

 diffuse out into the cavity, that is to say, a ' secre- 

 tion ' would take place. This is indeed the manner 

 of digestion in many invertebrates. Molluscs, for 

 example, possess a ' digestive gland,' ' hepato- 

 pancreas ' or ' liver ' as it is called, and there are 

 accounts of the ' secretion' from this organ and of 

 the reactions of the fluid said to be so produced. 

 There is really no evidence of the production of a 

 secretion in the sense in which we employ the word 

 in the physiology of the mammal. The tests made 

 involve the pulping of the gland and the examina- 

 tion of the product, and the latter certainly contains 

 the intra-cellular enzymes of the cells of the gland. 

 The latter is only an extension of the cavity of the 

 intestine of the mollusc. 



There is of course no reason why the preparation 

 of the food by the digestion of captured organisms, 

 and its preparation by the absorption of dissolved 

 food-stuff should not proceed simultaneously in an 



