518 W. B. HARDY. 



different regions of those animals in such a way that the de- 

 mands of one part may be met by the discharge of stored 

 nutritive material from other regions. Such a fluid, which 

 AUmann has called the " somatic fluid/' would not only con- 

 tain the immediate results of digestion, but also elaborated 

 material from the store of reserve nutriment possessed by 

 certain cells discharged in response to any special demand 

 in some particular region. In other words, it is not merely a 

 fluid for the distribution of the immediate products of digestion ; 

 it is more than that, and we shall see reason to think that it is 

 a true metabolic link between one part of the body and another 

 strictly comparable to the blood of higher forms. 



In Allmann^s account of digestion in Hydroids he thus 

 describes the '^somatic fluid : " — "Its basis is a transparent 

 colourless liquid, and in this solid bodies of various kinds are 

 suspended. These consist partly of disintegrated elements of 

 the food ; partly of solid coloured matter which has been secreted 

 by the walls of the somatic cavity ; partly of cells, some of 

 which have undoubtedly been detached from those walls, 

 though it is possible that others may have been primarily 

 developed in the fluid; and partly of minute irregular cor- 

 puscles, which are possibly some of the eff'ete elements of the 

 tissues." 



Leaving the Coelenterates and turning to the Turbellaria, we 

 have a striking instance of how the enteric cavity, the gut, may 

 serve as an organ for the distribution of nutriment. The case 

 of the Turbellaria also enables us to contrast cases 1 and 

 2, for in the Rhabdocoels we have animals with a simple gut 

 surrounded by a tissue, the mesenchyme, with numerous cleft- 

 like spaces, which may be so far developed as to form a fairly 

 well-marked space round the gut, as in Mesostoma tetra- 

 gonum, and to a less extent in Microstomum lineare. 

 That such a space or system of spaces facilitates the distribu- 

 tion of material derived from the gut hardly needs stating, 

 whether we consider the distribution to be brought about by 

 difl'usion or by the agitation of the contained fluid as a result 

 of the muscular movements of the animal. 



