202 THE INDUCTIONS OF BIOLOGY. 



shared by the endoderm. That the functions of accumnlation 

 and expenditure are here very incompletely distinguished, 

 may be admitted without affecting the position that this is 

 the first specialization which begins to appear. These 



two most general and most radically-opposed functions 

 become in the Polyzoa, much more clearly marked-off from 

 each other: at the same time that each of them becomes 

 partially divided into subordinate functions. The endoderm 

 and ectoderm are no longer merely the inner and outer walls 

 of the same simple sac into which the food is drawn : but the 

 endoderm forms a true alimentary canal, separated from the 

 ectoderm by a peri-visceral cavity, containing the nutritive 

 matters absorbed from the food. That is to say, the function 

 of accumulating force is exercised by a part distinctly divided 

 from the part mainly occupied in expending force: the 

 structure between them, full of absorbed nutriment, effecting 

 in a vague way that transfer of force which, at a higher stage 

 of evolution, becomes a third leading function. Meanwhile, 

 the endoderm no longer discharges the accumulative func- 

 tion in the same way throughout its whole extent; but its 

 different portions, oesophagus, stomach and intestine, perform 

 different portions of this function. And instead of a 

 contractility uniformly diffused through the ectoderm, there 

 have arisen in the intermediate mesoderm some parts which 

 have the office of contracting (muscles), and some parts 

 which have the office of making them contract (nerves and 

 ganglia). As we pass upwards, the transfer of force, 



hitherto effected quite incidentally, comes to have a special 

 organ. In the ascidian, circulation is produced by a muscular 

 tube, open at both ends, which, by a wave of contraction 

 passing along it, sends out at one end the nutrient fluid 

 drawn in at the other; and which, having thus propelled 

 the fluid for a time in one direction, reverses its movement 

 and propels it in the opposite direction. By such means 

 does this rudimentary heart generate alternating currents in 

 the nutriment occupying the peri-visceral cavity. How the 



