158 THE muDcnoNs ojf biologic. 



derm, whicii absorbs nutriment, and the ectoderm, which, by 

 its own contractions and those of the tentacles it bears, pro- 

 duces motion. That the functions of accumulation and ex- 

 penditure 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 

 space 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 function 

 in the same way throughout its whole extent ; but its differ- 

 ent portions, sesophagus, stomach and intestine, perform 

 different portions of this function. And instead of a con- 

 tractility uniformly diffused through the ectoderm, there 

 have arisen in it, some parts which have the office of con- 

 tracting (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 

 molluscs, circulation is produced by a muscular tube, open at 

 both ends, which, bj 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 



