No. 494] ZOOLOGICAL PROGRESS 



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under the influence, if not the control, of nerves, and these 

 cases represent, so to speak, the second step in the differ- 

 entiation of a neuro-muscular mechanism. In such jelly- 

 fishes, groups of cells especially open to stimulation by 

 light, pressure, etc., occur on the edge of the bell, and from 

 these sense bodies nerve fibers pass to the muscular sheet 

 on the under face of the bell. These sense bodies evidently 

 act as triggers by which the muscular mechanism can be 

 brought into action and in that way render it more deli- 

 cately responsive than if it relied entirely upon direct 

 stimulation. The relation of such a system may be de- 

 scribed as that of a set of sense organs directly connected 

 with a musculature, for there is nothing here that can be 

 fairly described as a central nervous organ. As the sense 

 organs are in the ectoderm and the muscles represent in- 

 cipient mesoderm, it is obvious that the future develop- 

 ment of these two layers will in this respect be most 

 intimately bound together. 



The last organ, in my opinion, to appear in this chain 

 of development is the central nervous organ, the brain 

 and its subordinate centers. This originated on the line 

 of connection between the sense organ and the muscle, 

 but rather from the sensory than the muscular end, as is 

 shown by its anatomical relations in adult animals and 

 by its invariable origin in the embryo from the ectoderm. 

 It is scarcely recognizable in the simple multicellular ani- 

 mals and begins to be an obvious organ in such inter- 

 mediate forms as the worms. Here it serves chiefly as a 

 means of freer and more extended communication between 

 the sense organs on the one hand and the musculature on 

 the other, and thus lays the foundation for the marvelous 

 development that it shows in the higher animals, where, as 

 a storehouse of race and individual experience, its sig- 

 nificance is unparalleled. The paramount importance of 

 the brain in fact is so fully recognized that it is usual to 

 treat the sense organs as appendages of it, but, if the 

 view that I have advanced is correct, just the reverse is 

 true; the sense organs of a bilaterally symmetrical animal 



