CONCLUSION 



201 



are not only applied to the muscle cells but form among 

 themselves a network of communication whereby the im- 

 pulses that arise from a few receptive cells may be trans- 

 mitted to many muscle cells instead of being limited to a 

 restricted group. Here is to be seen the first trace of the 

 nerve-net, an organ that in present forms makes the re- 

 ceptor-effector system immensely more responsive and in 

 past forms harbored the germ of 

 a central nervous or adjuster ap- 

 paratus that in the end profoundly 

 affected the whole scheme of neuro- 

 muscular organization. 



The simplest type of receptor- 

 effector system with a simple form 

 of nerve-net is such as is seen in the 

 tentacles of many actinians where 

 the receptive cells with their central 

 branches and the muscle cells con- 

 stitute the whole complex (Fig. 46). 

 A more complicated type of this 

 system and one that is more usual with the coelenterates is 

 that in which, in addition to the two kinds of cells men- 

 tioned, a third appears and from its position in the midst 

 of the nerve-net adds by its numerous branches im- 

 mensely to the intricacies of this structure (Fig. 47). 

 This third type of cell was long ago recognized by the 

 Hertwigs (1879-1880), who designated it a ganglion cell 

 and attributed to it central functions such as were for- 

 merly supposed to be of necessity associated with such 

 elements. Cells of this kind, according to Havet (1901), 

 intervened between the sensory cells and the muscles, and 

 he, therefore, denominated them motor cells. ' As they 

 are not massed together into ganglia and as they may 



FIG. 46. Diagram of a 

 simple type of receptor-ef- 

 fector system such as is seen 

 in the tentacles of sea-anem- 

 ones. It consists of recep- 

 tors r or sense cells whose 

 basal nerve-net connects them 

 with the deep-seated muscle 

 cells m. 



