84 THE ELEMENTARY NERVOUS SYSTEM 



ing that nettle cells are discharged through nervous ac- 

 tion. In the sea-anemone Metridium these cells are abun- 

 dantly present on the tentacles and especially on those 

 delicate filaments from the interior of this animal, the 

 acontia, Nevertheless, from both these structures the 

 nettling threads can be discharged only under direct 

 stimulation and this continues to be true even after the 

 part in question has been thoroughly and completely an- 

 esthetized with magnesium sulphate or chloretone. As 

 these drugs temporarily abolish all traces of nervous ac- 

 tivity and yet in no apparent way affect the activity of 

 the nettled cells, it is most probable that these cells, like 

 the mucous cells and the ciliated cells, are independent 

 effectors and not under nervous influence (Parker, 

 1916 a). 



Of the nervous control of the fourth type of effector 

 in actinians, the muscle, there is abundant evidence. The 

 muscles of these animals were regarded by the earlier 

 workers as more or less continuous sheets that gave to 

 the animal as a whole some tiling of the character of a 

 contractile sac. But after the publication of the impor- 

 tant paper by the Hertwigs (1879-1880) on the structure 

 of these animals, it became evident that their muscula- 

 ture was more differentiated than had been previously 

 supposed and that a considerable number of well denned 

 muscles or groups of muscle could be distinguished. 

 These muscles, as already stated, occur in the deep por- 

 tions of the ectoderm and the entoderm and may in some 

 instances even invade the supporting lamella. 



In Metridium it is possible to distinguish at least thir- 

 teen such sets of muscles. Their positions are indicated 

 in Fig. 20. Of the thirteen only two are found in the ecto- 

 derm, the remaining eleven being in the entoderm. They 



