THE ELEMENTARY NERVOUS SYSTEM 



brate nervous system exhibit in one fundamental par- 

 ticular a most striking similarity. 



Havet also claimed to have demonstrated a much 

 closer relation between the ectodermic and entoderniic 

 nervous layers than was suspected by the Hertwigs. He 

 believed that he could show by means of the Golgi method 

 that nervous fibrils pass from the ectoderm through the 

 supporting lamella to the muscles of the entoderm and 

 thus establish a direct union between structures that, ac- 

 cording to the Hertwigs, were only indirectly united 

 through the esophagus. This claim has been abundantly 

 confirmed by Parker and Titus ( 1916) , who have shown by 

 a special technique for nervous tissue that the supporting 

 lamella of the actinian Metridium contains an abundant 

 meshwork of branching neurofibrils that can be traced 

 from the ectodermic side of this layer through its sub- 

 stance to the more important systems of muscles in the 

 mesenteries. Moreover, the supporting lamella can be 

 seen to contain a great number of branching cells which 

 have all the appearances of true nerve cells and which 

 presumably form an essential part of the conducting sys- 

 tem between the ectodermic sensory areas and the ento- 

 dermic musculature. 



These observations revive in a way the opinion early 

 advanced by von Heider (1877, 1895) that the supporting 

 lamella in a number of actinians contains nervous ele- 

 ments, a claim that, notwithstanding the opposition of 

 Wolff (1904) and of Kassianow (1908), has been sup- 

 ported by the conditions found in the soft corals by Hick- 

 son (1895), by Ashworth (1899), and by Kiikenthal and 

 Broch (1911). Thus, though the details of the nervous 

 organization in the actinians is only just beginning to be 



