NEUROMUSCULAR MECHANISM IN METRIDIUM 443 



pole, but maintains that the ectoderm of the oesophagus, not that 

 of the oral disc, is the region of chief concentration. For him 

 the oesophagus is the central nervous organ of the actinian. 



In opposition to the view that the actinian nervous sj^stem 

 is centralized is that which holds it to be diffuse. Tliis view 

 is the natural outcome of the earher studies by Nagel ('92), 

 Loeb ('95), Parker ('96), and others and has been supported 

 more recently by the work of Bethe ('03) and especially of Jor- 

 dan ('08, '12). The same view has also been urged by Havet 

 ('01), whose histological studies have led him to conclude that 

 the nervous elements in Metridium are not sufficiently concen- 

 trated to justify the expression centralised. They are diffusely 

 scattered. 



But Havet ('01, p. 411) has not only declared for a diffuse 

 nervous system in actinians; he has also claimed grounds for 

 changing in certain important details the scheme of nervous 

 interaction proposed by the Hertwigs. According to Havet 

 the so-called ganglion cells described by the Hertwigs, are really 

 motor nerve-cells which receive impulses from the sense cells 

 and transmit them to the muscles. Thus the actinian nervous 

 system, according to Havet, contains in miniature the essential 

 sequence of cells as found in an organ like the vertebrate spinal 

 cord. Havet also claimed to have demonstrated a much closer 

 relation between the ectodermic and entodermic nervous layers 

 than was suspected by the Hertwigs. According to him ('01, 

 p. 400) nervous fibrils can be shown by means of the Golgl 

 method to pass from the ectoderm through the supporting lamella 

 to the muscles of the entoderm and thus establish a direct union 

 between structures that, accorcUng to the Hertwigs, were only 

 indirectly united thi'ough the oesophagus. Moreover, the sup- 

 porting lamella in Metridium is said to contain ganglion cells 

 (Havet, '01, p. 395). 



These ideas revive in a way the opinion early advanced by 

 von Heider ('77, '95), that the supporting lamella in Sagartia, 

 Zoanthus, and other actinians contains nervous elements, a claim 

 that has been supported by the work of Hickson ('95, p. 371), 

 of Ashworth ('99, p. 277) and of Kiikenthal und Broch ('11, 



