446 G. H. PARKER AND E. G. TITUS 



fig. 510) in the tentacles of Anemonia. If such cells do occur 

 in the ectodermic nerve layer of Metridium, they are certainly 

 extremely rare; in the vast majority of cases therefore the basal 

 branches from the sense cells must connect directly with muscle 

 fibers, that is, without the intervention of ganglion cells. Havet's 

 scheme of nervous connections, sense-cell to ganglion cell and 

 ganglion cell to muscle cell, assuredly does not hold for the 

 tentacular ectoderm of Metridium. Here the organization is 

 obviously a simpler one, sense cells through their basal fibrillae 

 connecting with muscle cells. 



In sections of the tentacles of Metridium, in which the nervous 

 layer in the ectoderm is easily demonstrated, it was impossible 

 to be certain of such a layer in the entoderm. This holds true 

 irrespective of the direction in which the tentacle is cut. The 

 limits of the entodermic epithelium of the tentacle are well 

 defined. Next its free edge the cells are filled with densely 

 staining protoplasm, but as the base is approached they become 

 more open in structure and their termination is in conjunction 

 with the thin sheet of circular muscle fibers without the inter- 

 vention of a nervous layer. According to the Hertwigs ('79- 

 80, p. 493, Taf. 18, fig. 6) the nervous layer in the tentacular 

 entoderm of Tealia occurs some distance from the circular 

 muscle; Schneider ('02, p. 623) describes this layer in Anemonia 

 directly in contact Mdth the muscle, and a similar situation is 

 claimed for it by Wolff ('04, p. 249) in Heliactis. 



Havet ('01, p. 410) makes no mention of a nervous layer in 

 the tentacular entoderm of Metridium dianthus and states 

 that in this situation sense cells are extremely rare. In Metri- 

 dium marginatum, as already stated, we have been unable to 

 identify any entodermic nervous layer whatsoever and we have 

 therefore been led to doubt if such is really present. 



Another portion of the body of Metridium whose nervous 

 structure is important is the column wall. The ectoderm of 

 the column wall in actinians is said by the Hertwigs ('79-80, 

 p. 500) and most recent investigators (Schneider, '02, p. 627) 

 to contain only an insignificant amount of nervous tissue and 

 yet scarcely any portion of the body of Metridium is more sen- 



