442 G. H. PARKER AND E. G. TITUS 



face of the cross arm of the T instead of on the sides of its shaft, 

 probably a mistake in observation, as pointed out by Carlgren 

 ('93). 



111. NERVOUS STRUCTURE 



Modern conceptions concerning the nervous system of the 

 coelenterates and especially of the actinians date from the publi- 

 cation of the well known researches by the Hertwigs in 1879 and 

 1880. According to these investigations the epithelial covering 

 of the actinians, to turn at once to that group, contains many 

 sense cells whose deep ends branch often and form thus a nerv- 

 ous meshwork of the finest fibrils resembling the punctate sub- 

 stance described long ago by Leydig as one of the constituents 

 of the central nervous organs in invertebrates. The nervous 

 fibrils originating from the sense cells are supplemented by others 

 from certain, large, deep-seated cells, the so-called ganglion 

 cells, and it is through this combination of fibrils that the still 

 deeper layer of muscle fibers is brought into action. This type 

 of nervous structure according to the Hertwigs, pervades much 

 of the body of the actinian, entoderm as well as ectoderm. It 

 is stated by them to be best developed in the ectoderm of the 

 oral disc and to form there a primitive central nervous organ 

 from which connections pass to the tentacles, oesophagus, etc. 

 On the column wall and foot this nervous mechanism is said 

 to be much reduced. The ectodermic constituent is believed 

 to be completely separated by the supporting lamella from the 

 entodermic part except at the inner margin of the oesophagus 

 where ectoderm and entoderm are continuous. Here the union 

 is supposed to be especially mediated by the mesenteric fila- 

 ments, by means of which nervous impulses are believed to 

 reach the mesenteric muscles and the acontia (Hertwig, '79-80, 

 p. 49). 



Almost exactly this view of the organization of the actinia'n 

 nervous system has been recently expressed by Wolff ('04, p. 274). 



Groselj ('09, p. 302), who has studied the nervous composi- 

 tion of the sea-anemones by means of methylen blue, also claims 

 that the nervous system is distinctly centralized about the oral 



