Reactions of Sponges 35 



covery by Kleinenberg ('72) of the so-called neuromuscular cells 

 in hydra. These cells, which have since been found in great 

 abundance in many other coelenterates, were believed by Kleinen- 

 berg to contain the germ of the nervous and muscular systems of 

 the higher metazoans. According to him the elongated basal 

 process of the neuromuscular cell was the contractile or muscular 

 element, and the cell-body that reached from the exterior to the 

 muscular part was the receptive and transmitting or nervous part. 

 In his opinion this cell became divided into two, one cell to become 

 purely muscular, the other purely nervous, and these two cells, 

 thus derived directly from the primitive neuromuscular cell, were 

 supposed to be the forerunners of the muscular and nervous 

 systems of the higher animals. Kleinenberg thus conceived mus- 

 cular and nervous organs to have had a common origin and to 

 have undergone a simultaneous differentiation. The neuro- 

 muscular-cell theory was favored by Van Beneden ('74), who 

 claimed that in Hydractinia the intermediate condition between 

 a neuromuscular cell and its two derivatives was to be seen, but 

 Bergh ('78) showed this claim to be based on inaccurate obser- 

 vation. 



The study of the nervous system and sense organs of marine 

 coelenterates led Oscar and Richard Hertwig ('78) to the con- 

 clusion that the so-called neuromuscular cells were not nervous 

 but merely muscular, and they proposed for these elements the 

 name epithelial muscle-cells. They also pointed out that the 

 nervous system of the coelenterates consisted of sense-cells and 

 ganglion-cells and they believed that these two kinds of cells to- 

 gether with the epithelial muscle-cells were simultaneously differ- 

 entiated from among the elements of the cnelenterate epithelium. 

 Thus they did not trace the origin ot nervous and muscular tis- 

 sue to a single cell but to a layer of cells from which the three 

 types just named were supposed to arise by simultaneous differ- 

 entiation. This view, though slightly modified by such workers 

 as Ha vet ('01), who declared that what the Hertwigs called gang- 

 Hon-cells were more strictly speaking motor-cells, has been more 

 or less tacitly accepted by most modern students of the neuromus- 

 cular mechanism of cielenterates (Schaeppi, '04; Wolff, '04; 

 Hadzi, '09; Groselj, 'og). 



