VII CONDUCTION OF STIMULI IN PROTOZOA 321 



division of labour in the province of nervous life, and it 

 is impossible to see why the nucleus should not be con- 

 sidered as central nervous organ in unicellular forms, if 

 it can be shown that it plays this part in the multi- 

 cellular. 



However, the question can only be decided in the future. 

 Possibly special fibrils for the conduction of nerve stimuli 

 will yet be demonstrated by more exact researches directed 

 to this point,^ and will at the same time lead us to the 

 central organ. In any case, nerves like those of the multi- 

 cellular forms are not present, and yet the animals evidently 

 receive stimuli by means of the cilia which are connected 

 with the protoplasm, which stimuli must be conducted along 

 definite paths to the central organ. These paths can be 

 nothing but strands of protoplasm ; probably they are formed, 



^ Cf. Engelmann, Zar Anatomie iind Physiologie der Flimmerzellen, PjlUr/ers 

 Archiv, Bd. xxii. 1880, p. 505. This investigator believes he has seeu such 

 fibrils in Stylonychia passing into cilia which vibrate at diflerent rates. I may 

 also remark that I have repeatedly found the ciliated cells of the surface of the 

 mantle and gills in Anodon to be most distinctly connected with nerves which 

 obviously fulfil the same purpose. After treatment with bichromate of potash 

 the cells can easily be isolated still in connection with the nerves, I have 

 hitherto failed to see a connection between the nerves and the fine protoplasmic 

 threads Avhich I was able in these cells to trace downwards below the nucleus, 

 and which are continued above into the cilia, but such a connection is very prob- 

 able (cf. Eimer : Weitere Nachrichten uber den Bau des Zellkerns, nebst Bemer- 

 kungen iiber Wimperejnthelien, Archiv fur mikroskop. Anatomie, Bd. xiv. 1877). 

 I consider these threads as nervous paths, as nerve-plasma, and the knowledge of 

 these was the basis of my view that in the Infusoria also the protoplasmic threads 

 form nervous paths. From the analogy of the relations of the fibrils from the 

 cilia in the ectoderm-cells of MedusEe which I have described (cf. Die Medusen, 

 Taf. iv. ), it may be assumed that these threads in Infusoria also pass into the 

 nucleus. On the other hand, that such threads pass into nerve-fibrils, I have 

 shown in the ectoderm-cells of Carmarina hastata (Cf. Medusen, Taf. xii. fig. 

 8, 19 ; Taf. xi. etc.) These cells appear distinctly striated longitudinally ; their 

 protoplasm is clearly transformed into nerve-plasma, and the fibrils representing 

 the latter, diverging from one another below, are continued directly into the 

 nerve-fibrils of the ring-nerve (" brush cells "). The fact that each of these ecto- 

 derm-cells is thus in connection with numerous nerve-fibrils, would seem to show 

 that they still act as central nerve-cells, although ganglionic nodules have already 

 been developed beneath the epithelium (cf. below : the first development of the 

 central nervous system, especially Hydra). 



Y 



