340 ORGANIC GROWTH 



epidermis-cells which give off internally a transversely- placed 

 fibrillar process, so that a fibrous layer is formed between the 

 ectoderm and endoderm, as neuromuscular-cells, 1 that is, as 

 epidermis-cells whose outer part is capable of receiving nerve 

 stimuli, while the internal process is of muscular nature, and 

 transforms directly the stimuli received by the former into 

 motion. 



The Hydra is simply a gastrula-sac consisting of two 

 layers, these neuromuscular-cells (the ectoderm) and digestive 

 cells (the endoderm), and provided with prehensile arms. 

 When a direct reaction ensues upon stimuli falling on the 

 ectoderm - cells, the movements of the animal are reflex 

 actions, and it unconsciously performs movements on stimu- 

 lation, just as the Mimosa folds its leaves when they are 

 touched. 



The question how the Hydra can exercise volition as it 

 certainly does, without a separate central nervous system is 

 not discussed by Kleinenberg. Yet, when we observe the 

 motions and general behaviour of many free - swimming 

 ciliated larvae, the conviction is forced upon us that although 

 they consist only of epiblast and hypoblast, they pursue 

 definite purposes, that their actions are in some degree 

 directed by a will. It must therefore be assumed, whether 

 nervous cells are somewhere present in Hydra or not, that 

 ectoderm cells in the lower Metazoa are the seat of volition. 

 But a morphologically recognisable nervous system consisting 

 of separate nerve-cells and nerve-fibres must, according to 

 these considerations, in its most primitive form lie immediately 

 beneath the epidermis, connected on the one hand with the 

 latter, on the other with muscles. Moreover, it probably 

 extended at first as a layer all over the body. 



These predictions were completely fulfilled by my 

 researches upon Beroe ovatus. 2 The further conclusion was 



1 Kleinenberg, Hydra, 1872. 2 Cf. Beroe ovatus, loc. cit. 1873. 



