STRUCTURAL CHARACTERS OF ANIMAL ORGANISMS. 39 



at the same time the most interesting modifications are those in 

 the special sense organs, where the cells are in immediate connec- 

 tion with nerves, and aid in forming the special nerve terminals.* 



FIG. 16. 



FIG. 15. Epithelial cells, some of which are filled with mucus (d), form- 

 ing goblet-like cells. (Cadiat.) 



FIG. 16. Neuro-muscular cells of hydra, ra. Contractile fibres. (Klein- 

 enberg.) 



Nerve Tissue. The great nervous centres are formed from 

 certain cells of the outer germinal layer, which, in the earliest 

 days of the embryo, dip in as a furrow, and are gradually cut off 

 from the parent tissue by the rapid growth of the middle germ-layer. 

 In looking for special conducting tissue in animals possessing the 

 irost simple structure, we find cells which would seem to possess 

 certainly a two-fold, and possibly a three-fold function, one of 

 which is conduction. In the so-called " neuro-muscular " cells of 

 the hydra, processes are described as found to pass off from them, 

 and to unite beneath the ectoderm with other fibre-like processes, 

 which are eminently contractile. Here we find for the first time 

 a portion of protoplasm specially devoted to acting as a conductor 

 of impulses, and attached by the one end to a contractile fibre, 

 and by the other to a surface (sensory) cell. The intimate rela- 

 tion between the development of nerve and muscle fibres is thus 

 established, and we have the first attempt at a nerve mechanism, 

 viz., a cell capable of receiving impressions, and a fibre capable 

 of transmitting the results of these stimuli. As further differen- 

 tiation proceeds, each of these parts becomes more distinct from 



* A fuller account of the Histology of these tissues will be found in the 

 chapters specially devoted to these subjects. 



