380 ZOOLOGY. 



definite kind (pressure on the cnidocil) unlocks a store of 

 potential energy, and causes an amount of work to be done 

 quite disproportionate to the pressure itself. In view of 

 the fact that pressure on some one part of a Hydra will be 

 followed by both muscular contraction and the firing-off of 

 nematocysts through a much larger region even over the 

 whole body it is evident that some sort of a nervous system 

 must exist for the transmission of stimuli to a distance. 

 And in fact we find, scattered among the other ectoderm 

 cells, a number of multipolar nerve-cells, whose processes 

 enter into intimate association with the sensory-motor 

 cnidoblasts, with one another, and with the muscle-tail- 

 cells. The exact nature and extent of this association is 

 not fully known the processes have been described as con- 

 tinuous between one cell and another, but modern methods 

 of histological research have so constantly shown such 

 supposed continuity not to really exist, that we may well 

 feel doubtful about the condition in Hydra. It is not even 

 known whether the distinction between axis-cylinder and 

 dendrons, so characteristic of nerve-cells in the higher 

 animals, is found here. However this may be, the scattered 

 condition of the nerve-cells, which are not compacted into 

 any central nervous system, is -a more primitive condition 

 than we have in any of our other types which possess a 

 nervous system. 



6. Gland-cells. The only other modification of the 

 ectoderm cells that need be mentioned at present is the 

 glandular character of those at the attached end, or " foot " 

 as it is often called. These secrete a sticky substance 

 which causes firm adhesion. In this region interstitial 

 cells are absent. 



7. The Endoderm (fig. 195, B) consists for the most part 

 of larger cells than those of the ectoderm. Each of these 

 contains a large vacuole (i.e. a space containing fluid), 

 which differs from the food-vacuoles and contractile vacuoles 

 of Protozoa, in being a comparatively permanent structure 

 in the cell, The end of the cell which is next the mesoglcea 

 is extended into a muscle-tail, less conspicuous than that of 



