ECTODERM QI 



cell, forming a distinct layer on the outer side of the structureless 

 lamella. It has, however, been claimed recently that these ap- 

 parent fibres are mere crystalhsations in the mesogloea, and have 

 no connection with the cells. Over the greater part of the body the 

 surface layer of the protoplasm is a firm pellicle, but in the disc this 

 is absent. The cells in this region are also peculiar in containing 

 granules of a substance secreted by the protoplasm which is 

 used to fix the animal to the surface it hangs from. Each musculo- 

 epithelial cell has a large oval nucleus in one of its pillars. In the 

 tentacles these cells are less tall than elsewhere. Between the 

 pillars are spaces which contain small, rounded interstitial cells. 

 These form a reserve from which, in 

 various circumstances, any of the other 

 cells of the body can arise and they 

 thus retain the undifferentiated nature 

 of the germ cells. Between the pillars 

 stand also peculiar cells known as 

 cnidoblasts, which project through the 

 surface protoplasm. These are very 

 numerous in the tentacles, where they 

 lie in groups or batteries (Fig. 57), but 

 absent from the basal disc. Each of 

 them has a pear-shaped body with the 

 narrow end at the surface of the 

 animal, where there projects from it a 

 short process known as the cnidocil. On 

 this side the cell contains a pear-shaped 

 sac, called the nematocyst, with a lid, 



the operculum. The narrow outer end of the sac is tucked in and 

 produced into a long, hollow thread, which hes coiled up in the 

 sac. The space between the thread and the wall of the sac contains 

 a fluid. On stimulation the thread is expelled, being turned inside 

 out in the process, but Httle is known of how this is brought about. 

 There is no nervous control and there is no evidence for the 

 common statement that the cnidocil is a sense organ. The 

 nematocysts are discharged by tactile stimuli preceded by a 

 chemical stimulus, the latter facihtating the effect of the former, 

 and a strong mechanical stimulus is effective by itself. Bursting 

 of the operculum is probably caused by the absorption of water. 

 The nematocysts are of four kinds— a large kind with a straight 

 thread provided with barbs at the base, a small kind with a 



Fig. 58. — A transverse section 

 of Hydra, stained and seen 

 under the low power of the 

 microscope X c. 120. 



ect. 



Ectoderm; end., endoderm ; st.l., 

 structureless lamella. 



