132 CCELENTERA. 



female organs may occur on the same animal, either at one 

 time or at different times, but often they occur on different 

 individuals. The buds have the same structure as the 

 parent body, but in origin they appear to be wholly ecto- 

 dermic. 



Minute structure. — The outer layer or ectoderm includes the 

 following different kinds of cells : — 



(i) Large covering or epithelial cells, within or between some of 

 which lie the stinging cells. The epithelial cells are somewhat conical, 

 broader externally than internally, and in the interspaces lie interstitial 

 cells. By certain methods, a thin shred can be peeled off the external 

 surface of the ectoderm cells. This is a cuticle, i.e. a pellicle no longer 

 living, produced by the underlying cells. 



(13) Many of these large cells have contractile basal processes, or 

 roots, running parallel to the long axis of the body, and lying on a 

 middle lamina which separates ectoderm from endoderm (Fig. 57, E). 

 The cells themselves are contractile, but there is special contractility 

 in the roots. Like the muscle cells of higher animals, they contract 

 under certain stimuli, and are often called "neuro-muscular." But the 

 discovery of special nerve cells (Jickeli) shows that even in Hydra 

 there is a. differentiation of the two functions of contracility and 

 irritability. 



(2) Small stinging cells or cnidoblasts occur abundantly on the upper 

 parts of the body, especially on the tentacles. Each contains a pro- 

 trusible nematocyst. This consists of a sac, the neck of which is 

 doubled in as a pouch, usually bearing internal barbs, and prolonged 

 into a long, hollow, spirally coiled filament or lasso. This lasso is 

 bathed in a fluid, presumably poisonous. On its free surface the sting- 

 ing cell usually bears a delicate trigger hair or cnidocil. Under stimulus, 

 whether directly from the outside or from a nerve cell, the cnidoblast 

 explodes and the nematocyst is thrown out. Besides the ordinary 

 stinging cells, there are others of small size which do not seem to 

 explode. 



(3) Scattered about there are minute nerve cells, with fine connec- 

 tions, especially with the muscular and the stinging cells (Fig. 57, B). 



(4) Small interstitial or indifferent units fill up chinks in the ecto- 

 derm, and seem to grow into reproductive, stinging, and other cells. 



(5) Granular glandular cells on the basal disc or "foot" probably 

 secrete a glutinous substance. They are also said to put out pseudopodia, 

 and so move the animal slowly. 



The inner layer or endoderm is less varied in structure, as is to be 

 expected from the fact that it is not, like the ectoderm, exposed to the 

 varying action of the environment. Its cells are pigmented, often 

 vacuolated, and most of them are either flagellate or amceboid. The 

 pigment bodies in H. viridis seem comparable to the chlorophyll cor- 

 puscles of plants ; in H. fusca they are brownish and without chloro- 

 phyll. The active lashing of the flagella causes currents which waft 

 food in and waste out. If some small animal, stung by the tentacles, 

 is thus wafted in, it may be directly engulfed by the amceboid processes 



