146 PHYLUM C(ELENTERA. 



middle lamina which separates ectoderm from endoderm (Fig. 69, 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 

 presence of special nerve cells shows that even in Hydra there is a 

 differentiation of the two functions of contractility and irritability. 



(2) Stinging cells or cnidoblasts occur abundantly on the upper parts 

 of the body, especially on the tentacles. Each contains a 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 lies in a gelatinous colloid, 

 presumably poisonous. On its free surface the stinging 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. With the help of the barbs they 

 penetrate through even a chitinous membrane, and the secreted fluid 

 has a solvent action. The victim is held fast and drawn closer. 

 Besides the ordinary stinging cells, there are others of small size which 

 coil into a spiral after explosion. 



(3) There is to the inner aspect of the covering cells a network of 

 ganglion cells and nerve processes. More superficially there are 

 minute sensory cells, some of them connected by fine fibres with the 

 ganglion cells. 



(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 pseudo- 

 podia, and so move the animal slowly. 



The endoderm is less varied. Its cells are pigmented, often 

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

 pigment bodies in H. viridis are like the chlorophyll corpuscles of 

 plants ; it seems almost certain that they are unicellular Algae. When 

 a green Hydra liberates an egg while kept in the dark, that egg gives 

 rise to a white Hydra, which is supposed to imply that the partner 

 Algae do not migrate into the egg when there is no light. In the other 

 species of Hydra^ the pigment is quite different from chlorophyll. 

 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 amoeboid processes of 

 some of the cells, and it has been noticed that the same cell may be at 

 one time flagellate and at another time amoeboid (cf. the cell-cycle, 

 p. 107). After this direct absorption the food is digested within the 

 cells, and while some of the dark granules seen in those cells may be 

 decomposed pigment bodies, others seem to be particles of indigestible 

 debris. Thus Hydra illustrates what is called intracellular digestion, 

 such as occurs in Sponges, some other Coelentera, and some simple 

 " worms." But experiments show that some of the food may be 

 digested in the gut cavity, and subsequently absorbed. Thus it seems 

 that both intracellular and extracellular digestion occur. 



Some of the endoderm cells have muscular roots like those of the 

 ectoderm. They lie on the inner side of the middle lamina, in a trans- 



