146 PHYLUM CELENTERA. 
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 Aydra 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 protrusible 
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 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 amceboid. The 
pigment bodies in A. wirédzs 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 
Algze 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 amceboid 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 amceboid (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 
débris. Thus Aydra illustrates what is called intracellular digestion, 
such as occurs in Sponges, some other Ccelentera, 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- 
