136 CCELENTERA. 



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. 



(la. ) 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. 40, 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 dis- 

 covery of special nerve cells (Jickeli) shows that even in Hydra there is 

 a differentiation of the two functions of contractility 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 structure called a nematocyst. This consists of a sack, 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, for it is able to paralyse 

 small animals. 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 contracts, and the pressure 

 of the fluid causes the forcible evagination of the barbed pouch and the 

 long lasso. 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. 40, 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 pseudoiDodia 

 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.fiisca they are brownish and without 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 ama-boid (cf. the cell cycle, p. 97). After 

 this direct absorption the food is digested within the cells, and while 

 some of the dark granules seen in these cells may be decomposed pig- 

 ment bodies, others seem to be particles of indigestible dt-hris. Thus 

 Hydra illustrates what is called intracellular digestion (T. f. Parker), 

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

 " worms." But, according to Miss C.reenwood, the food is digested in 

 the gut cavity, and subsequently absorbed. It seems likely that both 

 intracellular and extracellular digestion occur. 



