176 



PHYLUM CCELENTERA 



male organs appear as ectodermic protuberances a short 

 distance below the bases of the tentacles ; the ovary, with 

 a single ovum, is a larger bulging farther down. Both male 

 and female organs may occur on the same animal, either 

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

 different individuals. Abundant food favours the develop- 

 ment of female forms ; when food is scarce males are more 

 abundant. The buds have the same structure as the parent 

 body ; in origin they appear to be mainly due to multiplica- 

 tion of interstitial cells. 



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. 



(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. 88, E). 

 The cells themselves are contractile, but there are these special con- 

 tractile 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. 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 Alga;. When 

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



