Evolution 



to come in contact with them. These ' nettle cells ' occur in 

 much greater numbers in the tentacles than elsewhere, and here 

 they are brought into play against the animals, such as minute 

 Crustaceans, which form the Hydra's prey. Coming in contact 

 with the tentacles, such creatures are caught, paralysed by means 

 of the stinging cells, and are gradually transferred into the mouth 

 by a slow contraction of the tentacles. The Hydra reproduces, 

 for the most part, by a simple process of budding. Small lateral 

 outgrowths are formed, which gradually develop mouth and 

 tentacles of their own. Ultimately these separate and are carried 

 off by the water, later to settle down and become attached to 

 some fixed object. Sometimes, however, sexual reproduction 

 occurs. The reproductive cells are produced, male and female 

 on the same individual, among the ordinary cells of the outer 

 layer. These are set free, fertilisation occurs in the water, and 

 the egg develops in the same manner as that of the coral. The 

 Hydra is able, by means of the fibrous protoplasm of its outer 

 cells, to show well-marked movements. It can bend its body in 

 this direction or that, can contract its whole body into a small 

 oval mass, and is even able, by performing a number of slow 

 somersaults, to change its position. The structure and the 

 methods of reproduction in Hydra will be readily understood 

 from the illustrations of the creature on Figs. 33 and 34. 



If now we make a brief general survey of the group to which 

 the Hydra belongs, we find in it two somewhat strikingly different 

 types. On the one hand are sedentary forms that resemble, in a 

 general way, the Hydra ; that consist of a tube-shaped body, with 

 the mouth, surrounded by a ring of tentacles, at the upper end. 

 The sea-anemones and corals are examples of this type, in which, 

 however, the structure shows various complexities as compared 

 with that of the Hydra, which complexities we cannot here pause 

 to describe. On the other hand is the well-known Medusa form, 

 of which the common jelly-fish is a typical example. This 

 creature, as is well known, is mushroom shaped, with tentacles 

 round the edge. The mouth is in the middle of the lower aspect, 

 at the end of a short ' stalk.' This type is very different in 



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