ADDITIONAL ANIMAL STUDIES 



111 



comes in contact with the waving tentacles, the hydra ejects micro- 

 scopic threads from certain cells (nettling cells) in the tentacles. 

 The animal thus attacked is benumbed, and the hydra then uses 

 the tentacles to push its prey into a mouth epening in the center 

 of the circular row of tentacles. The food is drawn into the inside 

 of the column, which is simply a hollow tube (Fig. 124). Here 

 certain cells secrete digestive fer- 

 ments which dissolve the foods that 

 the animal has eaten, and the indi- 

 gestible matter is ejected from the 

 mouth. The digested food is then 

 absorbed by the cells lining the 

 cavity. Since the animal is bathed 

 outside and inside by water contain- 

 ing oxygen, the cells are able to 

 absorb oxygen from the water and 

 to give off carbon dioxid to the 

 water. Hence no breathing organs 

 are needed. 



It is evident that the tentacles 

 with the nettling cells also serve to 

 protect the hydra from too great 

 familiarity on the part of visitors 

 that might otherwise use it for food. 

 When the hydra moves from one 

 place to another, it bends over 



FIG. 125. The movements made 

 by hydra in locomotion. (Jen- 

 nings.) 



until the ends of the tentacles touch the surface on which it rests. 

 The tentacles then adhere to this surface, the bottom of the 

 column lets go, and the animal turns a somersault (Fig. 125) and 

 lands on the lower part of the column ; the process may then be 

 again repeated. 



Like the higher animals the hydra reproduces by means of eggs 

 and sperms. But it also has another interesting way of producing 

 new individuals. On the surface of the column one frequently sees 

 little bunches. These are called buds (Fig. 124). They keep on 

 growing outward till at last little tentacles and a mouth opening are 



