472 



CHARACTERS OF INVERTEBRATE ANIMALS 



is seen to make its appearance on the body-wall, and, becoming 

 gradually larger and larger, is shaped by degrees into a fresh indi- 

 vidual, which ultimately becomes detached. This process goes on 

 very vigorously in summer, and a Hydra may often be found 

 bearing several mature buds, which, in their turn, are giving rise 

 to a third generation. A temporary form of branching tree-like 

 form is thus produced. Late in the summer, when external con- 

 ditions as regards temperature and food become unfavourable, one 

 or more rounded projections make their appearance near the foot, 

 each of them containing an egg. After developing to a certain 



ENDODERM 



Fig. 289. Development of Hydra, greatly magnified 



i, Ovum. 2, Ovum, projecting from body-wall of parent. 3, Section through young 

 embryo, to show protective egg-shell. 4, Cross-section of older stage. 5, Longitudinal 

 section through embryo after rupture of egg-shell. 



extent, the egg surrounds itself with a firm horny coating, serving 

 as an efficient protection, and, invested in this, falls from the 

 parent animal into the mud at the bottom of the pond or stream, 

 where, in a dormant state, it is able to survive the winter, which is 

 not usually the case with the Hydra itself. The development is 

 completed in the following spring, when the egg-shell splits and 

 the young animal makes its way out. In many groups of the 

 animal kingdom, it is a common phenomenon for " winter eggs " 

 of this kind to be produced, and the extinction of the species 

 during the cold season is thereby prevented. A comparison may 

 well be drawn with the seeds of higher plants, in which dormant 

 embryos are contained that are able to resume growth with the 

 advent of warmer weather. 



An interesting property possessed by Hydra is its power of 

 regenerating parts which have been injured or removed. To such 



