58 



COELENTERATA 



[CH. 



pseudopodia emitted by the endoderm cells and bodily engulfed, 

 to be subsequently slowly digested in food vacuoles. Any insoluble 

 parts of the prey, such as cuticle, skeleton, etc., are ejected by the 

 mouth. Some of the endoderm cells also bear flagella, whose move- 

 ment doubtless aids the circulation of the fluid in the coelenteron. 



We have already seen that Hydra at certain seasons of the 

 year, viz., the late autumn, produces egg-cells (ova) and male 

 germs (spermatozoa). The latter are shed out into the water, and 

 eventually some of them reach the egg-cells and unite with them. 

 This process is called fertilisation, and the fertilised egg-cells 

 cover themselves with spiny coats and drop off into the mud. 

 Here they remain through the winter ; in the spring the hard coat 



cracks and out issues a minute Hydra. 

 But Hydra is by no means limited 

 to this method of sexual reproduction 

 in its power of multiplying itself. All 

 through the spring and summer, if it 

 be well fed, it buds or reproduces itself 

 by gemmation. A small swelling 

 makes its appearance on the side of 

 the body ; this is really a hollow pouch 

 containing a cavity in communication 

 with the coelenteron (4, Fig. 16). The 

 walls of the pouch are merely continua- 

 tions of the body- wall of the Hydra, 

 and hence consist of the same two 

 layers. The pouch rapidly lengthens, 

 and after a while a circle of tentacles 

 sprouts out from its free end, and a 

 mouth is formed in the centre. We 

 thus have a daughter Hydra still in 

 close connection with the parent, the 

 coelentera of the two being in open 

 communication; later, however, this 

 communication becomes closed and the 

 offspring separates from the parent 

 and leads a free existence. A third 

 method of reproduction, which probably 

 rarely occurs except artificially, is 

 fission. If a Hydra be divided into 

 two halves, each half will grow up into a new individual. 



FIG. 23. Obelia helgolandica 

 x 1. From Hartlaub. This 

 is the hydroid generation, 

 natural size as it appears 

 to the naked eye. 



