INVERTBBRATA. 383 



our metazoan types in having no permanent organs of 

 reproduction. It does, however, reproduce itself freely if 

 well fed, and the common method can easily be watched. 

 It is known as budding, and is not unlike the branching of 

 a plant : in fact it simply differs from ordinary growth by 

 being localized and eventually discontinuous. Both ectoderm 

 and endoderm grow rapidly at one point, and a protuberance 

 containing a diverticulum of the en teron. appears at the 

 surface. This increases in length, tentacles develop one by 

 one at its distal end, and a mouth is presently opened in 

 the centre of them. Gradually the bud grows into the 

 form of a Hydra, the attachment to the parent becomes 

 constricted and the enteric cavities separated, and finally 

 the young Hydra is detached to look after itself. Several 

 buds may often be seen at once on a Hydra. In this way 

 Hydra will multiply fifty-fold in a few months. 



Again, if a Hydra be cut across, each half will soon grow 

 so as to restore its lost parts, and two Hydrae will thus be 

 formed. 



In both these cases, it is to be noted that ectoderm-cells 

 give rise only to ectoderm-cells, and endoderm to endoderm. 

 No exchange of character can take place between them. 

 One of the earliest observers of Hydra tried the ex- 

 periment of turning Hydra inside out, and thought the 

 animal could live as well as before in this new condition. 

 This is now known to be a mistake, as a knowledge of 

 the detailed structure of ectoderm and endoderm would 

 indicate. 



11. Sexual Reproduction. Asexual reproduction by 

 budding cannot apparently be continued indefinitely. As 

 in the case of Paramecium, complex nuclear changes seem 

 to be necessary (or, at least, advantageous) from time to 

 time : these are effected by sexual reproduction. The 

 gonads are developed from the interstitial cells of the 

 ectoderm, testes and ovaries being formed simultaneously 

 on the same Hydra the testes near the hypostome, the 

 ovary or ovaries nearer the foot. In each case there is 

 rapid multiplication of the interstitial cells till a slight 

 protuberance is seen on the surface ; this differs from a 



