no 



ELEMENTS OF BIOLOGY. 



For these phenomena we can find no parallel amongst 

 plants. If we imagine, however, a tree which could detach 

 its flowers, and if we suppose these to be organised for an 

 independent existence, and to be capable of increasing in 

 size after their liberation, we should have very much the 

 state of things which we observe in Clytia. 



Still more extraordinary phenomena have been observed 

 in some others of the Hydrozoa, as in the Lncer7iarida. In 

 these, the egg gives rise to a minute, free-swimming, ciliated 

 body (fig. 32, «), which consists of two layers enclosing a 

 central cavity. Soon it becomes pear-shaped, fixes itself to 

 some solid body by its tapering extremity, and develops a 

 mouth and tentacles at the other extremity. It is now 

 known as the Hydra-tuba (fig. 32), from its resemblance in 

 form to the fresh-water polype or Hydra. The Hydra-tuba 

 has the power of multiplying itself by gemmation, and it 



a 



Fig. 32. — Development of one of the Lucer/iarida {Aitrelia). a Free-swimming cili- 

 ated embryo; b Hydra-tuba; c Hydra-tuba undergoing transverse fission; 

 d The same with the fission further advanced. 



can produce extensive colonies in this way ; but it does not 

 obtain the power of generating the essential elements of re- 

 production. Under certain circumstances, however, the 

 Hydra-tuba enlarges, and its body becomes constricted by a 



