Hydra. 



33 



of the body, which consists of a simple stomach 

 cavity from which the effete matters are ejected by 

 the mouth. 



On watching the process of feeding we notice that 

 the arms exercise a power over living prey far greater 

 than we could anticipate from their size ; and on 

 close inspection these tentacles (as the arms are called) 

 are seen to be covered with minute oval sacs, whose 

 outer thin walls (fig. 19 B), are easily burst by pres- 

 sure, and when this occurs a long whiplash-like 

 filament (c) which lay coiled within the cell, is 

 suddenly projected, thus ren- 

 dering the tentacle a formidable 

 organ for seizing prey, their action 

 being either mechanical, or by 

 virtue, possibly, of some poisonous 

 fluid. 



The cells of the outer layer in 

 Hydra have, projecting inwards or 

 towards the endoderm, slender 

 thread-like contractile processes, 

 acting like muscular fibres. The 

 ectodermal cells resemble nerve A, 

 cells, and hence to this stratum the 

 name neuro-muscular cell-layer is 

 given. 



Reproduction. Hydras in 

 early summer send off from near the base of the body 

 small buds (fig. 2), which grow rapidly, each deve- 

 loping a mouth at its free end, together with a crown 

 of tentacles, then, being detached, it assumes a sepa- 

 rate existence. Sometimes a second crop of buds 



D 



Planula or earliest 

 stage of a hydroidonits 

 emission from the egg. 

 B, Thread-cell undis- 

 turbed. C, the same 

 with the filament pro- 

 truded. 



