300 



THE BIOLOGY OF HYDRA : 1961 



no more. The photograph of Campamdaria (Fig. 4) shows that all 

 hydranths are of the same size. The bud of the hydranth which will 

 be produced next is at the top, and proximal to this is the begin- 

 ning of the outgrowth of the next pedicel. 



Some species of both thecate and athecate hydroids are solitary, 

 and there are other species in which all hydranths arise only 

 from the attaching stolon (e.g. Htjclr actinia). Yet another pattern 

 of colonial growth, in which the growth zones are apical, is seen 

 in sertularians and plumularians— presumably the most advanced 

 of the thecate hydroids. These too provide challenging problems 

 for experimental morphologists but cannot be considered here. 



Fig. 3. Pattern of a small portion of a 

 colony of Pennaria. From a color photo- 

 graph by Charles Wyttenbach. 



Fig. 4. (right). Pattern of growth for 

 Campanularia flexuosa. From a color photo- 

 graph by Charles Wyttenbach. 



