THE HYDROMEDUSAE 



and can be thickened by the addition of layers from without 

 inwards. It can be reabsorbed by the agency of the ectoderm 

 cells which secreted it ; this occurs when a new bud grows out- 

 wards from the coenosarc, 



POLYMORPHIC MODIFICATIONS OF THE HYDROID. A blastostyle 

 (Fig. 1 6, m) is a hydroid which exhibits a greater or less simplifica- 

 tion of structure, in correlation with its special function of giving 

 origin to medusoids by budding. It may have a few small tentacles 



Fio. 19. Portion of colony of Bougainvillea magnified. 

 (From Lubbock, after Allman.) 



(Podocoryne), or the tentacles may be reduced to mere knobs 

 (Hydractinia, Fig. 22, b) or absent (Eudendrium). The mouth is 

 very small or absent. There seems to be no reason to deny the 

 name blastostyle to the elongated tubes which spring from the 

 hydroid of Tubularia, each of which buds numerous medusoids 

 (Fig. 24, b). The blastostyle may spring from the hydrorhiza 

 (Podocoryne), from the hydrocaulus (most Eudendrium), or from 

 the hydroid (Tubularia). 



A false blastostyle (Allman, 1 ; Weismann, 10) is formed by the 



