GYMNOBLASTEA 



137 



blastic type to be described for Tubularia. The creeping hydrorhiza 

 gives off branches, one of which is seen in the figure, and from these 

 numerous individuals are budded. Most of these are polyps 

 (hydranths), distinguished from those of the Calyptoblastea by the 

 fact that the perisarc stops short at the base of the polyp and does not 

 form a hydrotheca. The medusoid individuals take their origin directly 

 from the coenosarc each as a simple bud, within which is developed 

 a single medusa which eventually divests itself of a thin covering, 

 breaks from its stalk and swims away. Several may spring from the 

 same stem, but this may also bear normal polyps. There is here no 



4en.or. 



Fig. 115. Median vertical section through a polyp of Tubularia. hist, blasto- 

 style; can.c. circular canal; end.dig. digestive endoderm; end.vac. vacuolated 

 endoderm, forming supporting core of tentacles ',gnph. gonophores ; M. mouth ; 

 prs. end of perisarc; t. testis; ten.ab. aboral and ten.or. oral tentacles. Slightly 

 altered from Kukenthal. 



blastostyle, or polyp modified for budding off medusae, and this con- 

 dition, in which polyps and medusae belong to the same grade of 

 differentiation from the coenosarc, is possibly to be regarded as 

 primitive, that of Obelia as secondary. In Eudendrium an intermedi- 

 ate stage occurs. Medusae are budded off from the stalk of a normal 

 polyp, and as soon as this budding commences the polyp loses its 

 tentacles, diminishes in length and may be said to become a blasto- 

 style. 



Tubularia (Fig. 115) occurs as a colony of large polyps with long stalks 

 springing from a hydrorhiza of insignificant extent. At the base of the 



