GYMNOBLASTEA 



159 



is very much constricted compared to Obelia or a free-swimming 

 anthomedusa. Another modification is that the eggs, which are large 

 and yolky, when Hberated from the gonad are fertiHzed in the sub- 

 umbrellar cavity and develop there through the planula stage into 



ten.or. 



\---prs. 



Fig. 117. 



Fig. 118. 



Fig. 117. Median vertical section through a polyp of T«6w/flna. 6/^f. blasto- 

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

 endoderm, forming supporting core of tentacles ; gnph. gonophores ; Mmouth ; 

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

 altered from Kukenthal. 



Fig. 118. Tiibularia with food. A, with stomach s, contracted and the basal 

 swelling sw. and the spadix of the gonophore sp. expanded. B, The reverse. 

 St. cavity of stalk. Arrows denote direction of fluid movement. From Beutler. 



an advanced larva called the actinula (Fig. 119B) which is really a 

 polyp of Tubularia with a short stem. At this stage it makes its way 

 out of the shelter of the gonophore and fixes by its aboral end. As a 

 rule, only one of these large eggs can be produced at one time and 



