138 THE INVERTEBRATA 



polyp the stalk forms a swelling; there the perisarc stops. There is an 

 oral cone surrounded by a ring of tentacles and also a ring of larger 

 (aboral) tentacles at the broadest part of the polyp. Both kinds of 

 tentacles are solid, with an axis of vacuolated endoderm cells placed 

 end to end, which have a skeletal value. The reproductive individuals 

 originate from hollow branched structures springing from the polyp 

 itself between the oral and aboral tentacles. Each polyp has several 

 of these branches, and from each branch about half a dozen repro- 

 ductive individuals arise. The branch is usually termed a blastostyle^ 

 although it is only part of an individual and not a modified polyp as 

 in Obelia, Each of the buds it produces, however, has the structure 

 of a medusa but remains attached to the parent polyp as long as it 



end. lam 



^ten.on 



Fig. 116. Longitudinal sections through gonophores of Tw^w/ana. A, Young 

 male. B, Female with larvae. In B the details of tissues are omitted; in A 

 ectoderm is black, endoderm cross-hatched, end.lam. endoderm lamella; 

 act. actinula larva; pla. planula larva with rudiments of aboral tentacles; 

 mb. manubrium; ov. ovum; stk. stalk of polyp. Other letters as in Fig. 115. 

 Original. 



lives. Like the free-swimming medusa of Bougainvillea it conforms 

 to the anthomedusan type, the depth of the medusa bell exceeding 

 the width and the gonads being situated on the manubrium. This 

 sessile medusa is called a gonophore. As seen in Fig. 116 A, the 

 radial and circular canals are formed as in Obelia^ and four very short 

 tentacles occur opposite the radial canals on the margin of the bell ; 

 but the entrance to the subumbrellar cavity is very much constricted 

 compared to Obelia or a free-swimming anthomedusa. Another 

 modification is that the eggs, when liberated from the gonad, are 

 fertilized in the subumbrellar cavity and develop there through the 

 planula stage into an advanced larva called the actinula (Fig. 116 B) 



