CNIDARIA GENERAL 



77 



then two symmetrical septa in the posterior division. The rise of the 

 other septa does not by any means occur in all Anthozoa after one 

 and the same simple plan. (In most Actinaria the other septa arise in 

 pairs, each pair in the space between two older septa.) We repeat 

 what we said above, that the septa project the farther towards the 

 chief axis of the Corals the older they are. The tentacles also arise 

 symmetrically 'with reference to the median plane over the interseptal 

 spaces of the gastric cavity. 



The mesenterial thickenings, or mesenterial filaments, are in- 

 serted in all Corals at the free inner edges of the septa. 



sf. 



Sk. 



FIG. 67. NausithoS. pr, Perradii ; ir, interradii ; ar, adradii ; sr, subradii ; rl, marginal lobes ; 

 t, tentacle ; gf, gastral filaments ; m, circular muscle of the subumbrella ; sk, sensory bodies 

 (rhopalia) ; g, sexual glands (goriades) ; in the middle the cross-shaped mouth. 



As the free- swimming Craspedote Medusa can be referred back to 

 the Hydropolyps or Hydrula, so the free-swimming Acraspede or Scypho- 

 medusa (Figs. 67 and 70, p. 85) can be referred back to the Scypho- 

 polyp or Scyphula. The lower Acraspeda, with deeply vaulted, often 

 cup-shaped body, are only slightly distinguished from the Scyplmla, 

 and we find among them forms still attached (e.g. the Lucernaria). 



The 4 radial gastric pouches separated by septa are still present, 

 or they flow together to form a great circumferential sinus, the septa 

 dwindling into 4 small points of connection between the sub- 

 umbrellar and exumbrellar gastric walls, between which the circum- 

 ferential sinus remains in wide-open communication with the central 



