S YSTEMA TIC SUR VE Y CTENOPHORA. 



165 



(b] Madreporaria. Stone or reef corals. 



Astraa, Madrepora, Fnngia, Mceandrina. 



(c) Antipatharia. " Horny " black corals, with an axial 



skeleton. Antipathes. 

 (2) Alcyonaria or Octocoralla. 



Alcyonium (Dead - men's - fingers), Tubipora (Organ-pipe 

 coral), Coralliiim (Red coral), Gorgonia^ Pennatula (Sea- 

 pen), Monoxenia (non-colonial). 



Class IV. CTENOPHORA. 



Delicate free - swimming organisms, generally globular in form 

 moving by means of eight meridional rows of ciliated plates, or comb- 

 like combinations of cilia. The stinging cells are almost always 

 replaced by "adhesive cells." The mouth is at one pole, and leads 

 into an ectodermic gullet. The gastric cavity is usually much branched. 



FIG. 80. Hydroctena. A medusoid with hints 

 of Ctenophore structure. 



al'.o., aboral sensory organ ; 7\, retractile tentacle ; 

 v., velum ; M., mouth ; ST., stomach. 



The mesenchyme is very well developed, and includes muscular and 

 connective cells. At the aboral pole there is a sensory organ, including 

 an "otolith," which seems of use in steering. Here, also, there are 

 two excretory apertures. Except in Beroc and its near relatives, there 

 are two retractile tentacles. All are hermaphrodite. The development 

 is direct. They are pelagic, very active in habit, carnivorous in diet, 

 and often phosphorescent. According to Lang, they have affinities 

 with Planarian " worms," but this is very uncertain. 



F.xamples : 



(a) With tentacles, Cydippe and the ribbon-shaped Venus' Girdle 



{Cesium veneris). 



(b] Without tentacles, Beroe. 



