CCBLENTERATA. 23 



Order 1. Stauromedusse. 



Conical or vase-shaped umbrella. No tentacu- 

 locysts. (Tessera.) 



Order 2. Peromedusse. 



Conical umbrella with transverse constriction. 

 Four inter-radial tentaculocysts. (Pericolpa.) 



Qrder 3. Cubomedusse. 



Four-sided umbrella. With per-radial tentacu- 

 locysts. Velum present. (Charybdea.) 



Order 4. Discomedusae. 



Saucer-shaped umbrella. Per-radial and inter- 

 radial tentaculocysts. (Aurelia.) 

 CLASS 3. Actinozoa. 



With a stomodseum, and with mesenteries ex- 

 tending into the coelenteron. Fixed forms. 

 Subclass 1. Zoantharia. 



Mesenteries and tentacles usually very numerous. 



Order 1. Actiniaria. 



Usually single. No skeleton. (Metridium. Sa- 

 gartia.) 



Order 2. Madreporaria. 



Usually form colonies and always have calcare- 

 ous exoskeleton. (Astrangia, Orbicella, Mean- 

 drina.) 



Order 3. Antipatharia. 



Tree-like. Mesenteries and tentacles compara- 

 tively few. Chitinoid skeleton. (Cirripathes.) 

 Subclass 2. Alcyonaria. 



Mesenteries and tentacles eight in number. Ten- 

 tacles branched. 



Order 1. Alcyonacea. 



Skeleton in the form of small, irregular bodies, 

 frequently calcareous spicules. (Alcyonium, 

 Tubipora.) 



Order 2. Gorgonacea. 



Tree-like, with calcareous or horny exoskeleton. 

 No syphonoglyphes. (Gorgonia.) 



Order 3. Pennatulacea. 



Colony with one end usually embedded in the 

 sea-bottom. (Pennatula, Renilla.) 

 CLASS 4. Ctenophora. 



Single. Pelagic. Eight rows of meridional 

 swimming plates. No nettle cells. 



