HYDRA 311 



nies of these animals can produce enormous stony barriers (reefs) in 

 the sea. Many corals are of beautiful colors. 



Order Antipathidea.— An order composed of branching colonies 

 whose individuals are joined by a branched tubular axis which is 

 covered by an epidermal layer. Cirripathes and Antipathes are typi- 

 cal examples. 



Many authors recognize Zoanthidea, C erianthidia, and Edwardsi- 



idea as orders in this subclass also. 



Subclass Alcijonaria.—The features of this division include eight 

 hollow, feathered tentacles, eight mesenteries, and one siphonoglyphe. 

 Colonial and polymorphic forms are not uncommon. 



Order Alcyonacea. — A colonial group which has calcareous spicules 

 but lacks an axial rod. Body walls of individuals fuse together as 

 one. Organ pipe coral belongs in this order. 



Another order, Stolenifera with Cornulariella as an example, is 

 frequently set apart. 



Order Gorgonacea.— This is another colonial coral which is sessile 

 and has a calcareous axial rod. The common sea fan. Gorgonia, as 

 well as the precious Corallium ruhrum are well known examples. 



Order Pennatulacea. — Another colonial form whose body is modi- 

 fied so that one portion is submerged in the substratum. The colony 

 takes a bilateral form, and the individuals are born on a disc or axial 

 stem which is supported by a hard skeleton. Renilla and Pennatula, 

 sea pens and sea feathers, are typical examples. 



Habitat and Behavior of Hydra 



Hydra (ChloroJiydra) viridissima is likely the most common hydra 

 available. It is the small green hydra which is very active 

 and has short tentacles. Most of the hydras are found in cool fresh 

 water, attached to the surface of plant leaves, smooth sticks, debris, 

 or even the surface film of the water. The brown hydras, such as H. 

 americana, H. carnea, and Pelmatohydra oligactis, are sluggish and 

 have longer tentacles than the green ones.* 



Hydra is a sedentary kind of animal and may remain stationary 

 for a considerable period of time if living conditions are uniformly 

 good. When the environmental conditions are changing, and the 

 animal is in need of food, it becomes quite active, moving about 



♦Recent taxonomic information concerning Hydras of the United States may be 

 found ?r the papers of Libbie H. Hyman. published in the Transactions of the 

 American Microscopical Society, Vols. 48, 49, and 50. 



