CCELENTERATA 75 



several years. One kept in an aquarium in England 

 lived to be more than sixty years old. 

 There are two subclasses : 



1. Zoantharia, including the sea anemones and the 

 stony corals, and, 



2. Alcyonaria, to which belong the organ-pipe coral 

 (Tubipord), sea fan (Gorgonia\ the precious red coral 

 (Corallium), and the sea pen (Pennatula). 



Zoantharia usually have numerous tentacles, generally 

 arranged in multiples of five or six, the tentacles being 

 unbranched and hollow, while in the Alcyonaria the 

 tentacles are finely branched and are always eight in 

 number. 



Zoantharia. The best-known representative of this 

 group is the Metridium, or sea anemone. It usually 

 leads a solitary life, though frequently several are found 

 together, some of which have arisen as buds from the 

 others. It is capable of a slow locomotion. Muscular 

 fibers run around the body, and others* cross these at 

 right angles. The tentacles, which often number over 

 two hundred, and the partitions, which are in reality 

 double, are in multiples of six. At night, or when 

 alarmed, the tentacles are drawn in, and the aperture 

 firmly closed, so that the animal looks like a rounded 

 lump of fleshy substance plastered on the rock. It 

 feeds on crabs and mollusks. It abounds on every 

 shore, especially of tropical seas. The size varies from 

 one eighth of an inch to a foot in diameter (Fig. 236). 



Alcyonaria. The most of the animals in this group 

 grow in branching colonies, the axis consisting of a 

 horny substance covered with flesh in which spicules 

 of lime are found. The polyps are usually small. 

 The sea pen (Pennatula} grows with one end em- 

 bedded in the mud and sand of the sea bottom. In 

 Gorgonia, the sea fan, the branches arise in the same 



