HYDRA 



309 



New England to the Gulf of Mexico. It may reach a foot in diam- 

 eter and has the appearance of a transparent umbrella. Many of 

 these animals produce a calcareous external skeleton called coral. 

 Both sexual and asexual reproduction are common. 



Class Anthozoa. — This class, of the polyp form, has two subclasses. 

 The group includes the corals and sea anemones. - 



Subclass Zoantharia. — This group has numerous paired septa, 

 typically occurring in multiples of six, and plain tubular tentacles. 

 It includes sea anemones and corals. 



CiJitsutiacQ. of body wall 



Stomach 



Qonad 



Sab-qenital p]t 



-\ Upper portion of 



manubrium 



r — Lateral mouth 



r^; Radial canal 



- - Jcib-umbrcl/a 

 space 



— Circular 

 muscle 



t^anubriurr)'" 

 (cut surface) 



Central mouth 

 Oral tentacles 



Fig. 108. — Cabbage-head jellyfish, Stomolophus, a very common form in the Gulf of 



Mexico. Bisected to show internal structure. 



Order Actinaria. — These anemones are usually solitary polyps; they 

 have many complete septa and numerous tentacles but no skeleton. 

 Sagartia, Ceria7ithus, and Metridium are common examples. 



Metridium usually lives attached to rocks or to solid bodies in 

 the water near shore, even in tide pools. They average about three 

 or four inches in height and two or two and a half inches in diam- 

 eter. The free end of the jar-shaped body is covered with tentacles 

 which are provided with nematocysts. The entire body can be 



