RADIATA. 



177 



and whose bodies are dilated. Some possess, also, a sac 

 filled with air, which acts as a float, as the Physalia, 

 (physa, a bubble,) or Portuguese 

 Man-of-war, whose purple-crest- 

 ed air-sac and long tentacles at- 

 tract such attention in tropical 

 seas, and whose thread-cells in- 

 flict such painful stings when 

 grasped by an incautious hand. 

 (Fig. 77.) The Porpita {porpc, 

 the ring of a shield) possesses 

 an internal skeleton, or flat 

 plate, of cartilaginous texture, 

 which is cellular and lighter than 

 water. Its lower surface con- 

 tains a beautiful fringe of blue 

 tentacles, or cirri. In the Vel- 

 ella (velella, a little sail) a second FlG> 77 '- physalia ' 



cartilaginous plate rises nearly at right angles from the 

 upper surface of the horizontal one, serving as a sail to 

 waft the little mariner from place to place. 



CLASS II. AcTINOZOA, (actin, a ray; zoon, an animal.) 

 This class embraces the Sea-anemones, the Corals, and 

 the Ctenophora, (kteis, a comb ; phero, I bear,) or comb- 

 bearing Medusae. The digestive cavity is suspended in 

 the body cavity, like a small bag within a larger one, by 

 vertical partitions, some of which extend from the body- 

 wall to the digestive sac, but others fall short of it. 

 Upon these septa, or mesenteries, are the organs of re- 

 production. The ectoderm is more highly developed 

 .than in Hydrozoa, and both mesenteries and body-walls 



