26 BRANCH CCELBNTERATA 



CLASS U. THE SCYPHOZO'A 



Jelly-fishes are soft umbrella-like creatures resembling molds 

 of jelly or gelatin, as one sees who picks them up along the beach, 

 where they have been cast ashore by the waves. Their tissues 

 are very watery, hence the scarcity of their fossil remains. 

 However, very perfect impressions of jelly-fishes are found in 

 the upper Jurassic Period. Most jelly-fishes are marine and 

 free swimming, though a few are temporarily attached. They 

 are most abundant in the tropics. Great schools of them are 

 sometimes seen. Sometimes they are phosphorescent. They 

 vary in size from about 4 mm., in the simple little, bell-like 

 Tessera, to 1 foot in the Aurelia, and 7 or 8 feet in diameter in 

 the Cyanea, whose tentacles sometimes reach the length of 

 130 feet. A small form, Gonionemus, found at Wood's Holl, 

 Mass., is green and about 1 inch in diameter. It grows on eel- 

 grass. All are carnivorous, feeding mostly upon crustaceans, 

 though some of the larger ones capture fishes of considerable size. 



The food is captured by the tentacles, which are suspended 

 from the margin of the umbrella and which are armed with 

 stinging thread cells. 



Locomotion is effected by the flapping of the umbrella-like 

 body, there being usually no velum. 



Minute colored " eye-specks " are around the rim. 



Multiplication usually is by alternation of generations, but 

 the young medusa or ephyra, as it is called, undergoes a meta- 

 morphosis or change of form as it matures. In some cases the 

 egg develops directly into the larval medusa and there is no 

 alternation of generations, but simple metamorphosis. 



CLASS m. ACTINOZO'A 



This class includes sea-anemones, sea-pens, and corals. 

 Only the polyp form is found in this class, no medusa being 

 known among them. They are exclusively marine. They are 

 usually fixed and many form permanent colonies. 



One point in their development is a step in advance of the 

 Hydrozoa, /. e., the development of a gullet, esophagus, or 

 stomodoeimi, the beginning of which is seen in the Scyphozoa. 

 The hypostome, which in Hydrozoa bore the mouth at its apex, 



