308 ESSENTIALS OF ZOOLOGY 



drorhiza and powerful nematocysts (stinging bodies). Millepora, the 

 staghorn or stinging coral, as it is called, is a good example. 



Order SiphonopJiora. — This is a pelagic order of colonial coelen- 

 terates with extreme polymorphism. A common tube of the coenosarc 

 unites the five kinds of individuals of the colony, and this cavity is 

 continuous from one individual to another. The blind end of the 

 coenosarcal tube is an air-filled, bladderlike float (pneumatophore) 

 with a superior crest. The polyps hang down into the water beneath 

 this float. Most of the individuals are specialized to such a degree 

 that they care for only limited functions. This specialization and 

 diversity of forms is such that the entire colony appears as a single 

 individual. Physalia, the Portuguese man-of-war, is a typical ex- 

 ample. Its sting is quite poisonous; bathers coming in contact with 

 the trailing tentacles, which bear batteries of nematocysts, suffer 

 severe pain. 



Class Scyphozoa. — The coelenterates belonging here are large 

 jellyfishes having an alternation of generation in which the medusa 

 form is dominant. There are records of individuals of this group 

 twelve feet in diameter, and possessing tentacles one hundred feet 

 in length. 



Order Stauromedusae. — Conical or vase-shaped medusae which visu- 

 ally lack marginal sense bodies (tentaculocysts). Tessera, Lucernaria, 

 and Haliclystus are usually cited as examples. 



Order Peromedusae. — These are cup-shaped, free-swimming forms 

 with four interradial tentaculocysts. They occur in the open sea. 

 Pericolpa and Periphylla. 



Order Cuhomedusae. — Forms which have rather cubical shape, four 

 perradial tentaculocysts, interradial tentacles, and are chiefly tropical. 

 Chiropsalmus and Charyhdea are examples. 



Order Discomedusae. — Scj^hozoa whose medusae are dominant, 

 saucer-shaped and almost transparent. Some of them are more 

 than seven feet in diameter. Tentacles are usually present also on 

 the margin of the bell. This is the most numerous and extensively 

 distributed group of Scyphozoa. Aurellia and Stomolophus are com- 

 mon examples. 



Aurellia* is the typical example, and, like most jellyfishes, is 

 composed largely of water. This is a common one and ranges from 



•This spelling is according to Mayer's monograph. The generic name first pro- 

 posed by Peronas Le Sueur was so spelled. 



