PHYLUM COELENTERATA 137 



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

 this float. The types of polyps include : gastrozooids (nutritive or 

 feeding), dactylozooids -with nests of nematocysts and having long 

 tentacles (tactile and protective), gonozooids which are male, repro- 

 ductive zooids, and others which produce ova-bearing medusae. Swim- 

 ming bells (nectocalyces) often occur just below the pneumatophore. 

 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 example. Its sting 

 is quite poisonous ; bathers coming in contact with the trailing ten- 

 tacles, 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. The scyphomedusa has an eight-notched margin, 

 lacks the velum (therefore acraspedote), and has gonads connected 

 with the endoderm. The polyps have four longitudinal endodermal 

 folds, called taeniolae, which form gastral tentacles or filaments in 

 the medusa. These jellyfish have a complex system of branched radial 

 canals and abundant marginal tentacles as well as oral tentacles. 

 Several of the representatives of this class are thought by some 

 zoologists to exist generation after generation only as medusae, but 

 it may be that the polyp form has not been discovered yet, if it exists. 

 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 usu- 

 ally lack marginal sense bodies (tentaculocysts). The tentacles are 

 distributed perradially and interradially. Lucernaria and Haliclystus 

 are usually cited as examples. 



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

 with four interradial tentaculocysts. The tentacles are adradial and 

 perradial. They occur in the open sea. Pericolpa and Periphylla. 



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

 perradial tentaculocysts, interradial tentacles, and are chiefly tropical. 

 Charyhdea is an example. 



Order Discomedusae. — Scyphozoa whose medusae are dominant, 

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

 than seven feet in diameter. There are usually eight or more ten- 

 taculocysts perradially and interradially distributed on the margin 



