PHYLUM CCELENTERATA 

 112 



orders on the other hand, the polyp has disappeared. In the order 

 Siphonophora, including Physalia, the Portuguese man-of-war. 

 the colonics float freely on the surface of the sea, and the polyps 

 are of many different forms with specialised functions. 



A 



B 



C 



D 



Fig. 75. — Diagrammatic vertical sections of polyps and medusae. All are shown 

 with the mouth below, whatever the natural position. Endoderm close-hatched ; 

 ectoderm open-hatched ; gonads in solid black. 



A, Hydrozoan polyp ; B, hydrozoan medusa ; C, hydridan polyp ; D, actinozoan polyp ; E, scyphozoan 



medusa. 



CLASS II—SCYPHOMEDUSM 



The medusa is the predominant phase, but there is also a polyp 

 which forms the medusae by transverse fission. The tentacles are 

 solid, but otherwise the characters are the opposite of those of the 

 Hydrozoa, that is to say, the enteron has vertical partitions ; 

 there is no velum or nerve ring ; the gonads are formed in the 

 endoderm ; and there is no skeleton. Beneath the gonads are 

 sub-genital pits in the ectoderm of the subumbrella. 



A typical member is the common jellyfish, Aurelia aurita, 



