PHYLUM CCELENTERATA 129 



Order 5. Hydrocorallinae. Colonial HYDROZOA with alter- 

 nation of generations and a massive or branching calcareous 

 skeleton into which the nutritive polyps (gastrozooids) and 

 protective polyps (dactylozooids) may be drawn. These HYDRO- 

 CORALLINE are often called corals and are found on coral reefs, 

 but they differ in structure fromvthe true corals (Figs. 86-91). 

 Example: Millepora. The staghorn coral (Millepora alcicornis) 

 occurs in Florida. 



Order 6. Siphonophora. Colonial free-swimming HYDRO- 

 ZOA with alternation of generations and highly modified (poly- 

 morphic) hydroid and medusoid members. Example: Phvsalia 

 (Portuguese man-of-war, Fig. 77). The hydroids and medu- 

 soids of the SIPHONOPHORA may be modified as shown in Tables 

 V and VI. 



3. CLASS II. SCYPHOZOA 



Most of the larger jellyfishes belong to the SCYPHOZOA. 

 They can be distinguished easily from the hydrozoan medusae 

 by the presence of notches, _ usually eight in number, in the margin 

 of the umbrella. They are called acraspedote (without velum 

 or craspedon) medusa? in contrast to the craspedote (with velum 

 or craspedon) medusae of the HYDROZOA. The SCYPHOZOA 

 range from an inch to three or four feet in diameter. They are 

 usually found floating near the surface of the sea, though some 

 of them are attached to rocks and weeds. There is an alterna- 

 tion of generations in their life-history, but the asexual stage 

 (the scyphistoma, Fig. 81, B) is subordinate. 



a. A Scyphozoan Jellyfish Amelia 



Aurelia (Fig. 79) is one of the commonest of the scyphozoan 

 jellyfishes. The species A^jlamdula^ ranges from the coast of 

 Maine to Florida. Members of the genus may be recognized 

 by the eight shallow lobes of the umbrella margin, and the fringe 

 of many small tentacles. 



In structure Aurelia differs from Gonionemus and other 



