204 



PHYLUM CCELENTERA 



and Halermita cumulans, which may turn out to be larval. The 

 hydroid stages of Pelagohydra and Margelopsis are free -swimming. 

 Examples : — 



Syncoryne sarsii, the free medusoid of which is called Sarsm tubulosa. 

 Bougainvillea ramosa liberates the medusoid Margelis ramosa. 



Cordylophora lacustris 

 and T uhularia 

 larynx have at- 

 tached gonophores 

 or sporosacs. 

 The second type includes 

 Campanularians and Ser- 

 tularians along one line ; 

 Halecids and Plumularians 

 along another line. The 

 protective perisarc sur- 

 rounding the colony is con- 

 tinued into little cups 

 (hydrothecae) enclosing the 

 polyps (calyptoblastic). 

 These hydrothecae are 

 stalked in Campanularians, 

 sessile in Sertularians and 

 Plumularians. The free 

 medusoids have their 

 gonads placed in the course 

 of the radial canals (Lepto- 

 medusae), and are either 

 " ocellate " or " vesicu- 

 late." 



Examples : — 



Plumularia, with hy- 

 drotheca; on one side 

 of the branches, and 

 S ertul aria, with 

 hydrothecae on both 

 sides of the 

 branches. 

 The Campanularian 

 Obelia geniculata 

 liberates the medu- 

 soid Obelia genicu- 

 lata. 



{b) Hydrocorallinae. — 

 Colonial forms which suggest the Hydractiniae in their polymorphism 

 and division of labour, but are distinguished by their power of 

 taking up lime, and so forming " corals." The colonies are com- 

 plex and divergent, the reproductive persons are either sessile 

 gonophores or simple medusoids. Millepora, Stylaster. 



(c) Trachymedusae. — These exist as a rule only in the medusoid form, 

 and are divided into two groups. Trachomtdusa! and Narcomedusae, 



Fig. 



no. — Campanularian Hydroid. — 

 After Allman. 



H., Hydrotheca or polyp-cup ; HY., hydranth or 

 polyp-head ; G., gonotheca, enclosing a repro- 

 ductive polyp producing medusoid buds ; M., a 

 liberated medusoid ; ST., basal stolon. 



