5 YSTEMA7IC SUR VE Y HYDROZOA. 



171 



(a) Hydrophora. Two types are included here. The first includes 

 the Tubularians, Hydractinia^ and other forms in which the polyps are 

 not enclosed in the protective perisarc which often surrounds the colony 

 (gymnoblastic), and in which the free medusoid forms, when present, 

 have their genital organs placed in the wall of the manubrium 

 (Anthomedusae), and are furnished with 



ocelli placed at the base of the tentacles. 

 Hydra and its allies may be included here. 



An unattached marine hydroid Hypolytus 

 peregrinus has been described, and as it 

 bore gonophores it was obviously mature, 

 which is doubtful as regards two other 

 unattached forms, Protohydra leuckartii and 

 Halermita cumulans, which may turn out 

 to be larval. The hydroid stages of Pelago* 

 hydra and Margelopsis are free-swimming. 

 Examples : 



Syncoryne sarstt, the free medusoid of 



which is called Sarsia tubulosa. 

 Bougainvillea ramosa liberates the 



medusoid Margelis ramosa. 

 Cordylophora lacustris and Tubularia 

 larynx have sessile gonophores or 

 sporosacs. 



The second type includes Campanularians 

 and Sertularians along one line ; Halecids 

 and Plumularians along another line. The 

 protective perisarc surrounding the colony 

 is continued into little cups (hydrothecse) 

 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 

 " vesiculate." 

 Examples : 



Plumularia, with hydrothecse on one 

 side of the branches, and Sertularia^ 

 with hydrothecae on both sides of the 

 branches. 



The Campanularian Obelia geniculata 

 liberates the medusoid Obelia geni- 

 culata. 



(b) Hydrocorallinae. Colonial lorms 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 complex 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, 



FIG. 88. Graptolites. 



I. Monograptus. 

 II. Diplograptus. 



