216 CCELENTERATA 



remain permanently attached to the colony. They then degenerate into 

 'sporosacs,' which always lack mouth, tentacles, and velum (fig. 179), often 

 also radial and ring canals, so that at last there remains only the manubrium 

 (spadix) and the sexual organs, the latter enveloped by the rudiments 

 of the umbrella. Since medusae and sporosac replace each other in closely 

 allied species, a common name, gonophore, has been applied to both. 



This developmental history may be modified in two ways: either the 

 polypoid or the medusan generation may be suppressed. In the first 

 case we have polyps which reproduce both sexually and asexually, in the 

 other medusae whose eggs develop directly into other medusae. (A few 

 medusae may bud new medusa?.) Thus we can have four conditions: 

 (i) Polyps which produce sometimes asexually, sometimes sexually, but 

 always polyps; (2) Medusae which always produce medusae; (3) Polyps and 

 medusae in alternating generations; (4) Polyps and sessile medusae (sporo- 

 sacs) united in a" polymorphic colony. 



FIG. 180. American Trachy- and Narcomedusre. A, Liriope scutigera (after 

 Fewkes). B, Cunocantha octon^ria (after Brooks). 



The Hydrozoa are almost exclusively marine. The colonial forms occur 

 mostly on rocky coasts down to a depth of 100 fathoms, but have been found 

 in water 4000 fathoms deep. The medusas belong to the pelagic fauna. For a 

 long time the only fresh-water species known belonged to the cosmopolitan 

 genus Hydra, but more recently both hydroid and medusan forms have been 

 found in various parts of the world. 



The Hydrozoa may be classified according to characters, derived either 

 from the hydroid or the medusan stage. The former gives four groups: (i) 

 HYDRARIA. Polyps with asexual and sexual reproduction; no persistent colonies, 

 no perisarc, no gonophores (fig. 169). (2) TUBULARLY. Mostly colonial, 

 with perisarc but without hydrothecae. Reproduction by gonophores (medusas 

 or sporosacs, figs. 94, 172). (3) CAMPANULARI^;. Colonial, with perisarc and 

 hydrotheca. Reproduction by gonophores arising in special perisarcal en- 

 velopes, the gonotheca (figs. 171, 173). (4) HYDROCORALLINA. Colonial, 

 with massive, calcified perisarc, resembling coral. Reproduction by sporosacs 

 or rudimentary short-lived medusas (fig. 174). 



The characters derived from the medusas give five groups: (i) ANTHOMEDUS^E 

 (Ocellatas). Gonads on the manubrium; no statocysts; eyes usually present; 

 polyp generation present. (2) LEPTOMEDUS.E. Gonads on radial canals; 

 usually velar statocysts; polyp generation present. (3) TRACHYMEDUS^:. 



