21C. 



CCELEXTERATA 



remain pcrmaiicntlv allached to tlic colony. Tlicy ihcii (Ici^cneratc into 

 'sporosacs,' which alwa\slack nioulh, tcniaclcs, and wlum i^lig. 170), often 

 also radial and riiij:; canals, so thai at last tliere remains only the manuhrium 

 (spadix) and the sexual organs, the latter eiwelopcd hy the rudiments 

 of tlte unrhrella. Since medusa- and sporosae rejilace each other in closely 

 allied species, a commoir name, i^oiio/^iiori-. has heen applied to liolh. 



This de\elopmental lrislor\- may he niodilied in two ways: citlrerthe 

 polypoid or the medusari Ljeneration. may he suppressed. In the hrst 

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

 other medus;v whose e,!;-gs de^•elop ilirectly into tuher medusa\ (A lew 

 medusx may luid new medusae) Thus we can ha\-e four conditions: 

 (i) Tolvps which produce sometimes asexually, sometimes sexually, luit 

 always polyps; (;) Medus;r which always produce medus;c; (3) Pol\-ps and 

 medusx in alternating generations; (4) Polyps and sessile medus;r (sporo- 

 sacs) united in a polymorphic colony. 



Fic 



-American Tracbv- and Xarcomc<Ui^:y. .1, Liriopc scittii^cra (after 

 Fe\\"l^es). B, CunucaiUlia ocloih-ria i,aftcr Brool^s). 



The Hydrozoa arc almost cxelusiyely marine. The colonial forms occur 

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

 in water 4000 fathoms deep. The niedu.sa- belong to the pelagic faiuia. For a 

 long time the only fresh-water species known lielongcd to the eosnio]iolitan 

 genus Hydra, but more receiulv both liydroid and medusan forms haye been 

 found in various parts of the world. 



Tlie Idydro.'.oa may be elassilied according to cliaraelers, derived either 

 from the h)'droid or the medusan sta,i,'e. The former j;i\es four groups: (i) 

 HvDtJARi.A. Polyps with asexual and sexual reproduction; no persistent colonics, 

 no pcrisarc, no gonophores ttii,'. lOg). (2) Tina i..\1nI.k. Mostly colonial, 

 with pcrisarc but without hydrotheeax Iveproduetion liv gonophores (medu-sjc 

 or sporosacs, figs. 04, 172). (:;) C.\»ii'Axui..Ai^ri-. Colonial, with pcrisarc and 

 hydrotheea. Reproiluetion by ,L,'onophorcs arising in special |>erisareal en- 

 velopes, the gonotheca (figs. 171, 173'). (4) ll\iiROcoR.\rrtJv.\. Colonial, 

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

 or rudimentary short-li\"ed mcdusx (fig. 174). 



The characters derived from the medusa- gi\e five groujis: (i) .VnttiOiMFDI'S.T', 

 (Ocellala-). Conads on tlie manubrium; no staloc\sts; ca'CS usually present; 

 polyp ,L;eneration [iresent. {2) LEPTOiMrnius.r:. Cionads on radial canals; 

 usually velar slatoc\-sts; polyp generation present. (_3) TR.f\CHYUKDiis.*:. 



