72 ZOOLOGY. 



fhread-ring araiind the disk, and wiXh ganglia near the sense-organs. In 

 Hydra the n^rtous sysUm, is represeriied by nerco-m/ascle reUs ; sense- 

 organs usually present, represented by simple eyes arid auditory tesides 

 (Uthoeysts), the two not tLsually coexuitlng. Xeltling organs (nematocysU) 

 usually present, and especiaMy characteristic of the class, being most abun- 

 dant in the tentacles. 



The sexes rarely united, uswiUy distinct. Often a high degree of poly- 

 morphism in the individvM hydrosome, the animal being differe atinted not 

 only into polypites and gonosomes, but, in the free-sicimming forms, into 

 locomotive zooids. Heproduction takes place by haddiag, and by fertilised 

 eggs developed in glands attached to or dependent from the primary ra- 

 diating canals. Tlie species undergo either a sUghl or marked metamor- 

 phosis, the free gonophares being Tnedv-^m (or m^diisoids), which produce 

 eggs, from, which in some Discophora {such as Aurelia) arise successif:ely 

 a m/rrula, gaMrula, filanvla, sayphistoma, strobHa, and adult medusa, 

 representing distinct stages of grmclh. 



Order 1. Eydroidea. — The individual either not differentiated into 

 zooids, as in Proiohydra and Hydra, or consisting of nutri- 

 tive and reproductive zooids forming a compound, station- 

 ary, branching, moss-like body (hydrosome), the medusa- 

 buds remaining on the gonosomes or becoming free medusae, 

 with usually four simple radiating canals, a velum, manu- 

 brium, and naked eyes. Hydrosome either naked or as in 

 SertuUiria, etc., protected by a homy sheath, or forming, as 

 in Mittepora and HduHHes, a massive corallum. Suborder 1. 

 r«4«?(77^iE (Hydra. Clava, Hydractinia, Millepora.Tubnlaria). 

 Suborder 2. CarnpanularitB (Plumularia, Dynamena, Cam- 

 panularia, JDquorea, Zygodactyla). 



Order 2. IHscophora.—y[.(iAwsss. like those of the Hydroids. but with 

 the four primarj- radiating canals usually subdividing into 

 numerous branches, the eyes more or less covered by a flap ; 

 the velum often absent ; often four genital pouches, dis- 

 charging eggs into the gastro-vascular cavity ; usuallv of 

 large size, and developing either directly from eggs, or, as 

 in Aurdia, passing through a gastrula, scyphistoma, and 

 strobila stage, not being developed from a hydra-like poly 

 pite. Suborder 1. Trachymeduscs (^gina, "Cunina, Gery- 

 onia, Chaiybdaea). Suborder 2. Lucernarim (Luce'rnaria). 

 Suborder 3. Aealephw (Pelagia, Cyanea, AureUa, Rhizos 

 toma). 



Orders SipJionophora. —Free-smmmiag. polymorphic hydrosomes. 

 with nutritive, feeding, reproductive and locomotive zooids! 

 Suborder 1. Physophorw (Agalma). 2. PhysalicB (Ph\-salia)! 

 3. Calycophorw (Diphyes). 4. Discoidea (TeleUa. Porpita). 



