72 ZOOLOGY. 



thread-ring around the disk, and with ganglia near the sense-organ*. In 

 Hydra the nervous system is represented by nerw-muscle cells; sertee- 

 organs usually present, represented by simple eyes and auditory vesicles 

 (lithocyste), the two not usually coexisting. Nettling organs (nematocysts) 

 usually present, and especially characteristic of tJie class, being most abun- 

 dant in the tentacles. 



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

 morphism in the individual hydrosome, the animal being differentiated not 

 only into polypltes and gonosomes, but, in the free-swimming forms, into 

 locomotive zooids. Reproduction takes place by budding, and by fertilized 

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

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

 phosis, the free gonophores being medusae (or medusoids), which produce- 

 eggs, from which in some Discophora (such as Aurelia) arise successively 

 a morula, gastrula, planula, scyphistoma, strobila, and adult medusa, 

 representing distinct stages of growth. 



Order 1. Hydroidea. The individual either not differentiated into 

 zooids, as in Protohydra 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 medusas, 

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

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

 Sertularia, etc., protected by a horny sheath, or forming, as- 

 in MiUepora and Heliolites, a massive corallum. Suborder 1. 

 T-ubularm (Hydra, Clava, Hydractinia, Millepora.Tubularia). 

 Suborder 2. Campanularim (Plumularia, Dynamena, Cam- 

 panularia, ^Equorea, Zygodactyla). 



Order 2. Discophora. Medusfe like those of the Hydroids, but with 

 the four primary 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 ; usually of 

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

 in Aurelia, passing through a gastrula, scyphistoma, and 

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

 pite. Suborder 1. Tratfiymedusas (^Egina, Cunina, Gery- 

 onia, Charybdsea). Suborder 2. Lucernarm (Lucernaria). 

 Suborder 3. Acalephos (Pelagia, Cyanea, Aurelia, Rhizos- 

 toma). 



OrderS. Siphonopliora. Free-swimming, polymorphic hydrosomes, 

 with nutritive, feeding, reproductive and locomotive zooids. 

 Suborder 1. Physophorce (Agalma). 2. Physalice^Phys&lial. 

 3. Calycophoro) (Diphyes). 4. Discoideai (Velella, Pcrpita). 



