72 ZOOLOGY. 



thread-ring around the disk, and with ganglia near the sense-organs. Tn 

 Hydra the nervous system is represented by nervo-muscle cells ; sense- 

 organs usually present, represented by simple eyes and auditory vesicles 

 (lithocysts), the two not usually coexisting. Nettling organs (nematocystx) 

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

 dant in tlie tentacles. 



Tlie sexes rarely united, usually distinct. Often a high degree of poly- 

 morphism in the individual hydrosome, tlie animal being differentiated not 

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

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

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

 diating canals. Tlie species undergo eitlier 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 medusae, 

 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 Millepora and Ileliolites, a massive corallum. Suborder 1. 

 TubularioR (Hydra, Clava, Hydractinia, Millepora, Tubularia). 

 Suborder 2. Campanularm (Plumularia, Dynamena, Cam- 

 panularia, ^Equorea, Zygodactyla). 



Order 2. Discophora. Medusa? 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. TrncJiymedusce (^Egina, Cunina, Gery- 

 onia, Charybda?a). Suborder 2. Lucernarice (Lucernaria). 

 Suborder 3. AcalephcR (Pelagia, Cyanea, Aurelia, Rhizos 

 toma). 



OrderS. Siphonophora. Free-swimming, polymorphic hydrosotnes, 

 with nutritive, feeding, reproductive and locomotive zooids. 

 Suborder 1. Physophorm (Agalma). 2. Physalifp (Physalia). 

 3. Calycophorce (Diphyes) 4. Dixcnidew (Velolla, PrrpitaV 



