n 



ZOOLOGY. 



^read-ring a/round the disk, and with ganglia near the aeni>e-<rrgans. In 

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

 organs usuaUy present, represented by simjple eyes and amditory vesicles 

 (lith^cysts), ihe two not usually coexisting. NetUin^ organs {nematoeysts) 

 usually present, and especmWy chan-acteristie of the class, being most abun- 

 dant on the tentacles. 



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

 morphism in the individual hydrosome, the animal bein/j 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 attached to or dependent from the primary ra- 

 diating canals. Ttts species undergo either a slight or mairked metamor- 

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

 eggs, from which in some Disoophora (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, brapching, 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 

 kertularia, etc., protected by a horny sheath, or forming, as 

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

 2VJmZ(m^ (Hydra, Clava, Hydractinia, Millepora,Tubularia). 

 Suborder 2. Gampanularice (Plumularia, Dynamena, "Cam- 

 panularia, ^quorea, Zygodactyla). 



Ordenr 2. IHscophora.—MednssB 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. Trachymedusm (Mgms,. Cimina, Gery- 

 onia, Charybdsea). Suborder 2. iMcemarim (Lucemaria). 

 Suborder 3. Acalephm (Pelagia, Cyanea, Aurelia, Ehizos- 

 toma). 



Order S SipJtonophora. —FTee-swimming, polymorphic hydrosomes, 

 with nutritive, feeding, reproductive and locomotive zooids. 

 Suborder 1. Physophoros (Agalma). 2. Physalios (Physalia). 

 8. CaZycop^oTffi (Diphyes) 4. iJiscowfoffl (Velella, Pcrpita). 



