G2 ZOOLOGY. 
thread-ring around the disk, and with ganglia near the sense-organs. In 
Hydra the nervous system is represented by nervo-musele cells ; sensé- 
organs usually present, represented by simple eyes and auditory vesicles 
(lithocysts), the two not usually coexisting. Nettling organs (nematocysts): 
usually present, and especially characteristic of the class, being most abun- 
dant in the tentacles. 
The sexes rarely united, usually distinct. Often a high degree of poly- 
morphism in the individual hydrosome, the animal being differentiated not 
only into polypites and gonosomes, but, in the free-swimming forms, into 
locomotive zovids. Reproduction takes place by budding, and by fertilized: 
eggs developed in glands attached to or dependent from the primary ra- 
diating canals. The species undergo either a slight or marked metamor- 
phosis, the free gonophores being meduse (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 meduse, 
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 Heliolites, a massive corallum. Suborder 1. 
Tubularie (Hydra, Clava, Hydractinia, Millepora, Tubularia). 
Suborder 2. Campanulariw (Plumularia, Dynamena, Cam- 
panularia, Aiquorea, Zygodactyla). 
Order 2, Discophora.—Meduse 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. Trachymedusw (A@gina, Cunina, Gery- 
onia, Charybdea). Suborder 2. Lucernarie (Lucernaria).. 
Suborder 8. Acalephe (Pelagia, Cyanea, Aurelia, Rhizos. 
toma). 
Order 8. Siphonophora. —Free-swimming, polymorphic hydrosomes,, 
with nutritive, feeding, reproductive and locomotive zooids, 
Suborder 1. Physophor@w (Agalma), 2, Physalie (Physalia). 
3. Calycophore (Diphyes). 4. Discoidee (Velella, Porpita). 
