22 CCELENTEEATA. 



With a calcareous polypary. 



Polypary, when present, chitinous ... HYDROCORALLIN.S. 

 With a hydriforrn trophosome. 



Not permanently attached ELEUTHEROBLASTEA. 



Permanently attached. 



No hydrothecae nor gonangia GYMNOBLASTEA. 



Hydrotheoae and gonangia CALYPTOBLASTEA. 



Without a hydriform trophosome HAPLOMORPHA. 



Order I. ELETJTHEROBLASTEA. 

 HYDRIDA. GYMNOCHROA. PCECILOMORPHA. 



Hydrosome consisting of a single polypite, not permanently 

 fixed. Nutritiye buds at maturity discharging themselves and 

 then growing independently as free organisms. 



Only one genus is known, containing the common Hydra viridis 

 and two or three other species. They are found in fresh water, 

 and if cut up each piece will develop in a few hours into a perfect 

 animal. The body is tubular, capable of great extension ; and 

 its proximal end is furnished with a hydrorhiza, by which it can 

 attach itself at will to any foreign body ; the opposite end is pro- 

 Tided with tentacles, by which it secures its prey. 



In the sexual mode of reproduction ova are formed near the 

 fixed end, and spermatozoa, which are liberated at the same time, 

 are formed at the base of the tentacles. Hydra is sometimes 

 monoecious, sometimes dioecious. 



Hy&ridm. 



Hydra. 



Order II. GYMNOBLASTEA. 



CORYNIDA. TUBULARINA. ATHECATA. TuBULARIAN HYDROIDS. 



Polypites aggregated. No hydrotheeae nor gonangia present, 

 either for the polypites or the gonophores. 



These are delicate plant-like marine organisms, except Cvrdylo- 

 phora, attached to various foreign bodies, and developing buds 

 [gonophores], which often becoming detached, float away into a 

 free existence [planoblasts, or gonozoids], and are then known 

 under the general name of Medusas. 



The Medusoid gonophore is composed of a swimming-bell 

 (nectocalyx) with its inner margin produced into a delicate mem- 

 brane called the "velum," its outer margin bearing the tentacles. 



