20 CCELENTEEATA. 



Class II. HYDKOZOA. 



HYDROMEDUSJS. 



Simple or compound organisms, the individual (polypite or 

 hydranth) consisting of a sac composed of an outer (ectoderm) 

 and inner membrane (endoderm), and enclosing a stomach-sac 

 not differentiated from the general body-cavity, the opening fur- 

 nished with tentacles. 



These organisms are nearly all marine ; they are almost in- 

 variably soft and gelatinous, occasionally with a chitinous covering 

 (perisarc). Reproduction is either by ova or by zooids, partially 

 independent organisms produced by gemmation or fission ; but 

 sexual communication is requisite at a certain period, or new sex- 

 organs may, after several generations, he developed [alternation 

 of generations]. The reproductive organs are exterior to the 

 body. The digestive cavity communicates directly with the 

 general body-cavity, the outer wall of which is in contact with 

 the water in which the animal lives. 



The Hydrozoa are, if we exclude the Ehabdophora, divided 

 into three subclasses : 



Attached to foreign bodies, sometimes in fresh 



water HYDROIDA. 



Free and oceanic. 



Polypites attached to a disk, float, or body- 

 sac SIPHONOPHORA. 



A single polypite suspended from the disk... DISCOPHOKA. 



Subclass I. HYDKOIDA. 

 HYDROPHORA. 



flydrosome fixed, consisting of numerous polypites united 

 together in a branched or tree-like form, and originating from 

 a single polypite, rarely the polypite maintaining a solitary 

 existence. The reproductive elements mostly medusiform. 



With a very few exceptions, the polypites'are united to form a 

 community or composite organism, which may include 100,000 

 individuals or person ae. 



The character of Hydroida is, according to Dr. Allman, "never 

 with a hydriform strophosome united with the gonosome into a 

 natatory column," which is directly opposed to the character of 

 Siphonophora. 



A community of Hydroids has to discharge two dissimilar 



