II CNIDARIA 67 



Order 1. Hydridaa (Fresh-water polyps). 

 Single individuals or small stocks without envelopes consisting of a few similar 

 individuals. Reproduction asexual by gemmation, and sexual. Hydra develop direct 

 from the egg. Hermaphrodite. Sydra, in fresh water. 



Order 2. Hydromedusse. 



Hydroid colonies, which are at least dimorphic, since, besides the sterile nutritive 

 polyps, there arise by gemmation sexual persons, which either detach themselves as 

 Craspcdote Mcdusm and swim about freely, or remain united with the colony as 

 medusoid gonophores. In one series of Hydromedusce the attached Hydroid form is 



Pig. 62.— Bougainvillea ramosa (after AUman), with tudding Medusa, li, Nutritive polyps ; 

 mfc, Medusa buds ; m, detached young Medusa {Margelis ramosa). 



suppressed, as the Craspedote Medusa develops direct from the fertilised egg into 

 another ilcdusa. The systematic relationship of single fonns is naturally determined 

 both by the Hydroid and by the Medusa forms. As the whole life-history and 

 development of only a minority of the many species is available, and in many species 

 only the Hydroid form is known, in others only the Medusa form, a natural system 

 of the Hydromedusce is still a desideratum. 



Hydroid form. Medusa form. 



Sub-Order 1. Hydrocorallia. 

 Hydroid stocks, with calcified peri- AVanting. 



derm skeleton. The sexual products are 

 produced in gonophores. Stylastcr, 

 Millepora. 



