Glass I. Hydrozoa. Order 1. Hydromedusce. 107 



tlie power of budding, but never forms a colony, since tlie new individuals soon 

 separate from tlie parent. Tbe medusoid generation is represented by wart-like 

 outgrowths from the body-wall in which ova or spermatozoa develop. Hydra 

 is noted for its enormous power of regeneration ; an individual may be cut 

 into several pieces, each of which can develop into a complete animal. 



Order 2. Siphonophora. 



The Siphonophora are closely allied to the Hydromedusse, 

 differing from them, primarily, in that the colonies are free-svsrinuning. 

 The colonies correspond with those of the Hydroids, and like them 

 consist of variously modified polyps. Further, they also bear 

 medusae or medusa-buds, which again may develop in various 

 ways. These diverse individuals are borne upon a common stem 

 (hydrosoma), which is usually either a long tube or a flat disc ; at 

 the upper end it is furnished with an air sac (pneumatocyst), or 

 if it is discoid, it encloses several small air spaces, which enable 

 the colony to float. The hydrosoma is to be regarded either as a 

 much elongated, or a much flattened polyp ; of which the air sacs 

 are invaginations, each communicating by a flne aperture with the 

 exterior. 



The polypoid generation occurs in the following principal 

 forms : — 1 . Nutritive persons (gastrozooids) , saccular polyps, 

 with a mouth, and with only a single tentacle* arising close to its base ; 

 which, however, attains a considerable length, and is provided with 

 lateral branches and numerous "batteries" of nettle cells; this 

 tentacle may be absent. The gastrozooids of the same stock may 

 sometimes vary considerably in size. 2. Peelers (hydrocysts), 

 with tentacles like the gastrozooids, but without a mouth. 

 3. Tentacle-like persons (dactylozooids), which arise 

 independently from the stem, and must not be confused vn.th 

 tentacles ; they have no mouth and are furnished with nettle 

 cells.t 



The medusoid generation arises either from the hydrosoma 

 of the colony at the base of the hydrocysts, or of the gastrozooids; 

 it appears in the following forms: (1) Fertile medusae (gono- 

 zooids), or medusoid buds with sexual cells, corresponding exactly 

 to the medusoid generation of the Hydromedusse ; if the medusas are 

 set free — a rare occurrence among the Siphonophora — they look 

 exactly like the free Medusae of the Hydromedusee (they are provided 

 with a velum, etc.) ; as a rule however, they remain in situ 

 throughout life, and then resemble the sessile medusae of the 



* In certain Hydroids also each polyp possesses only a single tentacle. 



f Suoh tentacle-like individiials occur at the margin of the discoid stem in the 

 genus Porpita (Fig. 63). 



