142 CCELENTERATA OR STINGING-ANIMALS. 



the hydro-polypes like Hydra, and the scypho-polypes like 

 Actinia, with each of which a medusoid type is associated, 

 while both are referable to a gastrula-like ancestor. Bud- 

 ding is common, alternation of generations is frequent, and 

 both the great orders include " Corals." 



A. Class Hydrozoa : — Orders Hydridse, Hydromedusas, 



Siphonophorae. 



B. Class Scyphozoa : — Sub-classes Scyphomedusse or 



Acraspeda, Anthozoa or Actinozoa. 



C. Class Ctenophora. 



A. Class HvDROzoA. 



There are two types, polypoid and medusoid, which 

 may be combined in one life-history. The mouth leads 

 directly into the gastric cavity. The mesoglcea is simple 

 and without migrant cells. The reproductive cells are 

 usually ectodermic. 



1. Order Hydridse. Solely polypoid, — Hydra, Protohydra, Micro- 



hydra, Polypodium. 



2. Order Hydromedusse. Polypoid colonies with special sexually 



reproductive persons, which are often liberated as medusoids. 



(a) Tubularians, in which the polypes are not enclosed in the protec- 



tive sheath which may surround the colony (gymnoblastic). 

 The sexually reproductive persons are often liberated as 



' ' ocellate " medusoids, Anthomedusse ; 

 or sometimes remain attached to the colony as fixed gono- 

 phores. 

 Examples : 



Syncoryne sarsii liberates the medusoid Sarsia tttbulosa. 

 Bougainmllea ramosa liberates the medusoid Margelis 



ramosa. 

 Cordylophora lacustris has no free medusoid. 

 Tubularia larynx has no free medusoid. 



(b) Campaiiularians, in which the polypes are enclosed in cups, 



continuous with the protective sheath which surrounds the 

 colony (calyptoblastic). 



The sexually reproductive persons are often liberated as 



" vesiculate " medusoids, — Leptomedusse ; 

 or sometimes remain attached to the colony as fixed gono- 

 phores. 

 Examples. — The hydroid Campanularia geniculata liberates 

 the medusoid Obdia geniculata. 

 Laomedea caliculata has no free medusoid. 



