SYSTEMATIC SURVEY — CTENOPHORA. 



153 



(Ascones), the simplest Ccelentera {Hydra)) and even by the simplest 

 "worms" (Turbellarians). 



If we begin in our survey with such a gastrula-like ancestor, the 

 probabilities are certainly in favour of the supposition that it was a free- 

 swimming organism. A gradual perfecting of the locomotor character- 

 istics might yield the two medusoid types of which we have already 

 spoken. But we know that the common jelly-fish Aurelia has a 



GENERAL SCHEME OF CCELENTERA. 



Predominantly Passive. 



Predominantly Active. 



C. Ctenophora, e.g. Beroe, 

 Venus' Girdle. 

 (Active climax.) 



B. 



ScYPHO-* 

 ZOA. 



II. Anthozoa or Actinozoa. 

 (Zoantharia) Sea-anemones 



and related corals. 

 (Alcyonaria) Dead - men's- 



fingers and related 



corals. 



The embryos are free -swimmers, and 

 a few adults also are locomotor. 



t iScyphomedusa: or 

 ' "^Acraspeda. 

 c. Adult Lucernarians 



usually attached. 

 b. Sedentary larval stage. 

 a. No fixed stage. 



c. Free embryos. 



h. Aurelia type of jelly-fish. 

 a, Pelagia type of jelly-fish. 



ANCESTRAL 



GASTR.EA. 



A. 



Hydro-- 



ZOA 



No fixed stage. 

 No fixed stage. 



Many Hydroid colonies. 

 (Campanularians and 

 Tubularians.) 

 Many Hydroid colonies, 

 whose reproductive per- 

 sons are not liberated. 

 Coralline Millepores. 



1. Trachy medusas (always locomotor). 



2. Siphonophora (locomotor colonies of 



modified medusoids}. 



3. Liberated reproductive "persons" 



of these colonies. 



4. No free stage, except as embryos. 



5. A male medusoid is known. 



V6. Hydra without any specially locomotor stage. 



prolonged larval stage which is sedentary, vegetative, and prone to bad. 

 If we suppose with W. K. Brooks that many forms, less constitutionally 

 active than others, relapsed into this sedentary state, with postponed 

 sexuality, and with a preponderant tendency to bud, we can understand 

 how polypes arose, and these of two types, one nearer the jelly-fish and 

 Lucernarians and leading on to sea-anemones and corals, the other 

 nearer the swimming bell type and leading on to a terminus in Hydra. 



