2o8 



PHYLUM CCELENTERA 



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 

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

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

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

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

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



«! — J?> i-V. '-^t»- f :> » n -DK ' 



■ Fig. 113. — Commensalism of sea-anemones and hermit-crab. 



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

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

 It is certainly suggestive that we have jelly-fish wholly free {Pelagia), 

 jelly-fish with a sedentary larval life {Aurelia), jelly-fish predominantly 

 passive (Lucernaria), and related polyps (Sea-anemones, etc.), which 

 only occasionally rise into free activity ; while in the other series we 

 have medusoid types always free (Trachymedusaa), others which are 

 liberated from (Campauularian and Tubularian) sedentary hydroida, 

 other (Sertularian and Plumularian) zoophytes whose buds though often 

 medusoid-like are not set free, and finally Hydra, which, though it 



