1 90 



PHYLUM CCELENTERA 



This development illustrates alternation of generations. From the 

 fertilised ovum a fixed asexual Scyphistoma results. This grows into a 

 Strobila, from which transverse buds or Ephyrae are liberated. Each 

 of these grows into a sexual jelly-fish, producing ova or spermatozoa. 



Relatives of Aurelia. — The Medusa?, or true jelly-fish, include 

 forms which agree with the Anthozoa in relative complexity of 

 structure as compared with Hydrozoa, and in the possession of 

 an ectodermal gullet, but differ in possessing ectodermal septal 

 muscles and in some histological features. If Goette's discovery of 

 rudimentary ectodermal muscles in the larvae of certain sea-anemones 

 be confirmed, however, it would greatly increase the probability of 

 a close relationship between the two sets. Among the Scyphomedusae 

 closely allied to Aurelia some, e.g. Pelagia, have a direct development 

 without the intervention of Scyphistoma or Strobila stages, but this 

 may occur exceptionally in Aurelia. Cyanea is often very large, 

 " it may measure j}, ft. across the bell, with tentacles 120 ft. long." 

 Chrysaora is hermaphrodite, and has diffuse sperm sacs even upon 

 the arms. In the Rhizostomae, e.g. Cassiopeia and Pilema, the 

 mouth is obliterated, and replaced by numerous small pores on the 

 four double arms. Lucernaria and its allies are interesting sessile 

 forms which have been compared to sexual Scyphistomas, that is, are 

 regarded as persistently larval forms (Figs. 96 and 97). 



Contrast between Medusoids {Hydromedusce) and 

 Medusce {Scyphomediisce) 



Medusoids. (Craspedota.) 



The majority are small " swimming- 

 bells." 



A flap or velum (craspedoii) projects 

 inwards from the margin of the bell. 



No taeniolcB, nor gastric filaments. 



A double nerve-ring around the margin . 



Naked sense organs either optic or 

 "auditory." They are usually derived 

 from the skin, but the " auditory " 

 sacs may be modified tentacles. 



Reproductive organs on the radial canals 

 or by the side of the manubrium. 

 The reproductive cells are usually 

 derived from the ectoderm. 



With the exception of the Trachy- 

 medusae, all arise as the liberated 

 reproductive personae of liydroid 

 colonies. 



{N.B. — "Auditory" organs are prob- 

 ably for balancing or equilibration.) 



Medusa. (Acraspeda.) 



Many are large " jelly-fish." 



No velum. (The velarium of Aurelia 

 is a mere fringe, very inconspicuous 

 in tlie adult, and not inturned.) 



In the Scyphistoma there are four 

 taeniolffi, from part of which the 

 gastric filaments of the adult grow. 



Eight separate nervous centres beside 

 the sense organs, and a sub-umbrellar 

 nervous plexus. 



Sense organs are modified tentacles, 

 and probably have almost always 

 a triple function. They are usually 

 protected by a hood. 



Reproductiv'e organs in special pockets 

 on the floor of the gastric cavity. 

 The reproductive cells arise in the 

 endoderm. 



Have no connection with hydroids, but 

 may have a small sedentary polyp- 

 stage (or Scyphistoma) in the course 

 of their life-history. 



Probably more nearly related to 

 Anthozoa than to Hydrozoa. 



