21 



each of which developed into a free- swimming Medusa-like 

 form, in which reproductive elements were produced. We 

 see this ancestral process repeated at the present day in the 

 life-history of the Discomedusse {e.g., Aurelia), where the 

 Scyphistoma stage (hydriform person) produces by trans- 

 verse fission a number of Ephyrae (medusiform persons). In 

 some Scyphomedusae {e.g., Pelagia), the hydriform stage has 

 become suppressed, and the ovum developes into a Medusa 

 directly, just as it does in the case of the Trachylarida. 



The ancestral Scyphomedusa must have developed solid 

 tentacle-like filaments projecting from the body wall into the 

 enteric cavity, as we find such gastral filaments* present in 

 all members of the Scyphomedusae. The Lucernarida are 

 probably more in the direct line of development than any 

 of the other Scyphomedusae, as they have retained an inter- 

 mediate condition between a Hydra and a Medusa form. 

 They do not produce Medusae by transverse fission (strobila- 

 tion), but develop genital organs from the endoderm. 



The Discomedusae are the typical large Medusae, with 

 often very complicated sense-organs (modified tentacles) 

 around the margin of the bell. The mouth may remain 

 simple or become greatly complicated (in the Khizostomag). 



The Cubomedusae and Peromedusae are two small groups, 

 which differ from the Discomedusae and from each other 

 mainly in the arrangement of the sense-organs and the 

 enteric cavities. They probably diverged from the base of the 

 Discomedusae. 



The Actinozoa were in all probability derived from 

 the ancestral Scyphomedusae, but they have attained a 

 more advanced condition both as regards their internal 

 cavities and also their general histology than is found in any 

 of the Hydrozoa (Hydromedusae and Scyphomedusae). Con- 



* Haeckel's "Phacellae," '• Challenger Zoological Reports, vol. iv, part xii. 

 The Deep-Sea Medusae, lutroduction, p. Ixxiii. 



