CCELENTERATA. 



153 



of the adelocodonic gonophores can only be explained on the assump- 

 tion that they are retrograde forms of the phanerocodonic gonophores, 

 and that the opposite view, that the phanerocodonic gonophores are 

 derived from the adelocodonic, leads to a series of untenable positions. 



The Trachy medusas, as has been shewn above, develop directly. They 

 are probably derived from gonophores in which the trophosome has 

 disappeared from the developmental cycle. 



To sum up, three types of development are found amongst 

 the Hydromedusae. 



(1) No alternations of generations. Permanent form, a sexual 

 trophosome. Ex. Hydra. 



(2) Alternations of generations. Trophosome fixed, gonophore 

 free or attached. Ex. Gymnoblastic and Calyptoblastic Hydroids, 

 and Hydrocoralla. 



(3) No alternations of generations. Permanent form, a sexual 

 Medusa. Ex. Trachymedusse. 



Siphonophora. In the Siphonophora alternations of generations 

 take place in the same way as in the Hydromedusss, but the starting 

 point appears to be a Medusa. The gonophores may remain fixed or 

 become detached. 



Acraspeda. With the single exception of Pelagia, in which the 

 development involves a simple metamorphosis, all the Acraspeda 

 undergo a form of alternations of 

 generations. The ovum, as already 

 described, develops into a fixed form 

 the Scyphistoma which increases 

 asexually by normal budding, and can 

 even form a permanent colony. 



The formation of the sexual Me- 

 dusa form takes place by a kind of 

 strobilization of the body of the fixed 

 Scyphistoma. A series of transverse 

 constrictions becomes formed round 

 the body below the mouth, dividing- 

 it up into corresponding rings, each 

 of which eventually gives rise to a 

 Medusa known as an Ephyra (fig. 

 85). In each of these rings is a dila- 

 tion of the stomach, and a section of 

 each of the four rudimentary mesen- 

 teries described in connection with the 

 development of the Scyphistoma. As 

 the constrictions become deeper the segments of the body between 

 them become disc-like, and their edges are produced into eight lobes 

 containing prolongations of the gastric cavity (fig. 85 C). The lower 

 surface of each disc, which forms the future aboral surface of the 

 Medusa, becomes convex, in part owing to the development of 

 gelatinous tissue. On the opposite surface a muscular layer becomes 



A 



B 



FIG. 85. THREE STAGES IN THE 

 ALTERNATIONS OF GENEEATIONS OF Au- 

 RELIA AUEITA. (From Gegenbaur.) 



A. Polype stage. 



B. Commencing strobilization. 



C. Completed strobilization. 



