CCELENTERATA. 185 



mcdusiform gonophores are really only modified trophosomes, or rather that 

 the trophosomes and gonophores are both modifications of some common 

 type, and (2) that the fixed so-called adelocodonic gonophores are retrograde 

 derivatives of the free mcdusiform gonophores. Unless these points can be 

 established it might be maintained that the Medusae were special zooids, 

 developed dt novo and not by a modification of trophosome zooids. To 

 demonstrate these propositions at length would carry me too far into the 

 region of simple Comparative Anatomy, and I content myself with referring 

 the reader to a discussion of the Hertwigs (No. 146, p. 62) where the first 

 point appears to me fully established. With reference to the second point I 

 will only say that the structure and development of the adelocodonic gono- 

 phores can only be explained on the assumption 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 Trachymedusae, as has been shewn above, develop directly. They 

 are probably derived from gonophores in which the trophosome has dis- 

 appeared 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, gono- 

 phore free or attached. Ex. Gymnoblastic and Calyptoblastic 

 Hydroids, and Hydrocoralla. 



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

 sexual Medusa. Ex. Trachymedusae. 



Siphonophora. In the Siphonophora alternations of gener- 

 ations take place in the same way as in the Hydromedusae, but 

 the starting point appears to be a Medusa, The gonophores 

 may remain fixed or become detached. 



Acraspeda With the exception of Pelagia and Lucernaria, 

 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 Medusa 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 





