1 84 ALTERNATIONS OF GENERATIONS. 



Such Medusas are divided into two great groups, the Ocellata and 

 Vesiculata, according to the characters of the marginal sense organs. 

 In the Ocellata the sense organs have the form of eyes, and in the Vesiculata 

 of auditory vesicles. The latter seem to be usually budded off from the 

 Campanularia stocks, and the generative organs extend in folded bands over 

 the radial canals. These bands .have been regarded by Allman as composed 

 of rudimentary gonophores, and he called the Medusae which give rise to 

 them blastochemes. He regards them as representing a more complicated 

 type of alternation of generations with three instead of two generations in 

 the series. The Hertwigs have brought what appear to me conclusive 

 grounds for rejecting this view, and have demonstrated that the generative 

 organs of these types resemble those of ordinary Medusas. 



In many forms the medusiform buds though fully developed 

 do not become detached ; whether detached or not they are 

 known as phanerocodonic gonophores. In other forms 

 again buds which begin as if they were going to form Medusse 

 never reach that condition but remain permanently in an unde- 

 veloped state. They have been called by Allman adelocodonic 

 gonophores. 



In all the above cases two generations at the least interpose 

 between the successive sexual periods, viz. : 



(1) A trophosome produced directly from the ovum. 



(2) A gonophore budded from this. 



In a very large number of types the gonophores do not 

 develop directly on the hydroid stem, but arise on specially 

 modified zooids resembling rudimentary trophosomes which 

 have been named blastostyles by Allman. On the sides of 

 each blastostyle a series of gonophores usually becomes de- 

 veloped. The blastostyles either remain exposed as in all the 

 Gymnoblastic or Tubularian Hydroids, or as in all the Calypto- 

 blastic Hydroids (Sertularidae and Campanularidae) they become 

 invested by a special case known as the gonangium which 

 is formed of perisarc lined by epiblast. In the forms with 

 blastostyles three generations interpose between the successive 

 stages of sexual reproduction, (i) the trophosome developed 

 directly from the ovum, (2) the blastostyle budded from this, (3) 

 the gonophore budded from the blastostyle. 



Such being the main facts, in order to prove that the existing condition 

 of polymorphism amongst the Hydromedusas is to be explained as hypo- 

 thetically suggested above, it is still necessary to shew that (i) the free 



