152 ALTERNATIONS OF GENERATIONS. 



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

 Vesiculata of auditory vesicles. The latter seem to be usually buckled 

 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 blastochernes. He regards them as repre- 

 senting 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 Medusae. 



In many forms the medusiform buds though fully developed 

 do not become detached ; whether detached or not they are known as 

 phanerocodouic gonophores. In other forms again buds which 

 begin as if they were going to form Medusae never reach that condition 

 but remain permanently in an undeveloped 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 developed. The blastostyles 

 either remain exposed as in all the Gymnoblastic or Tubularian 

 Hydroids, or as in all the Calyptoblastic Hydroids (Sertularidae and 

 Campanularidse) 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, (1) 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 Hydromedusse is to be explained as hypo- 

 thetically suggested above, it is still necessary to shew that (1) the free 

 medusiform 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 medusiform gonophores. Unless these points can be 

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

 developed de novo and not by a modification of trophosorne 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 



