PORIFERA, CCELENTERATA, VERTEBRATA 91 



"In the hydroid polyps and their medusoids the 

 germ-cells always arise in the ectoderm ; in species 

 which produce sexual medusoids by budding, the 

 germ cells arise in the ectoderm of the manubrium 

 of these medusoids (Fig. 29, M, kz). But in many 

 species these sexual stages have degenerated in the 

 course of phylogeny into so-called gonophores, 

 that is, to medusoids which still exhibit more or less 

 complete bells, but neither mouth (m) nor marginal 

 tentacles {T)^ and which no longer break away 

 from the colony to swim freely about, to feed in- 

 dependently, and to produce and ripen germ-cells. 

 The degeneration of the 'gonophores' often goes 

 even farther ; in many the medusoid bell is repre- 

 sented only by a thin layer of cells, and in some even 

 this token of descent from medusoid ancestry is 

 absent, and they are mere single-layered closed 

 brood-sacs (Fig. 30, Gph). 



"The adherence of the sexual animal to the hydroid 

 colony has, however, made a more rapid ripening of 

 the germ-cells possible, and nature has taken advan- 

 tage of this possibility in all cases known to me, for 

 the germ-cells no longer arise in the manubrium of 

 the mature degenerate medusoid, that is, of the 

 gonophore, but earlier, before the bud which becomes 

 a gonophore possesses a manubrium. The birth- 

 place of the germ-cells is thus shifted back from the 

 manubrium of the medusoid to the young gono})hore- 

 bud (Fig. 29, M, kz). The same thing occurs in 

 species in which the medusoids are liberated, but live 

 only for a short time, for instance, in the genus 



