170 



Finally, the higher Hydroids, such as iEginopsis, iEgineta 

 Cunina and Lyriope have been found by Muller, Agassiz, McCra.ly, 

 Leuckart, Gegenbaur, Fritz Muller and others, to develop directly 

 from eggs and pass through a metamorphosis as medusa?. Diiring 

 the past year (1874) Metschnikoil' has published, with many fig- 

 ures, an account of the development of Geryonia, Polyxenia 

 (iEgineta) and iEginopsis. 



In these animals the egg passes through a morula stage ; an 

 outer layer of cells (blastoderm) splits off from the morula, form- 

 ing the ectoderm and entoderm. The embryo, then, as in Polyx- 

 enia, passes through a ciliated planula stage. The embryo may 

 remain spherical, as in Geryonia, or as in Polyxenia and J-Igin- 

 opsis, the body of the planula becomes greatly elongated anc" 

 bomcrang-shaped, and from each end are developed the first two 

 tentacles, then others, and after a slight metamorphosis the adult 



The life history of the Hydroids comprises, then, the following 

 phases in development : — 



1, a. Origin of young Hydra by budding. 



b. Origin of embryo from egg u i ;ii;/.< ■ ! by spermatozoa. 



3. Planula (Gastrula) stage. 



5. Medusa, free and discharging eggs. 



II. THE MEDUSA (Discophora?). 

 Passing by the Lucernaria, beautiful and interesting, but of 

 whose early development we know nothing, we come to the common 



