INVERTBBBATA. 395 



a modified hydroid zooid. The mouth is homologous in 

 the two cases, and so likewise are the tentacles. The sub- 

 umbrella therefore corresponds with the hypostome, the 

 central part projecting as the manubrium. The oral and 

 aboral layers of endoderm, instead of being separated by a 

 continuous cavity, have come into contact to form the 

 endoderm lamella, except where the central cavity and the 

 radial and circular canals occur. The mesogloea is very 

 much thickened. The original cavity is thus divided up. 

 The hydroid is elongated along the axis of symmetry which 

 passes through the oral and aboral poles, the medusa is 

 shortened along this axis and expanded in the plane which 

 passes through the bases of the tentacles. The medusa 

 therefore is a modified zooid which separates from the 

 gemmarium. 



9. Generative Organs or Gonads. The generative organs 

 of the medusa are four in number, one in the middle of the 

 course of each radial canal. They are formed by ventral 

 diverticula of these canals, which, pushing the mesogloea 

 and ectoderm before them, form projections from the sub- 

 umbrellar surface. The sexes are separate. The germ-cells 

 have been traced in the early development of the medusa 

 on the blastostyle as originating in the ectoderm of the 

 manubrium, but they afterwards migrate to the endoderm, 

 and then to the gonads, where before maturity they leave 

 the endoderm and lie between the ectoderm of the sub- 

 umbrella and the mesogloea. When ripe they escape 

 through the ectoderm into the sea-water, where fertili- 

 sation and development take place. 



10. Development of Ovum. The fertilised ovum seg- 

 ments to form a hollow blastula, which becomes solid by the 

 delamination of cells from the blastula wall. There is no 

 invagination and therefore no gastrula. The outer layer 

 of cells becomes uniformly ciliated and the embryo is now 

 called a planula. A cavity appears by splitting in the 

 solid endoderm. The planula is easily converted into a 

 single hydroid zooid. It attaches itself by one end to stone 

 or weed, forms a mouth by rupture at the other, and then 



