CHAPTER VI. 



CCELENTERATA 1 . 



Hydroidea. THE most typical mode of development of the 

 Hydroidea is that in which the segmentation leads directly to the 

 formation of a free ciliated two-layered larva, known since Dalyell's 

 observations as a planula. The planula is characteristic of almost 

 all the Hydromedusse with fixed hydrosomes including the Hydro- 

 coralla (Stylasteridse and Millepora), the most important exceptions 

 being the genus Tubularia and one or two other genera, and the 

 fresh-water Hydra. 



In a typical Sertularian the segmentation is approximately regular* 

 and ends according to the usual accounts in the formation of a solid 

 spherical mass of cells. A process of delamination now takes place, 

 which leads to the formation of a superficial layer of cubical or 

 pyramidal cells, enclosing a central solid mass of more or less 

 irregularly arranged cells. 



The embryo, in the cases in which it is still contained within the 

 sporosack, now begins to exhibit slight changes of form, and one 



Hydroidea. 

 Tr achy medusa. 



*!. HYDEOZOA. 



1. Hydromedusse. 



2. Siphonophora. 



3. Acraspeda. 



II. ACTINOZOA. 



! Alcyonaria. (Octocoralla ) 

 2. Zoantharia. (Hexacoralla.) 



III. CTENOPHOEA. 



2 For a detailed description of the development of a single species the reader in 

 referred to Allman's description of Laomedia flexuosa, No. 149, p. 85 et seq. 



