CHAPTER VI. 



C(ELENTERATA '. 



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 plan u la. The planula is characteris- 

 tic of almost all the Hydromedusae with fixed hydrosomes 

 including the Hydrocoralla (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 8 and ends according to the usual accounts in the forma- 

 tion of a solid spherical mass of cells. A process of delamina- 

 tion 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 



1 I. HYDROZOA. 



\Hydroidca. 



1. Hydromedusse. \>/ rachymedu ^ 



2. SiphODOphora. \Cafycop**rM*. 



( Physophonda. 



3- Acraspeda. 



II. ACTINOZOA. 



1. Alcyonaria. (Octocoralla.) 



2. Zoanthaiia. (Hexacoralla.) 



III. CTENOPHORA. 



9 For a detailed description of the development of a single species the reader 

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



