CCELENTERATA — HYDROZOA 



III 



union of which gives rise to the sessile colony, — the hydroid. 

 This alternation of generation may be represented thus : — 



1ST Generation 



2D Generation 



3D Generation 



Colony 



feeding polyps 



reproductive polyps — medusae 



males <C spermatozoa 



-[- New Colony 



females <[ ova 



There is evidence in the order Leptolinae, that this alterna- 

 tion of the fixed nutritive colony with the free-swimming sexual 

 medusa is being gradually abandoned. The medusa generation, 

 whose special function it is to develop spermatozoa and ova, 

 gradually becomes reduced to a few cells which, remaining 

 attached within the gonotheca, produce the sexual products. In 

 Tubularia the medusae are almost fully developed, but never 

 break loose from the parent. In Sertularia pumila the medusa 

 form is entirely lost. 



Sense organs. — The medusae often have eye-spots (ocelli) 

 at the bases of the tentacles ; these are often only a few pig- 

 mented cells, but at times (as in Lizzia) a cuticular lens is present. 

 Otocysts, too, are present upon, or near, the tentacles ; these 

 are partially open or closed pits inclosing one or more otoliths, 

 organic or calcareous in nature, and are probably mainly balanc- 

 ing organs. Both ocelli and otocysts are ectodermal in origin. 



Development. — The fertilized egg is a single cell ; this splits 

 into two cells, which in their turn divide again, and so on, until 

 they give rise to the blastula (a single layer of cells surrounding 

 a central cavity), and this develops by true invagination as in 

 the sponge into the gastrula. The gastrula is vase-like in shape, 

 with two layers of cells ; between the outer (ectoderm) and the 

 inner (endoderm) layer, and deposited by them, is a stiff gelat- 

 inous, non-cellular secretion, the mesogloea. The ectoderm 

 becomes ciliated, enabling the minute embryo to swim freely 

 about. When it comes to rest, it affixes itself by root-like pro- 

 cesses (hydrorhizae). The opening (blastopore) of the vase-like 

 gastrula becomes the mouth of the polyp, and a circle of out- 

 pushings around it gives rise to the tentacles. The adult is 

 thus practically in the gastrula stage with the body wall consist- 



