Metazoa Obeli a. 2 3 1 



naiced, often amoeboid cell ; thespermhas the typical head and 

 flagellum which are so characteristic of the male sexual cell. 



The fertilised ovum or embryo segments into a large 

 number of cells, which arrange themselves into a hollow 

 sphere. This is known as a blastula, and the central cavity 

 as the blastoccele or segmentation-cavity. From the wall- 

 cells, however, many cells are given off by budding until the 

 wall has been made two layers thick. The embryo is now 

 known as a planula, and the cavity left or afterwards 

 formed in it is known as the enteron or primitive alimen- 

 tary canal. The outer layer is that which will give rise 

 to the ectoderm, and is called the epiblast ; the inner 

 layer gives rise to the endoderm, and is known as the 

 hypoblast The epiblast cells at this stage are ciliated. 



The planula is a free swimming organism, using its cilia 

 as organs of locomotion. After a time the cilia disappear 

 and the embryo settles down on one end. It then forms an 

 adherent disc and begins to branch over the rock or sea- 

 weed on which it has fixed itself, elongating and sending 

 out buds, which develop into zooids like those already 

 described in the adult fixed form with which we started. 

 The perisarc meantime forms, and by a branching of the 

 stolon in all directions a new colony is produced. 



There are several points of great importance to be looked 

 at before we pass on to the consideration of a higher type. In 

 the first place we may emphasise the appearance of a differ- 

 entiation of the cells formed by segmentation of the ovum, 

 into two or, in the medusoid, possibly three layers, which 

 give origin in process of development to entirely different 

 series of organs. We may note especially the development 

 of the nervous elements from the ectoderm and the lining 

 of the alimentary canal from endoderm. Again, the occur- 

 rence of marked alternation of generations in both kingdoms 

 is worthy of thought. The early differentiation of tissues in 

 the animal and the appearance of the sensitive system are 

 also points of great importance. With regard to the latter 



