Sub-Kingdom II. 



Metazoa. 



Phylum 1. Ccelentera. 



The Ccelentera are oMefly distinguished by tlie extreme simplicity 

 of their structure, and by the slight degree of differentiation in their 

 organisation. They occupy a more primitive position than any other 

 Metazoa known. 



Various types of Ccelentera are met with. In the simplest 

 case, the body consists of a longer or shorter sac, one end of which 



Fig. 58. Diagrammatic figures of the chief types of Ccelentera. A the simplest form, 

 B medusa, coral type, i endoderm, m mesoglsea, y ectoderm, n mouth, n' external 

 aperture of the stomadseum of a coral polyp. — Orig. 



is open, the other closed (Fig. 58 A). The body- wall consists of an 

 external layer of cells, the ectoderm; within this a thin 

 sheath of structureless jelly, the mesoglsea, and most internally 

 another cellular layer, the endoderm, lining the cavity of the sac ; 

 the ectoderm and endoderm are continuous at the open end of 

 the tube, the mouth. It is evident, therefore, that such a 

 Ccelenterate differs but little from a gastrula, the difference 



