74 



THE LIVING WORLD. 



lower marine and fresh-water animals, a type scarcely 

 more than such a two-layered sac. The Coelenterata 

 is perhaps in some respects the lowest group of 

 multicellular animals, and among this group some 

 orders have a structure which is almost exactly that 



FIG. 7. Diagram of the structure of Hydra. It differs from the gastrula of 

 Fig. 6 only in having the body around the mouth expanded into tentacles, t. 



above described as characterizing the first multi- 

 cellular animal. Hydra (Fig. 7) is readily seen to 

 be simply a two-layered sac, with an opening at one 

 end. The functions of the two layers of Hydra are 



