STRUCTURE OF HYDRA 83 



have lived as attached organisms, not moving from place to 

 place in search of food. As a result of the attached mode of life 

 radial symmetry is supposed to have developed because of 

 equal pressure on all sides. Another group of organisms, 

 the Echinodermata (star fish, sea-urchins, sand dollars, etc.), 

 have partially acquired through attachment at some time 

 and in some past age, similar radial symmetry, but their 

 phylogenetic history is far more complex than that of the 

 Coelenterata. Both cases, however, are excellent illustrations 

 of the effect of the mode of life upon body forms of animals. 



HISTOLOGY 



Several different types of cells make up the two layers, ecto- 

 derm and endoderm, of Hydra. Excepting the reproductive 

 cells these several types are not bound together into definite 

 aggregates or organs, but are distributed over the entire or- 

 ganism, forming diffuse tissues. Between the two layers is a 

 structureless and non-cellular, gelatinous intermediate layer 

 termed the mesogloea or supporting lamella (Fig. 34). The 

 mouth is at the top of a small rounded or conical prominence, 

 called the hypostome, and lies in the center of the crown of 

 tentacles. It opens directly into the digestive cavity, thus 

 corresponding to the blastopore which opens into the arch- 

 enteron of the gastrula. Hydra and the coelenterates in general 

 are often called the diblastic or two-layered animals, as dis- 

 tinguished from the triploblastic or three-layered higher animals 

 made up of ectoderm, endoderm and intermediate layer, the 

 mesoderm. 



The different types of cells of Hydra perform their functions 

 for the good of the entire organism, and represent, morpholog- 

 ically, the incipient stages of organ systems in more highly dif- 

 ferentiated animals. 



A. ECTODERM CELLS. Six types of cells are present in the 

 ectoderm: (i) epithelio-muscle (neuro-muscle) cells; (2) nettle or 

 stinging cells; (3) nerve cells; (4) sensory cells; (5) germ cells; 

 (6) formative or interstitial cells. The bulk of the body cover- 



