STRUCTURE OF HYDRA 81 



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 ex- 

 cellent illustrations of the effect upon body forms of animals of 

 the mode of life. 



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 somewhat 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 

 archenteron of the gastrula. Hydra and the coelenterates 

 generally are often called the diblastic or two-layered animals in 

 distinction to 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 morphologi- 

 cally, 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 covering 

 is made up of the epithelio-muscle cells, while sensory cells are 



