HYDEA 



115 



is composed of two layers of cells. The cavity of the sac (C) is 

 the digestive or gastral cavity (enteron), and it has only a single 

 opening to the exterior, the mouth (A), usually surrounded hy a 

 ring of tentacles (G). The wall of the sac is solid and there is 

 no body cavity or coelom surrounding the digestive tube as in 

 higher animals (Ccelornata). The outer cell layer (D) of the body 

 wall is the ectoderm (epiblast of the embryo), the inner (F) is the 



N, 



FIG. 58. A small portion of a thin longitudinal Section through the Body 

 Wall of Hydra viridi*, x 800. (From Marshall and Hurst's " Practical 

 Zoology.") 



A, a large ectoderm cell ; B, its nucleus ; C, its muscle process ; D, an undischarged 

 thread cell ; E, its trigger process ; F, a thread cell with discharged thread ; G, inter- 

 stitial cells ; H, mesoglcea ; I, endoderm cell ; K, vacuole ; L, nucleus of endoderm 

 cell; M, green algal cells living in the endoderm cells; N, flagellum of endoderm cell. 



endoderm (hypoblast of the embryo) and between the two is a 

 gelatinous, non-cellular supporting lamella, the mesogloea (E). 



Hydra itself is a very small form, but easily recognizable by 

 the naked eye. The body is long and slender or short and thick, 

 according to its state of contraction, and the same is true of the 

 tentacles, which may be visible as mere knobs around the mouth 

 at the unattached end of the animal, or as long slender threads 

 extended through the water like fishing lines and serving for the 

 capture of the minute organisms upon which the Hydra feeds. 

 The mouth is situated on the top of a conical projection, 



i 2 



