HISTOLOGY OF HYDRA 89 



la ted, and is frequently loaded with food substances taken into 

 the cell through the activity of pseudopodia which may also be 

 formed at the free ends. In Hydra viridis the nutritive cells 

 contain living algae (zoochlorellae) which give the green color 

 to the animal. At the basal ends the cells are developed into 

 muscle processes similar to those of the ectodermal cells, but 

 they are usually finer and shorter and run at right angles to the 

 long axis of the body. The contraction of these muscle processes 

 lengthens the animal while contraction of the longitudinal 

 muscle processes shortens it. 



2. Slime Cells. These are very numerous in the region of 

 the mouth and gullet where, as short cylindrical cells, they lie 

 between the nutritive muscle cells and usually at some distance 

 from the supporting lamella. They are usually filled with 

 granules which on discharge from the broader distal end form 

 a slimy secretion surrounding the prey taken in as food. 



3. Albumen Cells- -These are similar to the slime-forming 

 cells but are more widely distributed, and each cell is drawn out at 

 the base in a thin process which rests on the supporting lamella. 

 At the opposite free end they have, like the nutritive cells, two 

 or sometimes three long flagella. The large granules of secre- 

 tion usually lie in vacuoles. 



4. Sensory Cells. These fine thread-like cells are much more 

 numerous here than in the ectoderm. They may bear two 

 flagella, like nutritive cells, or only one flagellum, while oc- 

 casionally there is none at all. The basal ends of these cells 

 are prolonged into fine branching nerve processes which form, 

 with the nerve fibers, a plexus on the inner layer of transverse 

 muscle processes. 



5. Nerve Cells. These agree throughout with the ectodermal 

 nerve cells, and while less numerous are distributed in much the 

 same way, but are apparently absent from the endodermal 

 cell of the tentacles. 



6. The Formative Cells. These also are less numerous than 

 in the ectoderm, their chief reparative activity apparently being 

 the new formation of albumen cells, since nettle cells and re- 

 productive cells are absent. In regeneration after injury, how- 



