196 



ZOOLOGY 



happens near enough to touch one of the tentacles, it is seen 

 to stop suddenly as if paralyzed and is then grasped by the 

 tentacle and carried toward the opening. The hydra is simply 

 a hollow bag, the digestion of the prey taking place within it. 

 The waste products are passed out through the opening between 

 the tentacles where food is taken in. 



The hydra may change its position from time to time. The ani- 

 mal bends over, the base is unfastened, and the animal " walks '^ 

 on its tentacles to a new point of attachment. 



Structure of Body Wall. — The body wall of the hydra seen in a cross 

 section is found to be made up of two layers of cells. As in the sponge, the 

 inner layer serves for the purpose of taking in and digesting the food. This 

 layer is the endoderm. The outer layer of cells (called ectoderm) furnishes 

 the animal with weapons of offense and defense. This outer layer is also 

 provided with cells which are sensitive (sense cells). Between the inner and 

 outer layers of cells is a structureless substance called the mesoglea, in 

 which are found musclelike fibers, extensions of the protoplasm of the inner 

 outer layer. Scattered among these musclelike fibers are some of the cells of 

 the outer wall, irregular in shape, which have migrated in from the outer 

 layer. These cells are the nerve cells. They furnish the animal with path- 

 ways of sensation and provide a means of coordinated movement. 



Organs of Offense and Detense. — If the cells of the outer part 

 of the tentacle are examined under the microscope, we find how 



the animal is able to paralyze 

 its prey. Here are found many 

 cells, the bodies of which resem- 

 ble little bags. One end of the 

 cell projects from the outer sur- 

 face of the tentacle. These cells 

 are the cnidohlasts, or stinging 

 cells. Each cell has in its body 

 a small sac filled with an acid. 

 Attached to this bag and rolled 



Stinging cells (cnidoblasts); cnc, cnidocil; Up in it is a loug hollow dart, 



d., dart; n., nucleus; «., sac. (Drawing, Trrbiph of\r\ ViP p-5rr>pllprl whpn HiP 

 greatly enlarged, after Parker and Haswell.) ^^ICU CaU De expeiieQ WUeU tUe 



cell is " set off." This is done 

 by an animal or substance brushing against a triggerlike projec- 



