PHYLUM COELENTERATA (cNIDARIA). SIMPLE TISSUE ANIMALS 



109 



higher animals in that they are not polar- 

 ized. 



The nerve cells of the epidermis lie just 

 external to the longitudinal muscle layer, 

 between the bases of the supporting cells, 

 and their processes interlace to form the so- 

 called nerve net which extends over the en- 

 tire body from the tip of the tentacles to 

 the basal disk. Formerly it was thought that 

 these processes were continuous from one 

 cell to the next, but careful studies have 

 shown that although the endings of fibers 

 lie very close to each other, they do not join. 

 Occasionally, however, nerve endings ap- 

 pear to be fused with other nerve cells, mus- 

 cle processes, or bodies of epitheliomuscular 

 cells. 



Response to stimuli. The nervous system 

 of higher animals is synaptic, that is, the 

 nerve cells are usually separate, and the im- 

 pulse must go from the endings of one nerve 

 cell to those of another. In a synaptic sys- 

 tem the direction of the nerve impulse is 

 controlled and is normally conducted in one 

 direction. In the hydra, there is probably 

 little directional control of nervous impulses, 

 and the resulting reactions are similar to 

 those that would take place if there were 

 only a protoplasmically continuous network 

 of nervous tissue. 



The greatest concentration of nerve ele- 

 ments occurs around the hypostome, where 

 the fibers pass in a circular direction to form 

 a loosely organized nerve ring. Another 

 somewhat similar concentration of nerve 

 fibers appears in the foot. 



The sensory cells (Fig. 53) consist of 

 slender, threadlike, specialized nerve cells, 

 lying in both epidermis and gastrodermis, 

 between the epitheliomuscular cells. They 

 frequently bear a hairlike process or some 

 other specialized structure at their tips. 

 Basally they usually divide into two or more 

 fibers which may connect either with the 

 nerve plexus or with muscle fibers. The sen- 

 sory cells of the gastrodermis are said to be 

 more abundant toward the foot region. The 

 epidermal sensory cells are found mainly on 



the hypostome and inner part of the tenta- 

 cles, and on the basal disk; these parts are 

 the most sensitive to external stimuli. 



The nematocysts 



These nematocysts (stinging capsules) 

 (Fig. 54) are present on all parts of the 

 epidermis of the hydra except on the basal 

 disk; they are most numerous on the tenta- 

 cles. Each is formed inside an interstitial 

 cell, which is then known as a cnidoblast. 

 On the general body surface, cnidoblasts are 

 mostly wedged in between the outer edges 

 of the supporting cells, but on the tentacles 

 and hypostome, cnidoblasts lie within the 

 bodies of the epitheliomuscular cells, which 

 are then known as host cells. On the tenta- 

 cles the host cells are large and each con- 

 tains a battery of stinging capsules, consist- 

 ing of one or two large nematocysts (pene- 

 trants) surrounded by a number of smaller 

 types. 



Four kinds of nematocysts occur in the 

 hydra as follows: (1) The largest is the 

 penetrant, which, before it is discharged, is 

 pear-shaped and occupies almost the entire 

 cnidoblast in which it lies. Within it is a 

 coiled tube, at the base of which are three 

 large and a number of small spines. Three 

 rows of minute spines spiral along the out- 

 side of the thread when discharged. (2) 

 Volvents are small pear-shaped nematocysts, 

 containing a thread, which, when dis- 

 charged, coils tightly around the hairs or 

 bristles of its prey. (3) The oval glutinant 

 is large and has a long thread that bears 

 minute spines. (4) Tlie small glutinant is 

 a straight unarmed thread. The first two 

 types are of special help in capturing prey; 

 the others secrete a sticky substance pos- 

 sibly used in locomotion as well as in food 

 getting. 



Projecting from the cnidoblast, near the 

 outer end of the nematocyst, is a hairlike 

 process, the cnidocil. Nematocysts may be 

 exploded by adding a little acetic acid or 

 methyl green to the water. The lid forming 

 the apex of the cyst is thrown off, and the 



