THE CQALENTERATES 31 
provided with barbs near its base. On the outer extremity 
of the nettle-cell projects a delicate bristle-like process, the 
trigger hair. These cells are especially abundant on the 
tentacles (Fig. 16, A, D), forming close, knob-like eleva- 
tions or “batteries,” thus rendering it practically impossi- 
ble for any free-swimming organism to avoid touching them 
in brushing against the body. In such an event the dis- 
turbances conveyed through the trigger hair set up in some 
unknown way very rapid changes in the cell. This causes 
the sac to discharge the coiled thread and barbs into the 
body of the intruder, which is rendered helpless by the par- 
alyzing action of the fluid conveyed through the thread. 
Thus benumbed it is rapidly borne to the mouth and swal- 
lowed. In time new nettle-cells develop to take the place 
of those discharged and consequently worthless. 
32. Digestion of food.—Upon the interior of the body of 
Hydra and all of the ceelenterates the food, by reason of its 
large size, is incapable of being taken into the various cells. 
It is necessary, therefore, to break it up into smaller masses, 
and this is accomplished through the solvent action of the 
digestive fluid poured over it from some of the cells of the 
adjacent inner layer. When subdivided, the granules swept 
about the gastric cavity by the beating of the flagella (Fig. 
16, D) are seized by the processes on the free surfaces of the 
remaining inner layer cells, where they undergo the final 
stages of digestion ; then in a dissolved state they become 
absorbed and assimilated by all the cells of the body. 
33. Methods of multiplication.—Very frequently, espe- 
cially if the Hydra has been well fed, two or three pro- 
cesses arising as outpushings of the body wall may be 
noted upon the sides of the animal (Fig. 16, A, D). If 
these be watched ‘from time to time they are found to in- 
crease in size, and finally, upon their free extremities, to 
develop a mouth and surrounding tentacles. Up to this 
point growth has taken place as a result of the assimilation 
of nutritive substances supplied from the parent ; but a con- 
