138 MANUAL OF ELEMENTARY ZOOLOGY 



occur it can be averted, while old age cannot be averted in 

 the metazoan. Depression is an abnormal event, produced 

 by unnatural conditions of culture. In its earlier stages the 

 animals can be stimulated in such a way as to endow them 

 with a new lease of life. This can be done by shaking the 

 culture, or better by a change of diet, as by feeding with 

 beef-tea. After a time the stock can be put back to a diet 

 of hay bacteria and kept till there sets in a deeper depres- 

 sion, which is capable of being averted in the same way. 

 By one means and another (after a while beef-tea failed, 

 and brain and pancreas extracts had to be used) the life of 

 such a culture has been kept up for two years, but the effect 

 of unnatural conditions was in the long run too strong, the 

 recurring periods of depression became more and more 

 severe, and at last the whole brood died. 



Among the most beautiful forms of pond life are the 



bell-animalcules, of which the scientific name 

 vorticeiia ; j s Vorticella. Various species of these creatures 

 Features. may be found as minute, colourless bodies 



fastened to weeds by stalks which contract at 

 the slightest disturbance of the water. Some of them also 

 appear in infusions. The body of a Vorticella is outwardly 

 shaped like a bell, but has no hollow within, the bell being 

 filled with a mass of protoplasm. In the place of the 

 handle is a long stalk, by which the animal is fastened to 

 some solid object. Animals which are thus fixed are said 

 to be sessile. The bell can be bent upon the stalk. The 

 wide end of the bell has a thickened rim, within which is a 

 groove known as the peristome. On one side there passes 

 from the peristome, down into the mass that fills the bell, 

 a tube known as the gullet. The first part of this 

 is wider than the rest and is sometimes called the 

 vestibule. The part of the upper surface which is 

 encircled by the peristome is known as the disc. It 

 is not level, but slopes, being raised on the side where 

 the gullet lies. The disc can be retracted, and the 

 rim of the peristome drawn inward over it. Around 

 the edge of the disc and down into the gullet two rows 

 of cilia wind spirally, the inner row long and standing 

 upright, the outer short and standing outwards. In the 

 gullet the members of the outer row are fused to form an 



