266 THE FOOD OF ANIMALS 



collar-cells, from each of which projects a thread of living sub- 

 stance (protoplasm) which constantly describes whip-like move- 

 ments, and has therefore been termed a flagellum (L. ftagellum, 

 a whip). By the combined action of all the flagella, currents are 

 produced in the surrounding water, which flow through the holes 

 in the body-wall and then to the exterior through the mouth of 

 the vase (fig. 471). These currents bring with them food of mixed 

 vegetable and animal nature, as in so many other cases where 

 this mode of getting a living is adopted. Colonial Sponges 

 may assume all sorts of shapes, and their structure is often very 

 complicated, but in all cases the food is obtained as in the simple 

 case described. 



ANIMALCULES (PROTOZOA) (see vol. i, fig. 301) 



We have here a host of microscopic or minute forms, the chief 

 omnivorous species belonging to the two important groups of 

 Infusoria and Rhizopoda. 



INFUSORIA. These include forms which are more or less 

 covered with the short vibratile threads known as cilia, and also 

 others provided with one, two, or a small number of the longer 

 protoplasmic threads termed flagella. The former set of species 

 constitute the Ciliata, which are either simple or colonial, free- 

 swimming or fixed, and the same variety in character and habit 

 is presented by the flagellate species, embraced in the Flagellata. 



Among the Ciliata (see vol. i, p. 489) the free -swimming 

 Slipper Animalcule (Paramcecium) and the fixed Bell Animalcule 

 (Vorticella) will serve as examples. The Slipper Animalcule is 

 a small active creature of oval shape, which is just visible to 

 the naked eye. It moves actively about by means of the cilia 

 which cover its body, and on the under side is a ciliated de- 

 pression which conducts food particles to the mouth and thence 

 to the semifluid interior of the body. With each batch of 

 solids a small amount of water passes in, and the food-containing 

 globule is termed a " food-vacuole ". The food-vacuoles are 

 moved slowly round the inside of the animal, digestion and ab- 

 sorption going on meanwhile, and the undigested remnants are 

 cast out at a point not far from the mouth. The Bell Animalcule, 

 when fully expanded, is shaped like a blunt cone attached by a 

 stalk at the narrow end, while the broad part bears a wreath 

 of cilia arranged in a short spiral. These produce a sort of 



