PRIMITIVE ANIMAL FORMS 113 



at the bottom of the sea. They have formed, practically unaided, 

 deposits of great thickness at different epochs. In the less 

 important Rhizopods the pseudopodia do not fuse ; in the 

 Amoeba they take the form of simple rounded lobes. 



We come thus to the Infusoria, which alter their shape 

 but little, and move with the aid of permanent processes 

 known as vibratile flagella when they are long and few in 

 number, and waving cilia when they are short, numerous, and 

 arranged like a fleece or in fringes. The Infusoria have left no 

 fossil traces. Some Flagellates, however, are interesting because 

 certain bodies which resemble them in all respects, are charged 

 with the duty of promoting the circulation of water in the 

 internal canals of Sponges. This is the only case in which so 

 marked a resemblance has been found between free Protozoa 

 and cells forming an integral part of an organism. 



The ciliated Infusoria, in spite of their small size — the largest 

 hardly attain a length of more than a few tenths of a milli- 

 metre, present an interest of a different order ; their forms 

 already seem to obey the laws dominating the higher organisms. 

 The thin cuticle is pierced by two orifices functioning like those 

 found in the digestive tube of higher animals, one for the entry 

 of food, and the other for the expulsion of the residuum of 

 digestion. These orifices may be terminal, in which case the 

 form of the animal is disposed symmetrically to the axis uniting 

 them. The vibratile cilia form a continuous fleece (Holophrya) 

 or are arranged in a series of transverse rings (Didinium). 

 This is therefore essentially a swimming type, but most of the 

 Infusoria are able to move over the surface of Confervse, 

 minute Algae, or even directly over the ground. In this case 

 they have a flattened ventral aspect, to which the mouth has 

 been transferred. This latter is slightly eccentric. But for 

 this fact, the symmetrical currents, stimulated by the cilia, 

 would flow around it without directing any food to it. The 

 eccentric position of the mouth and the way in which the larger 

 lateral portion of that region of the body in front of it forces 

 the currents with the food particles they carry against it, 

 provides it with food. The vibratile cilia round about it are 

 at first similar to the others [Paramecium), but, as though 

 strengthened by the constant and intensive use the animal 

 makes of them, they grow larger than the others and form an 

 ado ral fringe that can twist itself in a spiral around the pseudo- 



