552 THE LOWEST ANIMALS. 



food can be taken in, and the remains ejected at any point, but the latter frequently 

 appear to be cast out at one spot behind the nucleus and contractile vacuole. 

 The animal reproduces by dividing into two halves, each containing a portion of 

 the original nucleus and a contractile vacuole, and each growing to the size of the 

 original. A unit particle of protoplasm capable of carrying on the functions of 

 life, namely, nutrition and reproduction, is termed a " cell." Formerly, a unit of 

 this nature was supposed to be a sac or vesicle, hence the name " cell," which is 

 retained, though many cells are solid and without a definite wall. The Pro- 

 tozoans are animals consisting of a single cell, or colonies of cells. In the latter 

 case each cell is more or less independent of the others, and capable of carrying 

 on all the functions of life. All animals above the Protozoa are composed of 

 many cells united into a whole, in which there arises the principle of division of 

 labour. 



The Protozoa are divided into two groups, the Rhizopoda and the Infusoria ; 

 in the former the body-substance is of more or less uniform consistence, and can 

 extend itself from any part of the surface in the form of pseudopods ; whereas, in 

 the latter, the outer layer is firmer and denser than the inner, and the animal has 

 a more or less definite shape. In place of pseudopods, the Infusoria develop on 

 their surface one or many fine processes in the shape of cilia or flagella, which 

 set up food-carrying currents, converging towards a definite mouth, and which 

 enable the animals to move rapidly about, when they are not fixed. 



The Root-Footed Group, — Class Rhizopoda. 



The simple organisms of this class take their name from their power of 

 protruding from the body the processes known as pseudopodia, which are often 

 branched like roots of a tree, and by means of which they creep about. The 

 group includes the amcebas, the foraminifers, the sun-animalcules, and radiolarians. 

 In the first the pseudopodia are simple and lobose ; in the second they are slender, 

 confluent, and reticulate; while in the two last they are simple, radiating, and 

 somewhat stiff. 



The Amcebas, — Order Lobosa. 



The chief character of this group consists in the usually broad lobose simple 

 form of the pseudopods, which flow out from the body in the shape of finger-like 

 processes. The simplest forms are apparently without even a nucleus, and on this 

 account have been separated from the rest as the Monera. As the first representa- 

 tive of the group, we may take the form known as Protomyxa, which forms 

 minute orange-coloured particles of jelly creeping over shells, and consists simply 

 of protoplasm containing granules, oil-globules, and food-particles. Occasionally a 

 specimen retracts all its pseudopodia, some of which are broad and others slender, 

 and becomes a quiescent sphere, the contents of which break up into numerous 

 portions, each of which forms a new individual. The amcebas are divided into 

 two groups, the shell-less (Nuda), and the shelled forms (Testacea). The common 

 amoeba, which has been described above, belongs to the former group, as also does 

 Pelomyxa, a large species found in the form of little white ovoid masses, about 



