30 MANUAL OF ZOOLOGY SECT. 



incomplete division (or, in other cases, budding) from an 

 original simple animal, is termed a colony, and the elements 

 or units of which it is composed are termed zooids. How 

 such a colony of unicellular Protozoa is to be distinguished 

 from a multicellular animal or Metazoan (p. 19) will be 

 explained at a later stage. It will at once be apparent that 

 the compound Foraminifera are of the nature of colonies of 

 unicellular zooids, each occupying one of the chambers of 

 the shell, formed as the result of a process of repeated 

 budding. 



In addition to the process of multiplication by fission, 

 reproduction also takes place in some Heliozoa by a 

 process known as spore-formation. In this process (a 

 form of which has already been referred to as occurring 

 in the Foraminifera) the protoplasm breaks up into 

 numerous small parts, each of which eventually develops 

 into the form of the parent. Usually the Protozoan 

 passes into a quiescent condition before this takes place ; 

 the pseudopodia are withdrawn, and the whole becomes 

 enclosed in a firm envelope or sporocyst ; this process 

 is known as encystation. The spores in some of the 

 Heliozoa, when set free, are provided each with two 

 flagella (Fig, 8, j, b} which subsequently become lost, 

 pseudopodia appearing in their place. 



The Radiolaria are marine Rhizopoda which have 

 exceedingly delicate, thread-like pseudopodia (Fig. 9, psd.) 

 and a skeleton usually composed of silica. This skeleton 

 may be composed of loosely woven needle-like bodies or 

 spicules ; more usually it is in the form of a globular, 

 conical, star-shaped, or disc-shaped shell, perforated by 

 numerous openings, and often supported by spines which 

 radiate out from the centre; sometimes (Fig. 10) there are 

 several such shells one within the other. In some 



