3° 



MANUAL OF ZOOLOGY 



But in some genera the process of fission under some circum- 

 stances remains incomplete, the two protoplasmic bodies 

 to which the fission gives rise remaining connected together 

 by a bridge or isthmus of protoplasm, instead of becoming 

 separated off in the shape of two independent animals, as in 

 Amceba. Further, these two bodies may each in turn divide 

 in the same incomplete way, so that four Heliozoans are 

 developed, all remaining connected together; and by further 

 repetitions of the same process a structure may be formed 

 consisting of a large number of units all connected together 

 by living substance. A structure of this kind, formed as a 

 result of repeated 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 

 multiplication also takes place in some Heliozoa by a pro- 

 cess known as the formation of spores. In spore-formation 

 (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 qui- 

 escent condition before this takes place; the pseudopodia 

 become withdrawn, and the whole becomes enclosed in a 

 firm envelope or sporocyst ; this process is known as encysta- 

 fton. The spores in some of the Heliozoa, when set free, 

 are provided each with two flagella (Fig. 8, 3, b) which 



