THE ORDER AMONG ANIMAL TYPES 99 



ample is Euglena, a cigar-shaped form common in fresh water 

 pools. Some Mastigophora are parasitic. Noctiluca is a spherically 

 shaped form that is phosphorescent; it occurs in unimaginably great 

 numbers in sea water and is chiefly responsible for the phosphor- 

 escence of breaking waves and in the wake of ships. 



Class III. INFUSORIA (Fig. 30). Some members of this class are 

 large enough to be detected with the unaided eye. All move by 

 means of numbers of fine fibres arising in the ectoplasm, termed 

 cilia. They, like Sarcodina and Mastigophora, are also quite com- 

 mon in stagnant water, where they feed on bacteria and other bits 

 of organic materials. Perhaps the most common form is Para- 

 mcechim caudatiim. Some members of the class are parasitic. 



Class IV. SPOROZOA (Fig. 31). The distinguishing character of 

 this class is that all members at some time during their life history 

 reproduce by the formation of considerable numbers of spores. All 

 members of the class are parasitic. Various species of the genus 

 Plasmodium are the cause of the several types of malaria (p. 74). 



Sub-Kingdom Metazoa 

 All animal types composed of many cells that are differentiated 

 into different tissues are placed in this sub-kingdom. Various au- 

 thorities recognize twelve or more phyla of the Metazoa. In order 

 of complexity of cellular arrangement these are: 



Phyhtm For if era 

 The Sponges. Vase-shaped, radially symmetrical animals con- 

 sisting of two layers of cells, the ectoderm or outer, and the en- 

 doderm or inner layers, enclosing a central cavity. The walls are 

 perforated by pores, hence the name Porifera (Fig. 32). The en- 

 doderm consists wholly or in part of cells termed choanocytes, 

 that have a characteristic collar in which is a beating fibre or 

 flagellum. Beating of these flagellar causes water to be drawn in- 

 ward through the pores and to pass outward through a single large 

 aperture termed the osculum. With some exceptions the ectoderm 



