PHYLUM PROTOZOA 33 



and Peranema (Fig. 19). In contrast with these stand the dino- 

 flagellates some of which have distinct shells made of a substance 

 closely allied to cellulose, covering the entire body. 



The members of this subclass include forms of extremely 

 diverse size. While some species are only about 2 microns in 

 length, others, such as the Noctiluca, one of the organisms caus- 

 ing phorphorescence in the ocean, reach as much as 1.5 mm. in 

 diameter. 



This subclass includes the much discussed Volvox family, the 

 most plantlike of all Protozoa. Members of this family illus- 

 trate all the important steps in colony formation and in the 

 history of reproduction. In Spondylomorum, reproduction is 

 wholly asexual, each of the sixteen cells of the colony by fission 

 producing a new colony. In Gonium, while asexual reproduction 

 occurs, a zygote may result from the permanent union of isoga- 

 metes. Male and female sex cells occur in Pandorina, Eudorina 

 (Fig. 5), and Volvox, the colonies of some species of the last two 

 being sexually differentiated into male and female colonies. 



The subclass Phytomastigina includes five orders, which are 

 listed and some of the distinctive characteristics given in the 

 table of systematic arrangement of the Protozoa at the close of 

 this chapter. Though motile fiagella are characteristic, many 

 species of Phytomastigina have the ability to discard their 

 fiagella and still live an active life. In this immotile state, 

 feeding and reproduction may continue normally and there is 

 no impervious wall formed as in instances of encystment. In 

 many of these plantlike flagellates the surface cytoplasm of the 

 cell has the ability to secrete a gelatinous substance which passes 

 through the pellicle and forms a matrix surrounding the cell. 

 Frequently the cells resulting from division remain associated. 

 Then the whole mass becomes enclosed as a colony in a gelatinous 

 matrix, only the fiagella extending through it to the outside. 



Another type of surface secretion results in the deposition of 

 cellulose or other materials to form a shell or test surrounding all 

 or part of the cell body. In some instances these tests are highly 

 ornamented and frequently they are composed of plates joined 

 together in specifically characteristic arrangement and number 

 so that species, in the dinoflagellates for example, may be differ- 

 entiated on the characters furnished by the test. 



The planthke fiagellates are very widely distributed in both 

 fresh and salt water. Many genera of the dinoflagellates are 



