MASTIGOPHORA 255 



to be combined in such a form as Ochromonas. In association with 

 chromatophores, there occurs refractile granules or bodies, the 

 pyrenoids, which are connected with starch -form at ion. Reserve 

 food substances are starch, oil, etc. (p. 113). 



In less complicated forms, the body is naked except for a slight 

 cortical differentiation of the ectoplasm to delimit the body surface 

 and is capable of forming pseudopodia. In others, there occurs a thin 

 plastic pellicle produced by the cytoplasm, which covers the body 

 surface closely. In still others, the body form is constant, being en- 

 cased in a shell, test, or lorica, which is composed of chitin, pseudo- 

 chitin, or cellulose. Not infrequently a gelatinous secretion envelops 

 the body. In three families of Protomonadina there is a collar-like 

 structure located at the anterior end, through which the flagellum 

 protrudes. 



The great majority of Mastigophora possess a single nucleus, and 

 only a few are multinucleated. The nucleus is vesicular and contains 

 a conspicuous endosome. Contractile vacuoles are always present in 

 the forms inhabiting fresh water. In simple forms, the contents of 

 the vacuoles are discharged directly through the body surface to 

 the exterior; in others there occurs a single contractile vacuole near 

 a reservoir which opens to the exterior through the so-called cyto- 

 pharynx. In the Dinoflagellata, there are apparently no contractile 

 vacuoles, but non-contractile pusules (p. 310) occur in some forms. 

 In chromatophore-bearing forms, there occurs usually a stigma 

 which is located near the base of the flagellum and seems to be the 

 center of phototactic activity of the organism which possesses it. 

 Asexual reproduction is, as a rule, by longitudinal fission, but in 

 some forms multiple fission also takes place under certain circum- 

 stances, and in others budding may take place. Colony-formation 

 (p. 174), due to incomplete separation of daughter individuals, is 

 widely found among this group. Sexual reproduction has been re- 

 ported in a number of species. 



The Mastigophora are free-living or parasitic. The free-living 

 forms are found in fresh and salt waters of every description ; many 

 are free-swimming, others creep over the surface of submerged ob- 

 jects, and still others are sessile. Together with algae, the Mastigoph- 

 ora compose a major portion of plankton life which makes the 

 foundation for the existence of all higher aquatic organisms. The 

 parasitic forms are ecto- or endo-parasitic, and the latter inhabit 

 either the digestive tract or the circulatory system of the host ani- 

 mal. Trypanosoma, a representative genus of the latter group, in- 

 cludes important disease-causing parasites of man and of domestic 

 animals. 



