142 A CENTURY OF PROGRESS IN THE NATURAL SCIENCES 



unicellular, colonial, or filamentous. The unicellular species may be naked or provided 

 with a wall — usually of unknown composition but in some instances known to be composed 

 of pectin and rarely also containing cellulose — or the naked cell may be enclosed in a 

 capsule (lorica) which is open at one end. In many, if not all, the Mallomonadaceae and in 

 Aurosphaera (Chrysosphaerales) siliceous scales are embedded in the pectic wall and 

 the scales may bear delicate, hinged silicified needles. In the Coccolithophorineae and in 

 Achrosphaera (Chrysosphaerales) the pectic wall contains discoid bodies of calcium 

 carbonate (coccoliths) which in some instances are provided with spinelike processes. 

 In the Silicoflagellatophycidae the naked cell contains an internal skeleton consisting of 

 a framework of variously arranged siliceous rods. 



The unicellular forms are either flagellated, or are consistently rhizopodial, or occur 

 as gelatinous aggregations of cells (palmelloid types) or as nonmotile cells enclosed by 

 a wall (coccoid types). Depending on the species, the flagellated cells have one, two equal 

 (isokont), two unequal (heterokont), or one short and two long flagella. As far as known 

 (Petersen, 1918, 1929; Vlk, 1938) the flagellum of the uniflagellate Chrysomonadales is 

 of the tinsel type whereas in the isokont Isochrysidales one of the flagella is of the tinsel 

 type and in the heterokont Ochromonadales the long flagellum is of the tinsel type. The 

 structure of the flagella in the triflagellate Prymnesiales has not yet been determined. 



The filamentous forms are simple or branched and have a firm cell wall which, at least 

 in Phaeothavuiion, is known to be composed of cellulose. 



The majority of the Chrysophyceae are photosynthetic. Some are colorless and are 

 either saprophytic or engulf solid food. Some of the pigmented forms also ingest solid 

 food. Food is stored as leucosin, a substance of unknown chemical composition (prob- 

 ably a carbohydrate), and oil. The cells usually contain only one or two chromatophores 

 which are parietal in position, and in some instances naked pyrenoidlike bodies are 

 present. The pigmented species have a golden-brown color owing to a preponderance of 

 carotenes and xanthophylls. As far as known the pigment complex consists of chlorophyll 

 a. beta-carotene, lutein, and fucoxanthin (Strain, 1951, p. 253). 



Contractile vacuoles are of common occurrence either in the vegetative stages or in 

 the reproductive cells of species representative of all the orders. 



The ordinary method of reproduction is by vegetative cell division. Some species 

 also produce zoospores. Sexual reproduction appears to be of extremely rare occurrence 

 and is isogamous. Up to the present a union of gametes has been observed with certainty 

 only in Ochrosjihaera (Schwarz, 1932) and Dinobryon Borgei (Skuja, 1950). The report 

 by Schiller (1926) of a fusion of gametes in Dinohryon sertularia is not entirely convinc- 

 ing and the observations by Mack (1951) with respect to Chrysolykos require confirma- 

 tion. Many of the species are known to produce cysts. 



The cysts constitute one of the most distinctive features of the class. They were first 

 observed by Cienkowski (1865b) and have since been studied in a large number of species 

 by Scherffel (1911, 1924), Conrad (1927, 1928), Doflein (1923), Pascher (1924, 1932) and 

 others. These resting stages are formed endoplasmatically and have a wall consisting of 

 two pieces which are usually of a different size. The larger piece is formed first and is 

 composed of cellulose which is impregnated with silica; and the outer surface is often 

 elaborately sculptured. The smaller piece is ordinarily in the form of a plug which seals 

 from the inside the terminal opening left in the larger piece. The plug usually contains 

 little or no silica and is dissolved at germination of the cyst or is separated from the wall 

 around the pore. These cysts contain almost all the original protoplasm of the cell and 

 leucosin, and at germination the contents ordinarily divide to form a number of motile 

 cells which escape through the pore. 



History: Hydrurus foetidus (Villars) Trevisaii is the first member of this chiss 

 to have been described with sufficient accuracy to be recognized by later investi- 

 gators. It was described by Villars in 1789 as Conferva foetida. C. Agardh in 

 1824 (p. xviii) erected the genus Hydrurus. It was not until the latter part of 

 the nineteenth century, however, that the relationship of Hydrurus with the 

 Chrysophyceae was established by Klebs (1892, pp. 283-285, 420-427) and 



