CLASSIFICATION 



87 



a very natural group. They are unicellular organisms, but may be 

 united into chains or filaments, or by the secretion of a gelatinous 

 matter they remain together in colonies of characteristic form, adher- 

 ing to plants and other objects. Chromatophores are always present. 

 These contain, besides chlorophyll, a golden-brown pigment, Diato- 

 min. They are all characterized by the formation of a rigid silicious 

 shell, which is composed of two pieces (valves), one of which fits 

 over the other. 



Cell-structure of Diatoms While the form of the Diatoms is 



extremely varied, the commonest of the fresh-water forms are oblong, 

 or somewhat boat-shaped in outline, e.g. Navicula (Fig. 60, A), Pin- 

 nularia. The cell is enclosed in a shell composed of silica, which is 

 in two parts, one fitting over the other like the cover of a pill-box. 

 Each of these valves consists of two parts : the top, and the margin 

 or girdle corresponding respectively to the top (or bottom) of the 

 pill-box, and the sides. Sometimes one or more plates are inserted 

 between the top of the valve and the girdle. The flinty shell is usu- 

 ally elaborately sculptured (Fig. 60, C), the markings often being 

 excessively fine, and sometimes used as tests for microscopic lenses. 



In the elongated forms, like Pinnularia, there is usually a median 

 straight or curved line, with an enlargement at the middle and end 

 of the valve. This line is known as the raphe., and has been shown 

 to form a cleft or system of openings communicating with the interior 

 of the cell. 



The cytoplasm in these elongated forms lines the cell-wall, and in 

 the middle of the cell forms a bridge across it, in which lies the 

 nucleus. In other forms the 

 nucleus may be imbedded 

 in the peripheral cytoplasm, 

 or even suspended in the 

 central vacuole by cytoplas- 

 mic threads extending to the 

 peripheral cytoplasm. Con- 

 spicuous oil-drops are often 

 to be seen within the cell. 



Chromatophores. The 

 chromatophores of the Dia- 

 toms are usually large plates, 

 most commonly two lying 

 parallel and extending nearly 

 the whole length of the cell. 

 Sometimes, e.g. Cocconeis, 

 but a single One is present, FIG. 61. MarineDiatoms. A,Licmophorasp. 

 whioh mflv bp vavirmslv (X 250). B, Tabellariasp., three individuals 

 may vailOUSiy connected by gelatinous joint. C, end view 



cut, and of irregular 'form. of a cell (x 260). 



