18 DIATOM EJE. 



The order Polydinida deviates in a high degree from the other Dinoflagellata, 

 not only by its many tranverse furrows, each with its own transverse cilium, and 

 by the absence of chromatophores, but also in having several cell-nuclei and a 

 Mnd of stinging capsule, which otherwise does not occur within the whole 

 class. It may therefore be questionable whether this order should really be 

 placed in the vegetable kingdom. 



Class 3. Diatomeae. 



The individuals each known as a frustule assume very various 

 forms and may be unicellular or multicellular, but present no 

 differentiation; many similar cells may be connected in chains, 

 embedded in mucilaginous masses, or attached to mucilaginous 

 stalks. Tjbe cells are bi-lateral or centric, often asymmetrical, 

 slightly dorsiventral and have no cilia ; those living in the free 

 condition have the power of sliding upon a firm substratum. The 

 cell contains 1 cell-nucleus and 1-2 plate-shaped or several disc- 

 shaped chromatophores. The colouring material " Melinophyl " 

 contains, in addition to a modification of chlorophyl, a brown 

 colouring matter, diatomin. 1 or 2 pyrenoids sometimes occur. 

 Starch is wanting and the first product of assimilation appears to 

 be a kind of oil (?). 



The cell-walls are impregnated with silica to such a degree that 

 they are imperishable and are therefore able to contribute in a 

 great measure to the formation of the earth's crust. The structure 

 of their cell- wall is most peculiar and differs from all other plants 

 (except certain Desmidiaceoe) ; it does not consist of a single piece 

 but is made up of two the " shells " (compare Exuviella and 

 Prorocentrum among the Dinoflagellata) which are fitted into each 

 other, one being a little larger than the other and embracing its 

 edge, like a box with its lid (Fig. 14 B). The two parts which cor- 

 respond to the bottom and lid of the box are known as valves. 

 Along the central line of the valves a longitudinal rib may often 

 be found, interrupted at its centre by a small cleft (perhaps 

 homologous with the cilia-cleft of the Dinoflagellata), through 

 which the protoplasm is enabled to communicate with the exterior 

 (Fig. 14 A). It is principally by reason of the valves, which bear 

 numerous fine, transverse ribs, striae or warts, etc. (Figs. 14, 15, 17), 

 that the Diatomege have become so well known and employed as 

 test objects in microscopical science. When the division takes 

 place, the two shells are separated a little from each other, and 

 after the cell-contents have divided into two masses, two new 

 shells are formed, one fitting into the larger valve, the other one 



