i66 



MORPHOLOGY. 



Class Peridineae. 



3580. These are peculiar one-celled organisms provided with two flagella 

 and show some relationship to the Flagellates. They usually are provided 

 with a cellulose membrane, which in some forms consists of curiously 

 sculptured plates. In the higher forms this cellulose membrane consists of 

 two valves fitting together in such a way as to resemble some of the diatoms. 

 Like the Flagellates, some have green chromatophores, which in some are 

 obscured by a yellow or brown pigment (resembling the diatoms), while 

 still others have no chlorophyll. The Peridinese are abundant in the sea, 

 while some are found in fresh water. 



Class Diatomaphyceae (Bacillariales, Diatomaceae). 



3586. The diatoms are minute and peculiar organisms believed to be 

 algae. They live in fresh, brackish, and salt water. They are often found 

 covering the surface of rocks, sticks, or the soil in thin sheets. They occur 

 singly and free, or several individuals may be joined into long threads, or 

 other species may be attached to objects by slender gelatinous stalks. Each 



Fig. 1710. 



A group of Diatoms: c and d, top and side views of the same form; e, colony 

 of stalked forms attached to an alga; f and g, top and side views of the form shown 

 at e\ h, a colony; *', a colony, the top and side view shown at k and , forming auxo- 

 spores. (After Kerner.) 



protoplast is enclosed in a silicified skeleton in the form of a box with two 

 halves, often shaped like an old-fashioned pill box, one-half fitting over the 

 other like the lid of a box. It is evident that in this condition the plant 

 cannot increase much in size. 



They multiply by fission. This takes place longitudinally, i.e., in the 

 direction of the two halves or valves of the box. Each new plant then has a 

 valve only on one side. A new valve is now formed over the naked half, 

 and fits inside the old valve. At each division the individuals thus become 

 smaller and smaller until they reach a certain point, when the valves are 

 cast off and the cell forms an auxospore, i.e., it grows alone, or after conju- 

 gation with another, to the full size again, and eventually provides itself 



