40 A HALF-HOUR WITH THE 



form as the other. The desmids are distinguished 

 from the diatoms by their bright-green colour, and 

 by their cells not depositing silex, or flinty matter, 

 as is the case with the latter. The desmids some- 

 times abound in ditches and small pieces of stand- 

 ing water. Amongst other objects in a drop of 

 water they are easily recognized by their beautiful 

 bilateral forms and dark-green colour. One of 

 the most charming of these is named Euastrum 

 (Fig. 25, PI. 2), and consists of two notched halves 

 of a bright-green colour, with darker green spots. 

 The green matter is composed of chlorophyle, the 

 same matter that produces the green colour of 

 leaves. Some of them assume a lunate form, and 

 are named Closterium (Fig. 26, PL 2). There are 

 various species of Closterium, all of the same 

 general form, and occasionally occurring in very 

 great abundance. Sometimes several of the cells 

 are attached together, forming a long chain, as in 

 the genus Desmidium (Fig. 27, PI. 2), from which 

 the family takes its name. These break up and go 

 on growing. When they grow, the new cells are 

 formed between the two halves of the parent cells 

 (Figs. 134 and 135, PL 5). In a genus called 

 Scenedesmus, several cells are united, and the two 

 last halves are furnished with horns (Fig. 29, PL 2) ; 

 at other times several cells unite, forming a glo- 

 bular mass, as in Pediastrum (Fig. 28, PL 2). In 

 this case each cell presents two projections, forming 

 objects of singular beauty. 



The diatoms are more numerous and widely 

 diffused than the desmids. The latter are decom- 

 posed, and their bodies perish when they die ; but 

 from the fact that the diatoms deposit silex in 

 their structure, they are almost imperishable. They 

 are found in great abundance in the mud of rivers, 

 ponds, and lakes. They are also present in those 



