MICROSCOPIC OBJECTS. 49 



Fig. 97. A Species of Confervoid Alga (Volvox globator), 



x40. 



It is only within the last few years that this microscopic 

 organism has been thoroughly examined, and found to be 

 one of the Confervoid Algse, before which time it was thought 

 to be an infusorial animalcule. On the examination of fresh 

 specimens (which may often be procured in pools, on open 

 heaths, fields, &c., and which appear to the naked eye as 

 minute globular bodies, moving with a rolling motion) under 

 a low power, say 1 inch, the cause of motion will be seen to 

 consist in a multitude of minute hairs (or cilia, as they are 

 called) with which the bodies are studded; these act as oars. 

 In the interior a number of small green bodies will be seen ; 

 these are the young volvoces, and, on the rupture of the mem- 

 branous sac of the parent plant, escape, grow, and increase. 

 They are preserved with difficulty so as to show the cilia. 

 They appear to keep tolerably well when mounted in a shallow 

 cell in weak alcohol and water. 



Fig. 98. A Diatom in its Natural State on a Conferva 

 (Diatoma vulgare), x 200. 



Diatoms belong to the Confervoid Algse, but are remark- 

 able in consisting of an extremely brittle structure composed 

 of silex. They are found in nearly all places where there is 

 water, either growing on other plants, or forming a dense 

 mass at the bottom. Their colour, being generally of a 

 yellowish brown, makes them very distinguishable when oc- 

 curring at the bottom of ponds &c. The species are very 

 numerous, and they form an order of plants that have been 

 much studied perhaps a little too much so, to the exclusion 

 of other minute and remarkable forms of vegetable and 

 animal life. 



If they are to be preserved in their natural state, they are 

 best mounted in a shallow cell in fluid; but the siliceous 

 valves or frustules are generally cleaned by boiling in nitric 

 acid for a short time, and the species separated by taking 

 advantage of their different specific gravities. They may 

 then be mounted dry or in Canada balsam as usual. 



