424 THE MICROSCOPE. 



Diatomacece. This author is of opinion that a soTcode 

 substance envelopes the external surface of the diatom, 

 and its movement is due to this agent exclusively. He 

 j)refers the P. angulatum, Plate 11. iSio. 38, lor examination 

 to the larger P. halticum, because the transverse markings 

 on its frustule do not impede to so great an extent the 

 observation of what is going on within. AVhen you have 

 a living specimen of P. angulatum under the microscope, 

 it always has its broad side turned to view, with one long- 

 curved "raphe" uppermost, and the other in contact with 

 the glass on which it is placed ; at the central part is seen 

 the thickened " umbilicus," Plate 2, No. 40. AYithin the 

 siliceous frustule is the yellow colouring matter, or "endo- 

 chrome," which fills the cavity more or less completely, 

 and is arranged in two longitudinal masses, to the right 

 and left of the raphe. In the broader part of the frus- 

 tule these bands of endochrome describe one or two com- 

 plicated windings. It is only possible in those specimens 

 in which the bands are narrow properly to trace their 

 foldings, and ascertain that only two exist, since an 

 examination of frustules richer in endochrome has led to 

 the impression that there are three or four of these bands. 

 ''The next objects which strike the eye on examining 

 a living Pleurodgma are highly refractive oil-globules. 

 These are four in number ; one pair near either end of the 

 the Diatom. They are not, however, all in the same place, 

 one globule of each pair being nearer the observer than 

 the other ; their relative positicju is best seen when a view 

 of the narrow side of the frustule can be obtained, so that 

 one raphe is to the left and the other to the right. The 

 blue-black colour, which is assumed by these globules 

 after tlie Diatom has been treated with hyperosmic acid, 

 demonstrates that they consist of oleaginous matter. The 

 middle of the cavity of the frustule is occupied by a 

 colourless finely granular mass, whose position in the body 

 is not so clearly seen in the flat view as in the side view. 

 IJesides the central mass, the conical cavities at either end 

 of the siliceous shell are seen to be tilled with a similar 

 granular substance, and two linear extensions from each of 

 the three masses arc developed, closely underlying that 

 X>art of the shell which is beneath the rapha3; so that 



