86 



EECHEATIYE SCIENCE. 



Imagine, now, that we have a common 

 round snuff-box before us ; we see that it is 

 in two pieces, the top or lid, and the bottom 

 part, or the box proper. This will give a 

 tolerable idea of many of the Diatomacese ; 

 such are " discoid," or of a disc-like shape. 

 Looking down upon our box, with the eyes 

 directly over its centre, we just see that its 

 shape is round, and of its form should from 

 such a point of view learn nothing more. If 

 now we turn it so as to display its edge, its 

 thickness, the form of the mouldings, and the 

 way in which the two pieces of which it is 

 composed fit together, are apparent. In a 

 somewhat similar way a Diatom is rdade Up 

 of two portions, or " valves," externally con- 

 vex, which when united enclose a cavity, as 

 may be rendered plainer by Fig. 1. 



^ / 



Fig. 1. — Coscinodiscus in outline, a, side view ; 

 6, front vie 



To get a ftdl knowledge of its outward 

 form, we must turn our Diatom 'upon edge, or, 

 as this can be seldom done, except with the 

 very largest, examples must be sought for on 

 the slide under examination which present 

 this aspect. In nearly all the Diatomacese, 

 the shells are marked with lines, or dots, 

 arranged in such wise as to form patterns of 

 the most elegant tracery. Of these we shall 

 have more to say presently. 



If now, instead of a round, we take a 

 long, oval box, we shall obtain an idea of the 

 shape of another considerable portion of the 

 Diatomacese, as in Fig. 2. 



The two valves of a Diatom, when in 

 union, constitute a "frustule;" they are gene- 

 rally alike, but in some few forms the upper 

 and lower valves are different. The flinty 

 nature of their shells or valves has already 



been mentioned. Closely applied within the 

 valve is a delicate lining membrane, of the 



fT\ 



( — 



Fig. 2. — Pinnularia cardinalis, in outliiie. «, side 

 view ; t, front view. 



greatest importance in the economy of the 

 little being ; to this, as the term is of com- 

 mon use in speaking of vegetable cell-growth, 

 it will be of advantage to give the correct 

 term, "primordial utricle," at once. This 

 membrane, from its extreme delicacy, can 

 seldom be distinctly seen until its collapse 

 and separation by death from the valve ; it, 

 too, is occasionally marked with dots or lines. 

 In or near the centre of every Diatom is a 

 small-rounded body, the "nucleus" connected 

 by delicate threads of "protoplasm," with 

 little patches of soft substance, generally of 



Fig. 3. — Surirella biseriata, from a living specimen, 

 a, side view ; 6, front view ; ?i, nucleus ; p, proto- 

 plasmic threads ; g, granules, moving towards and 

 from the nucleus. 



a bright brown colour, the " endochrome," 

 which means simply "internal colouring 

 matter." These points may be more clearly 

 understood by looking at Fig. 3. 



