200 DESMIDIACE.E : — GENERAL CHARACTERS. 



green colour, growing in fresh water ; generally speaking, the 

 cells are independent of each other (Figs. 110, 113, 114) ; but 

 sometimes those which have been produced by binary subdivision 

 from a single primordial cell, remain adherent one to another in 

 linear series, so as to form a filament (Fig. 115). This tribe is 

 distinguished by two peculiar features ; one of these being the 

 semblance of a subdivision into two symmetrical halves, divided 

 by a ' sutural line,' which is sometimes so decided as to have led 

 to the belief that the cell is really double (Fig. 113, a), though in 

 other cases it is merely indicated by a slight notch ; whilst the 

 other is the frequency of projections from their surface, which are 

 sometimes short and inconspicuous (Fig. 113), but are often elon- 

 gated into spines, presenting a very symmetrical arrangement 

 (Fig. 110). These projections are generally formed by the Cellulose 

 envelope alone, which possesses an almost horny consistence, so as 

 to retain its form after the discharge of its contents (Figs. 113, 

 B, D, 117, e), but does not include any Mineral ingredient, either 

 calcareous or siliceous, in its composition ; in other instances, how- 

 ever, they are formed by a notching of the margin of the cell 

 (Fig. 112), which may affect only the outer casing, or may extend 

 into the cell-cavity. The outer coat is surrounded by a very 

 transparent sheath of gelatinous substance, which is sometimes 

 very distinct (as shown in Fig. 115), whilst in other cases its 

 existence is only indicated by its preventing the contact of the 

 cells. The outer coat encloses an inner membrane or Primordial 

 Utricle, which is not always, however, closely adherent to it ; and 

 this immediately surrounds the Endochrome or coloured substance 

 which occupies the whole interior of the cell, and which in certain 

 stages of its growth is found to contain Starch-granules. — Many of 

 these Plants have a power of slowly changing their place, so that they 

 approach the light side of the vessel in which they are kept, and 

 will even traverse the field of the Microscope under the eye of the 

 observer ; by what agency this movement is effected has not yet 

 been certainly made out. 



201. A Circulation of fluid has been observed mClosterium, not 

 only (as in the cells of higher Plants, § 289) within the Primordial 

 Utricle, but also (it is asserted) between this and the Cellulose en- 

 velope. It is not difficult to distinguish this movement along the 

 convex and concave edges of the cell of any vigorous specimen of 

 Closterium, if it be examined under a magnifying power of 250 or. 

 300 diameters ; and a peculiar whirling movement may also be 

 distinguished in the large rounded space which is left at each end 

 of the cell by the retreat of the Endochrome from the Primordial 

 Utricle (Fig. Ill, a, b). By careful focussing, the circulation may 

 be seen in broad streams over the whole surface of the endochrome ; 



bf Mr. Ralfs's admirable Monograph of it in 1848. For later information 

 see the sections relating to it in Pritchard's " History of Infusoria," 4th 

 Ed., 1861. 



