16 DINOFLAGELLATA. 



Order 4. Hydruracese. The individuals are attached, without cilia, multi- 

 cellular, branched, and with apical growth. The cells are spherical, but in 

 the final stage almost spindle-shaped, and embedded in large masses of mucilage. 

 Asexual reproduction by zoospores which are tetrahedric, with 1 cilia, and by 

 resting akinetes. Hydrurus is most common in mountain brooks. 



Class -2. Dinoflagellata. 



The individuals are of a very variable form, but always uni- 

 cellular, and floating about in free condition. The cell is dorsi- 

 ventral, bilateral, asymmetric and generally surrounded by a colour- 

 less membrane, which has no silica embedded in it, but is formed 

 of a substance allied to cellulose. The membrane, which exter- 

 nally is provided with pores and raised borders, easily breaks up into 

 irregularly- shaped pieces. In the forms which have longitudinal and 

 cross furrows, two cilia are fixed where these cross each other, and 

 project through a cleft in the membrane ; one of these cilia projects 

 freely and is directed longitudinally to the front or to the rear, the 

 other one stretches crosswise and lies close to the cell, often in a 

 furrow (cross furrow). The chromatophores are coloured brown 

 or green and may either be two parallel (ExuvieUa), or several 

 radially placed, discs, which sometimes may coalesce and become a 

 star-shaped chromatophore. The colouring material 1 (pyrrophyl) 

 consists, in addition to a modification of chlorophyl, also of phyco- 

 pyrrin and peridinin ; this colour is sometimes more or less masked 

 by the products of assimilation which, consist of yellow, red or 

 colourless oil (?) and starch. Cell-nucleus one : in Polydinida several 

 nuclei are found ; contractile vacuoles many, which partly open in 

 the cilia-cleft (Fig. 13 gs). In some an eye-spot, coloured red by 

 hasmatochrome, is found. Pyrenoids occur perhaps in ExuvieUa 

 and AmpTiidinium. 



THE REPRODUCTION takes place as far as is known at present, 

 only by division. This, in many salt water forms, may take 

 place in the swarming condition, and, in that case, is always 

 parallel to the longitudinal axis. The daughter-individuals, each 

 of which retains half of the original shell, sometimes do not separ- 

 ate at once from each other, and thus chains (e.g. in Ceratium} of 

 several connected individuals may be formed. In others, the 

 division occurs after the cilia have been thrown off and the cell-con- 

 tents rounded. The daughter-cells then adopt entirely new cell-walls. 

 A palmella-stage (motionless division-stage) sometimes appears to 



