FLAGELLATA 



31 



show variability as to their nutrition, which is sometimes 

 holozoic. Under these conditions the cell can take in food- 

 particles as bulky as the eggs of Eotifers and Copepods, by the 

 protrusion of a pseudopod at the junction of the two grooves. 

 As in most coloured forms an eye-spot is often present, a cup- 

 shaped aggregation of pigment, with a lenticular refractive body 

 in its hollow. A contractile vacuole, 

 here termed a " pusule," occurs in 

 many species, communicating with 

 the longitudinal groove by a canal. 

 Nematocysts (see p. 246 f.) are 

 present in Polykrikos, trichocysts cr -^^ 

 (see p. 142) in several genera. 



Division is usually oblique, 

 dividing the body into two dis- 

 similar halves, each of which has 

 to undergo a peculiar growth to 

 reconstitute the missing portion, 

 and complete the shell. The in- 

 complete separation of the young 

 cells leads to the formation of 



chains, notably in Ceratium and Fig. t%.Peridinium divergens. 

 Polykrikos, the latter dividing 

 transversely and occurring in 

 chains of as many as eight. The 

 process of division may take place 

 when the cell is active, or in a cyst, as in Pyrocystis (Fig. 47). 

 Again, encystment may precede multiple fission, resulting in 

 the formation of a brood of minute swarmers. It has been 

 suggested that these are capable of playing the part of gametes, 

 and conjugating in pairs. 1 



The Dinoflagellates are for the most part pelagic in habit, float- 

 ing at the surface, and when abundant tinge the water of fresh- 

 water lakes or even ponds red or brown. Peridinium (Fig. 46) 

 and Ceratium (the latter remarkable for the horn-like backward 

 prolongations of the lower end) are common genera both in the sea 



a, 



Flagellum of longitudinal groove ; 

 b, flagellum of transverse groove ; 

 cr.v, contractile vacuole surrounded 

 by formative vacuoles ; n, nucleus. 

 (After Schiitt.) 



1 Conjugation of adults has been observed by Zederbauer (Ber. Deutsch. Ges. 

 xxii. 1904). A short connecting tube is formed by the meeting of outgrowths 

 from either mate ; their protoplasmic contents meet and fuse herein to form a 

 spherical resting-spore, as in the Conjugate Algae. 



