FLAGELLATA 



131 



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 Pobjkrikos, trichocysts cJ■.l^^,^^ 

 (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. 46, 

 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.^ 



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 



Peridinium divergens. a, 

 Flagellum of longitudinal groove ; 

 i, Wagellum of transverse groove ; 

 cr.v, contractile vacuole surrounded 

 by formative vacuoles ; n, nucleus. 

 (After Schiitt.) 



1 Conjugation of adults has been observed by Zederbauer {Ser. Deutsch. Ges. 

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

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

 spherical resting-spore, as in the Conjugate Algae. 



