A large vacuolar space, traversed by plasmic strings, separates the peripheral cytoplasm from the central, within which is the large nucleus. There are in most species one or more chromatophores, coloured by a yellowish or brownish pigment, which is a mixture of lipochromes, distinct from diatomin. In a few species the presence of these is not constant, and these species {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 Rotifers 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 (see p. 142) in several genera. [Illustration: FIG. 46.—_Peridinium divergens._ _a_, Flagellum of longitudinal groove; _b_, flagellum of transverse groove; _cr.v_, contractile vacuole surrounded by formative vacuoles; _n_, nucleus. (After Schütt.)] Division is usually oblique, dividing the body into two dissimilar halves, each of which has to undergo a peculiar growth to reconstitute the missing portion, and complete the shell. The incomplete separation of the young cells leads to the formation of chains, notably in _Ceratium_ and _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.[142] The Dinoflagellates are for the most part pelagic in habit, floating 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 {132}and fresh-waters. _Gymnodinium pulvisculus_ is sometimes parasitic in _Appendicularia_ (Vol. VII. p. 68). _Polykrikos_[143] has four transverse grooves, each with its flagellum, besides the terminal one. Many of the marine species are phosphorescent, and play a large part in the luminosity of the sea, and some give it a red colour. Several fossil forms have been described. _Peridinium_ is certainly found fossil in the firestone of Delitzet, belonging to the Cretaceous. A full monograph of the group under the name "Peridiniales" was published by Schütt.[144] [Illustration: FIG. 47.—_Pyrocystis fusiformis_, Murray. × 100. From the surface in the Guinea Current. (From Wyville Thomson.)]