_Volvox_, as we have seen, has attained a specialisation entirely comparable to that of a Metazoon, where the segmentation of the fertilised ovum results in two classes of cells: those destined {130}to form tissues, and condemned to ultimate death with the body as a whole, and those that ultimately give rise to the reproductive cells, ova, and sperms. But this is a mere parallelism, not indicating any sort of relationship: the oospores of the Volvocaceae show that tendency to an encysted state, in which fission takes place, that is so characteristic of Algae, and these again show the way to Cryptogams of a higher status. Thus, _Volvox_, despite the fact that in its free life and cellular differentiation it is the most animal of all known Flagellates, is yet, with the rest of the Volvocaceae, inseparable from the Vegetable Kingdom, and is placed here only because of the impossibility of cleaving the Flagellates into two. The Dinoflagellata (Figs. 46, 47) are often of exceptionally large dimensions in this class, attaining a maximum diameter of 150 µ (1/160") and even 375 µ (1/67") in _Pyrocystis noctiluca_. The special character of the group is the presence of two flagella; the one, filiform, arises in a longitudinal groove, and extending its whole length projects behind the animal, and is the conspicuous organ of motion: the other, band-like, arises also in the longitudinal groove, but extends along a somewhat spiral transverse groove,[140] and never protrudes from it in life, executing undulating movements that simulate those of a girdle of cilia, or a continuous undulating membrane (Fig. 46). This appearance led to the old name "Cilioflagellata," which had of course to be abandoned when Klebs discovered the true structure.[141] There is a distinct cellulose membrane, sometimes silicified, to the ectoplasm, only interrupted by a bare space in the longitudinal groove, whence the flagella take origin. This cuticle is usually hard, sculptured, and divided into plates of definite form, bevelled and overlapping at their junction; occasionally the cell has been seen to moult them.