130 PROTOZOA 



to form tissues, and condemned to ultimate death with the body 

 as a whole, and those that ultimately give rise to the repro- 

 ductive 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 differen- 

 tiation 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 1 5 /jl ( T J .~ f ) and even 3 7 5 jm ( G V0 * n Pyfocystis 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, 1 

 and never protrudes from it in life, executing undulating move- 

 ments 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. 2 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 over- 

 lapping at their junction ; occasionally the cell has been seen 

 to moult them. 



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 chro- 

 matophores, 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 



1 In the Adinidae there is no groove ; the two lashes arise close together, and 

 the one is coiled round the base of the other. 



2 In Unt. Inst. Tubingen, i. 1883, p. 233- 



