34 



PROTOZOA. 



generally present, and contain chlorophyll and diatomin*; but there 

 are colourless forms. Amylum, fat, red pigment, and stigmata may 

 also be present. There is always a longitudinal groove upon what we 

 call the ventral surface, and in the Dinifera there is a second groove 

 the transverse groove encircling the body ; the latter generally 

 has a slightly spiral course (so that the two ventral ends of it are 

 not quite at the same level, Figs. 27, 28) and in one genus makes 

 two complete turns. The two flagella arise, as a rule, close together, 

 where the two grooves cross one another. The transverse flagellum 

 lies wrapped round the body in the circular groove. The flagella 



project through a hole in the cuticle, 

 which in some genera at least ex- 

 tends as a slit along the left side of 

 the ventral groove, being enlarged 

 posteriorly where the longitudinal 

 flagellum projects. Reproduction 

 takes place by transverse fission. 

 Conjugation has been observed in 

 a few forms, and in a few cases 

 individuals have been observed to 

 unite in chains (? a form of conju- 

 gation). In Polykrikos, which has 

 four nuclei, there are eight trans- 

 verse furrows, each with a flagellum. 

 The presence of a mouth is doubtful. 

 Fresh-water and marine. 



FIG. 28. Shell of Ceratium tripos (after 

 Stein). If longitudinal furrow ; qf 

 transverse furrow. 



Sub-order 1. ADINIDA. 



Longish bilaterally -symmetrical forms with inclination to asymmetry ; ttoe 

 two flagella arise at the anterior pole, and the transverse furrow is not developed ; 

 with bivalved porous membrane ; two vacuoles near one another at the anterior 

 end ; chromatophores. 



Fam. 1. Prorocentrina, with characters of sub-order. Exuviaella Cienk. ; 

 Prorocentrtim Ehrb. 



Sub-order 2. DINIFERA. 



Longi- 



"\Vith a more or less distinct transverse furrow containing a flagellum. 

 tuclinal flagellum directed backwards. 



Fam. 2. Peridinida, with the transverse groove at or near the centre of the 

 body. Podolamjms Stein ; Fcridinium Ehrb. ; Goniodoma Stein ; Ceratium 

 Schrank (Fig. 28) ; Pyrophacus Stein ; Glenodinium Ehrb. (Fig. 27) ; Gymno- 

 dinium Stein, without cuticle ; Ccratocorys Stein. 



' By some naturalists there is supposed to be affinity between the Dino- 

 flagellata and the silicious Algre, the Diatomaccce. 



