136 THE PROTOZOA 



Diatomacese (Warming). The majority of observers are agreed, how- 

 ever, that a connecting link with the Flagellidia is seen in certain 

 species of the family Prorocentridas and represented among living 

 forms by Exuvicella and Prorocentrum (Fig. 66). Biitschli ('83) and 

 Klebs ('93) agree that these might be called true Flagellidia; for, as 

 Delage happily expresses it, they are little more than a chromomonad 

 in the shell of Phacotus. The shell is bivalve, and perforated by 

 minute apertures characteristic of the Dinoflagellidia, and there is an 

 entire absence of longitudinal and transverse furrows, while the flagella 

 are directed outward from the anterior end. Biitschli, Bergh, and 

 Klebs derive them from forms like Cryptomonas, where the two flagella 

 are pointed in the same direction and the chromatophores are yellow. 

 The transition from these primitive forms of Dinoflagellidia to the 

 more complex types with a shell composed of nicely articulated plates, 

 is much more difficult than the connection between the main groups. 

 Stein maintained that the simplest form is the unshelled Gymno- 

 dinium, but Biitschli showed that this view is not in harmony with the 

 other forms of Dinoflagellidia, it being much more obvious to con- 

 sider the shell of the Peridinidse, for example, as arising by the split- 

 ting up of the bivalve shell of the primitive type, than by the loss of 

 this shell and the subsequent formation of the articulated forms. 

 Evidence in Biitschli's favor is seen in the forms where there are but 

 few plates {e.g. Ceratocorys), although we are inclined to agree with 

 Klebs that a polyphyletic origin of the group is possible and that 

 Gymnodinium might have been derived from the Rhizomastigidas. 



The origin of the Cystoflagellidia, composed of Noctiluca and 

 Leptodiscus, is to-day generally conceded to be from the Dino- 

 flagellidia, and is supported by direct evidence in the development 

 of Noctiluca, where the swarm-spores are strikingly similar to Peri- 

 dinium. The relationship to the Dinoflagellidia, as first pointed out by 

 Allman, was based upon superficial resemblances only, and the first 

 conclusive observations must be credited to Pouchet ('83) and to Stein 

 ('83), while Biitschli ('85) first applied the theory on .the basis of the 

 swarm-spore as described by Cienkowsky ('73) an d Robin ('78). The 

 interesting form which Pouchet later described ('92) as Peridinium 

 pseudonoctiluca is now considered a young stage of Noctiluca. 



CLASSIFICATION 



Class II. MASTIGOPHORA. Protozoa of definite or indefinite form; naked, or 

 provided with a well-defined membrane. The nutrition is holozoic, parasitic, 

 holophytic, or saprophytic. The motile organs are flagella, which may vary in 

 number from one to many. Mouth, contractile vacuole, and nucleus are usually 

 present. They are usually small forms with a widespread tendency to colony- 

 formation. 



