F. Cavers. 
184 
like Gymnodinium. Whether Polykrikos forms a link between the 
Peridiniales and the Ciliate Infusoria is, of course, an open question 
in the absence of further transitional types. It is possible that the 
genus Eyytliropsis (Hertwig, 57 ; Delage, 37 ; Pavillard, 114) affords 
such a transition ; in this organism the irregularly sphericai body 
shows a longitudinal groove, a transverse groove with a wavy 
flagellum at the anterior end of the body, and a relatively thick 
contractile outgrowth at the posterior end, while there is a stigmatic 
apparatus comparable with that of Pouchetia. Hertwig regarded 
Erythropsis as an Infusorian allied to Vorticdla ; while Metchnikoff 
compared its appendage to the sucker of Acineta and placed the 
genus in the Suctorial Infusoria. It is probable that the resemblances 
to Infusoria presented by Polyknkos and Erythropsis are merely 
superficial or due to homoplasy ; in any case, both genera appear 
to be directly related to the Gymnodiniacese. 
The life cycle of the lower Gymnodiniacese, so far as known, 
is extremely simple. In some cases division occurs in the motile 
condition, but more usually after encystment, the cyst being covered 
by gelatinous envelopes or by a firm wall and its contents dividing 
into two or more cells. In Cystodinium (Fig. 9, 20-26) the motile 
cells resemble Gymnodinium in structure, but on becoming encysted 
they acquire an elongated and horned form, the contents then 
become rounded off and dividing to produce two or four motile cells. 
In Diplodinium (Fig. 9, 27-34) the life cycle is somewhat complicated, 
since the encysted cell divides to form sixteen secondary cysts, each 
of which gives rise to four, eight, or sixteen motile Gymnodinium- like 
cells ; to this genus Klebs refers Pyrocystis lunula and certain 
species which had previously been placed in the genus Gymnodinium. 
Finally, Hypnodinium (Fig. 9, 35-40) is known only in the resting 
stage; on becoming encysted, the protoplast shows Gymnodinium• 
like grooves and divides into two naked cells exactly like Gymno¬ 
dinium but without flagella—on being set free by rupture of the cyst 
these two cells acquire a membrane and soon form new cysts. 
In the genera Blastodinium and Apodininm, recently discovered 
by Chatton (21, 22), and perhaps best placed in a family (Biasto- 
diniaceae) distinct from but closely allied to the Gymnodiniacese, 
which live as parasites or commensals in the bodies of Copepods 
and other marine animals, the cell divides into two portions, of 
which one continues the ordinary vegetative cycle while the other 
divides into a number of cysts which are set free as biflagellate 
Gymnodinium- like cells. 
The genus Diplodinium leads from the Gymnodiniacese to the 
family Phytodiniaceae (Fig. 10, 1-13), which includes the old genus 
Pyrocystis (minus P.lunula, now transferred to the genus Diplodinium) 
and four new genera founded by Klebs (68). In this family the 
cells show Peridinean cytological features, though no grooves are 
present; reproduction takes place by simple division of the cell 
contents into two, but no motile cells have been observed. The 
simplest form is Phytodinium, with ovoid cells (Fig. 10, 7,2); in 
Pyrocystis (Blackman, 8) the protoplasm is radially arranged, and 
is massed together at one end of the cell, very much as in the 
primary cyst of Diplodinium lunula; in Tetradinium (Fig. 10,5-7) 
