Flagellata and Primitive Alga. 
179 
despite the voluminous literature that has accumulated as the result 
of the collection of plankton Peridiniales, at all times of the year, 
by numerous workers ; there is, for instance, no proof at present 
that a sexual process occurs in the group—unless we include in it 
the genus Noctiluca (see below). Zederbauer (157) observed 
individuals of Cerntium Hirundinella, a freshwater species, grouped 
in pairs and connected by protoplasm, while the contents of other 
individuals had been extruded in the form of a cyst—whence he 
inferred that zygospores had been formed as the result of a 
conjugation process. However, Jollos (62) and Wesenberg-Lund 
(145) observed the formation of similar cysts in Cerntium which 
had certainly arisen without copulation, and it would appear that 
the paired cells seen by Zederbauer had arisen as the result of 
abnormal cell division or that the process may be simply one of 
plastogamic fusion such as occurs in certain Protozoa. 
Schiitt (134) divided the Peridiniales into three families— 
Gymnodiniaceae, Prorocentraeege, and Peridiniaceae. In the 
Gymnodiniaceae the cell is naked or clad only by a thin mucilaginous 
or cellulose wall showing uniform structure and in most cases 
forming merely a delicate periplast like that of the majority of 
Flagellata. Of the two flagella, one is directed backwards in the 
longitudinal groove (sulcus) while the other (usually thrown into 
undulating curves) lies in the transverse groove (annulus). Both 
sulcus and annulus may be straight, meeting at right angles at one 
point where the flagella arise—in this case the annulus is either 
subequatorial (complete in Gymnodiniutn, a half-ring in Hernidinium ) 
or is near the anterior pole so that the anterior (prae-annular) 
portion of the cell is much smaller than the posterior and is rostrum¬ 
like (Aniphidinium) ; or the annulus may be spirally coiled and the 
sulcus slightly ( Spirodinium ) or markedly ( Cochlodinium , Pouchetin) 
spiral also, meeting the annulus at both ends, with the transverse 
flagellum inserted at the anterior end of the annulus and the 
longitudinal flagellum at the posterior end of the sulcus— Pouchetia 
is distinguished from Cochlodinium by having a complicated 
stigmatic apparatus consisting of a red or black pigmented body 
with one or more large spheroidal refractive lens-like bodies 
adjoining it. In this family Schiitt includes Pyrocystis (see below). 
In the Prorocentraceae (Fig. 8) there is a shell consisting of 
two biconvex valves, dotted with pores except on either side of the 
suture ; from an opening in this suture, at the anterior end of the 
cell, arise the flagella, of which one is directed forwards while the 
second either vibrates about the base of the first or is directed 
laterally. In the three genera included here by Schiitt, there are 
usually two large plate-like chromatophores, one lying within each 
valve of the wall, but sometimes these are deeply lobed or even 
divided into a number of radiating elongated chromatophores, as 
in the majority of Peridiniales. In Cenchridium the pore from 
which the flagella arise is continued inwards as a gullet-like tube; 
in Exuvicdla and, more markedly, in Prorocentrum, there are small 
projections of the wall close to the flagellum pore, which Schiitt 
suggests may represent the beginnings of the characteristic wing¬ 
like expansions of the margins of the flagellum groove in the 
Peridiniaceje, 
