50 Peridmiese 



longitudinal axis and sometimes pursue a spiral course about the axis of 

 progression. Its effect upon the general morphology of the PeridinieEe is 

 fundamental and a transverse furrow for its lodgment is almost universally 

 present throughout the group. The longitudinal flagellum also determines 

 the direction of the main morphological axis. That part of the body which 

 leads in locomotion is called the apical region. The side of the body on 

 which the two ends of the girdle are found is called the ventral side. 



Our knowledge of the life-history of the Peridiniefe is yet very deficient. 

 It is definitely known that certain forms periodically enter into an encysted 

 condition in the form of large resting-spores. Some become encysted in the 

 winter, others in the spring and summer, and yet others in the autumn. 

 Thus, in temperate climates some species are found in the active vegetative 

 condition in every month of the year. In warmer climates it has been 

 shown that the periodic forms of colder latitudes become perennial con- 

 stituents of the plankton. The duration of vegetative and encysted states 

 is very variable, depending largely upon temperature. Multiplication by 

 cell-division is often very rapid, and has been shown to occur in both the 

 motile vegetative condition and the encysted state. 



The Peridinieas, as generally understood, fall very naturally into three 

 divisions, concerning which there has never been any controversy. To these 

 it is apparently necessary to add a fourth group, the Pyrocystacese, in order 

 to accommodate a group of organisms which cannot with justice be included 

 in the other three. It will be convenient to consider these groups 

 separately. 



Class 

 Order PERIDINIALES. 



A. Cell-wall absent or consisting of a gelatinous covering. Vegetative condition 



motile. Fam. Gfymnodiniaceae. 



B. Cell-wall consisting of a continuous cellulose membrane. Vegetative condition 



non-motile. Fam. Pyrocystacese. 



Cell-wall consisting of a cellulose membrane which is composed of definite 

 articulated plates. Vegetative condition motile. 



a. Cells with a girdle, usually in the form of a conspicuous groove; cell-wall 



composed of three or more plates. Fam. Peridiniacex. 



This is the important group, the classification of which depends entirely 

 upon the morphology of the plates composing the wall. 



b. Cells without a girdle ; cell-wall composed of two plates only. 



Fam. Prorocentracete. 



To these four groups should probably be added the Phytodiniacese of Klebs (in part ; 

 see p. 57) and the Blastodiniaceee of Chatton (see p. 53). 



In the above arrangement the Peridiniales is the only order of the class Peridiniege, but 

 it may be expedient in the near future to establish a new order for some of the Phyto- 

 diniacese and perhaps for the Pyrocystacete. 



