276 BIOLOGY OF THE PROTOZOA 



Oxyrrhis, Povchetia, Erythropsis, Polykrikos, etc., are colorless types 

 with various structural peculiarities which distinguish them from 

 other dinoflagellates. Oxyrrhis marina has a rudimentary furrow 

 and no encircling flagellum; Erythropsis has a definite tentacle 

 capable of extension and retraction, while it, together with Pouchetia, 

 has a huge stigma provided with a distinct lens; Polykrikos has 

 several transverse furrows with flagella and one (P. auricularia, 

 Bergh) or several (P. schwartzi, Biitschli, P. kofoidi, Chatton) 

 axial flagella. This genus also is remarkable in having both tricho- 

 cysts and nematocysts the latter especially in P. schwartzi, have 

 been studied in detail by Chatton (Fig. 132). 



Tribe B. Peridinioid^, Biitschli, Bergh. — Family 1. Peridin- 

 idce are found both in fresh and in salt water, many of them being 

 deep-sea forms. Green-yellow or brown discoidal chromatophores 

 are usually present in large numbers but many deep-sea species are 

 entirely colorless. The main characteristic of the family is the 

 nature of the shell which is always made up of distinct plates the 

 number and arrangement of these plates aft'ording a basis for generic 

 and specific differences. The transverse furrow is covered by a 

 single annular girdle plate or cingulum which serves as a dividing 

 line between an anterior and a posterior part of the organism. The 

 former portion, called the epitheca, consists of apicals and dorsal 

 intercalaries and the latter, called the hypotheca, consists of pre- 

 cingulars, postcingulars and antapicals while the region of the 

 flagella origin is covered by one or more delicate and thin plates 

 forming a region which Stein called the "oral fissure" (Mundspalte) 

 and which Kofoid calls the "ventral area." 



Certain plates of both parts of the shell may be developed into 

 characteristic longer or shorter processes or horns (Fig. 131) those 

 from the hypotheca being usually solid while the single horn from 

 the epitheca is hollow and the distal end is open forming the apical 

 pore (Fig. 131, B, C). The antai)ical horns are often provided with 

 ribs, fins, or wings which are frequently developed asymmetrically. 

 The living organisms appear to have alternate periods of active 

 movement and of rest and according to Kofoid, when the flagella 

 are at rest the specific gravity of the shell and body causes the organ- 

 ism to sink from at or near the surface to greater depths. By 

 reason of the asymmetry due to the unequal development of horns, 

 fins and wings, the cell turns in its descent in such a manner that 

 the resistance to the water is increased by exposure of the broader 

 surfaces and sinking is correspondingly retarded. Such excrescences 

 on the shell are thus interpreted as organs of flotation and asym- 

 metry as an adaptation wherel)y forward movement is not impeded 

 while too rapid sinking is. 



In many species, e. g., Peridiniuvi dimrgcns, the plates are sepa- 

 rated by rather wide bands of intercalary stria? which, as Biitschli 



