KOFOID AND SWEZY: UNARMORED DINOFLAGELLATA 89 



scarcely organized, variously directed, posteriorly located, and functionally in- 

 efficient form, as found in P rot apsis (tig. LL), and culminating in the con- 

 centrated or more highly integrated, anteriorly directed, and functionally 

 efficient form of the organ. This is best seen in Poucketia and Erythropsis, 

 where the known species are sufficiently numerous to afford a range of t\'pes. 



In most of the species of Pouch efia the ocellus presents a high degree of 

 integration (tigs. 00, PP). It is in the genus Erythropsis, however, that this 

 peculiar organelle reaches its highest iy^e of development as well as its greatest 

 size, as in E. cornuta and E. pavillardi (figs. SS, 1, 3). Evidence of a lesser 

 degree of integration is also foimd in this genus, as in E. Jahnim (fig. SS, 6). 



Structitjal Evoli'Tiox. — The influence of the peculiar types of flagella 

 characteristic of the dinoflagellates and their peculiar position has undoul)tedly 

 played an important part in producing the other structural modifications of 

 that group. From this standpoint the most fundamental feature is the ri])bon- 

 like transverse flagellum. In the lower forms, such as Protodinifer, Haplo- 

 diniiun. and Prorocentrum (figs. R, 5, 7), both flagella arise at the extremity of 

 the body, and no profound structural modifications thereof result. AVith the 

 migration of the flagella from this point, however, changes begin to take place. 

 These consist of the formation of a groove or girdle (fig. B, gir.) aroimd the 

 body for the lodgment of the transverse flagellum, and another furrow which 

 we call the sulcus (fig. B, sulc.) in a longitudinal direction on the ventral surface 

 which contains the longitudinal flagellum. The sulcus also extends anteriorly 

 and sometimes posteriorly beyond the area of its actual functional relations to 

 the flagellum. 



The ])eginning of this process may be seen in Protodinifer with its poorly 

 developed, evanescent girdle (fig. R, 2). In Oxyrrhis it has extended com- 

 pletely around the body, but is still poorly developed, with ill-defined posterior 

 border (fig. R, 3). It reaches its highest development in the next group, the 

 G}Tiino dinii da e . 



The formation of both furrows is due to the direct influence of the activity 

 of the flagella on the surface of the body. In Protodinifer the transverse 

 flagellum has a greater length than the circumference of the body and lies in- 

 differently in several jjositions. Its activity would be felt on all parts of the 

 circle, but more strongly near its origin, where we find accordingly the Ix'gin- 

 nings of the girdle. The greater part of the flagellum in this species is not held 

 close to the body, as is the case in the higher members of the group. In the 

 Gymnodiniidae the flagella are frequently short, not more than 0.5 circumfer- 

 ence in length, especially in moril)und individuals. Many other species, how- 

 ever, especially in CocModinium (tig. HH, 3) and Pouchctia (figs. 00, 5 ; PP, 3), 

 have the transverse flagellmn equal in length to the girdle, and this is probably 

 the normal condition. 



The action of a broad, ribbon-like flagellum in constant motion with short, 

 transverse waves, passing through its entire length and held close to tlie body, 

 would have a profound influence in hollowing out for itself in the plastic body 



