10 MEMOIRS OF THE UNIVERSITY OF CALIFORNIA 



The shape of the body of the lower members of the dinoflageUates approaches 

 that of the t^'jDieal flagellate, that is. a slender pear-shape with the flagella 

 attached at the anterior end. This is shown in Haplodinium' (fig. R, 5). 

 Starting out from this simple t}^e the first changes are foimd in the gradual 

 shifting of the location of the flagella. which may have their origin at any point 

 lietween the anterior and jiosterior ends of the body (fig. R). With this Ijack- 

 ward shift of these organelles the fonn of the body responds to the change by 

 assuming a spindle-shape, which is the predominant one among the dinofla- 

 geUates, thongh often secondarily modified, as in the dorsoventral compression 

 of the body in many species of AmpJiidiniion. 



Further modifications of this prhnary shape are found in the extension of 

 the labile i^osteroventral sulcal area of CocModininm and Pouchetia. culmi- 

 nating in the prod of Erythropsis (fig. T). This latter genus is further modi- 

 fied by the thickening of the body, gi\'ing it a squat appearance typical of all 

 the members of the genus (pi. 12). A few species of Gijnniodinium (figs. X, 

 7, 8, 26) seem to have acquired a permanently rounded form. 



One of the most striking and characteristic features of the body is its 

 bilateral asAimnetry, following the rule obtaining throughout the Protozoa 

 generally, where complete bilateral SATinnetry is the exception outside of some 

 of the Radiolaria. This bilateral asynunetry is directly correlated with the 

 spiral course in locomotion, and may be one of the factors in the maintenance 

 of the organism near the surface of the sea. Kofoid's studies (1910&) on the 

 thecate dinoflageUates point to the conclusion that optinuim conditions of ex- 

 istence for the members of this group lie within the upper levels of more or less 

 ilhmiinated water, and that descent below this region is fatal for them. The 

 apparent lack of special organs for flotation, other than vacuoles, is compen- 

 sated for by the asynnnetry of the body, and in the thecate forms, where a 

 greater appreciable overweight of the body is present, by the formation of horns 

 and fins. These combined with the rotation of the body caused by its as^in- 

 metry impede the descent of the organism into lower regions in response to 

 gravity. Having a lighter specific gra'^dty and greater powers of locomotion, 

 the need for additional structures to meet this response is less insistent in the 

 naked dinoflageUates than in the thecate forms. 



An increasing torsion or twisting of the body, beginning with the genus 

 Gyrodiniiim (figs. CC-EE), reaches its culmination in the genus CocModininm 

 (figs. FF-HH), where the twisting of the body, as shown by the course of the 

 girdle, may be as great as four complete turns, as in C. aufiustum (fig. HH, 15). 

 This is correlated with the movements of the flagella, combined with the pressure 

 exerted by the water on the more plastic species of the genus. In the thecate 

 forms this backward reach of the distal end of the girdle has not developed 

 beyond the Gi/nniodiuinm and Gi/rodiiiiiiiii types, showing either a loss of plas- 

 ticity in the body structures accompanying the relatively slight locomotor powers 

 of the skeletal-bearing forms, or else indicating the origin of these forms from 

 ancestors sunilar in lack of torsion to these two genera in the G^nnnodiniidae. 



