58 MEMOIRS OF THE UNIVERSITY OF CALIFORNIA 



modifications of the fundamental tyipe, such as membranellae and cilia in the 

 ciliates ; and the modifications in the Dinoflagellata are no less peculiar and 

 tyiDical. 



The modifications characteristic of this group consist of two grooves at the 

 junction of which are located the openings into the pusules. The first of these 

 grooves is the transverse one or girdle, extending in a more or less spiral course 

 around the body (fig. B, gir.). Occupying the furrow of the girdle is the rela- 

 tively wide, ribbon-like flagellum. This is in constant motion with waves passing 

 from its proximal to or towards its distal end. This motion, combined with 

 the spiral course of the girdle, keeps a constant current of water passing over 

 tlie opening. The second fiirrow is longitudinal and joins the two ends of the 

 girdle. The fiagellum which arises from the posterior ])ore and occupies the 

 posterior portion of the longitudinal groove or sulcus is threadlike, and hence 

 offers the least possible obstacle to the free movement of the current of water 

 down the sulcus. Both of these grooves are bordered by lips usually wide and 

 well developed. In the thecate forms these are still further marked off by 

 broad lists or fins. The relatively narrow opening or pore into the anterior 

 pusule lies at the anterior junction of the girdle and sulcus at the base of the 

 groove. The posterior pore lies near the distal jimction. These pores are thus 

 located at the points which would give optimmn conditions for their functioning 

 as c>i:ostomal areas. 



The anterior pusule is somewhat more prominent than the posterior one 

 and is probably the one in which the function most closely approximates that 

 of the cytophar^Tix of other grouj^s or is most highly localized. Its close con- 

 nection with the anterior flagellum finds its parallel in the juxtaposition of 

 flagella and cytostome in the trichomonad flagellates (Kofoid and Swezy, 1915), 

 and more strikingly in CJnlomonas Paramecium, where the flagella emerge 

 directly from the cytostomal opening with their insertion near its base. The 

 same connection of flagelhun and pusular opening is found also in the longi- 

 tudinal fiagellum. The reason for this intimate connection is patently the need 

 of a propelling force to insure a constant current of water down the ventral 

 groove and over the openings. The constant wavelike motion of the transverse 

 flagellum, combined with the shape of the furrows, lends itself admirably to 

 this purpose. 



The full homology of the c}i:ostome of the simpler flagellates is, however, 

 to be found in the intercingular sulcus as a whole, and the two pores at the bases 

 of the two widely sej>arated flagella are to be regarded as the anterior and 

 posterior ends respectively of the primitive cytostome shifted from its original 

 anterior location to the midventral region and stretched out longitudinally be- 

 tween the migrating flagella. It is twisted about the body in Gijuuiodinium 

 and Coclilodinium and may be extended anteriorly in the anterior limb or loop 

 of the sulcus towards the apex and posteriorly in the posterior limb towards 

 the antapex. It is improbal)le that these extensions have any share in cyto- 

 stomal functions of the intercingular siilcus, from which they take their origin. 



