KOFOID AND SWEZY: UNARMORED DINOPLAGELLATA 23 



that are usually preseut iu the cytoplasm aud the rather complicated pusule 

 ap]5aratus. In the latter he distinguished four or more parts, including a sack 

 ])usule, a collecting pusule, smaller daughter and accessory pusules in the sur- 

 rounding cytoplasm, with a pusule canal extending to the periphery of the l)ody 

 from the main pusule. This entire structure he considered homologous with 

 the contractile vacuole of the ciliates and rhizopods. 



A superficial examination reveals a general similarity in appearance hetween 

 the two structures. A closer examination, however, brings to light striking 

 differences. Unlike contractile vacuoles, these pusules possess a distinct 

 envelope or membrane, and contractions have never been observed. 



This structure, moreover, has been shown by Kofoid's work (1909) on 

 Pen'di Ilium sfciiii to be connected directly with the intake of fluid into the ])ody, 

 and not, as might be expected from the function of similar organelles in the 

 ciliates, as a collecting pusule for the discharge outward of fluids within the 

 body. The actual process of filling uji from the surrounding medium has been 

 oliserved by us in some species of Gj/iiiiiodiiiiiiin, and in these cases did not occiir 

 gi-adually but with a sudden inrush that immediately inflated the pusule. The 

 varying sizes and degree of development of these structures are dependent upon 

 the periodicity of their functioning, and this evidently accounts for their 

 presence in one indi-^'idual and absence in another of the same species. 



The jH'ocess is essentially as follows : The fluid is taken in at the pores as 

 a result of the activity of the flagella, particularly of the waves of contraction 

 of the transverse flagelluni, whicli tends to carry a current of water along the 

 girdle to its proximal end and thence down the concavity of the ventral area, 

 thus batliing both pores. There may be a continuous, gradual accumulation of 

 fluid in the pusule or it may, by a sudden expansion, fill up with a rush. Vacuoles 

 similar in color and refractive index to the pusules soon l^egin to accuundate 

 in the plasma. In Peridiniuni steiiii, where this process was followed contin- 

 uously for nearly five hours (Kofoid, 1909), minute vacuoles appeared in a 

 layer aroimd the sides of the pusule, followed shortly by larger vacuoles in the 

 surroimding plasma. These collected in the pei-ipheral zone, the outer border 

 of which began to shrink away from the thecal Avail, the space thus left vacant 

 being filled with a fluid of the same rosy tinge as that found in the pusules and 

 vacuoles. This fluid seems to ])e discharged from the surface, probably ])y 

 osmosis. 



Various stages of this process may he seen in our figures of the Gynniodi- 

 nioidae, suggesting the sinularity of their fmictioning with that of the ])usules 

 in tlie thecate forms. In Gi/rodiiiiioii cupsiihil/uii the final stage of the process 

 is in ])rogress (pi. 5, fig. 54), with the vacuoles collected at the surface ready to 

 discharge their contents into the surrounding water. In some cases the ])lasma 

 is rather densely filled with these vacuoles (pi. 7, fig. 74), while in other cases 

 the vacuoles are eiitii-ely absent. The huge size of the pusules in some indi- 

 viduals (pi. 5, fig. 57) and their entire absence in others suggests a periodicity 



