KOFOID AND SWEZY: UNARMORED DINOFLAGELLATA 



75 



other types (figs. Q, 4, 5). The shape of it varies considerably, and may pos- 

 sibly be influenced by the relative size of the enclosed dinoflagellate or even be 

 the honiologue of a Pijroci/sfis stage. Thus in figure Q, 1, the outline is not 

 modified by the contained organism, while in figure Q, 6, considerable change 

 in shape results from the development of the relatively huge Gyrodinium 

 within it. 



In the third type we find the digestive cyst, or the one commonly enclosing 

 the t\"|3ical unarmored dinoflagellate when first observed under the microscope 

 (fig. Q, 5). This is thin- walled, hyaline, and varies in size from the closely 

 investing sheath that is first formed to one several times the length of the 

 enclosed organism. This cyst is always very delicate and easily ruptured, in 

 marked contrast to the other, stronger types. Successive cysts of this type may 

 be formed, one within the other (fig. Q, 7). 



Fig. Q. Types of cysts in the Gymnoilinioidae. 1. Gymnodininm sp. After Schiitt (1895, pi. 25, fig. 80,) 

 2. Gymnodinhim lurutla Schiitt. After Dogiel (1906, pi. 1, fig. 2). 3. Cochlodinium piruin (Schiitt). 4. Gymno- 

 dinium sp. 5. Gymiwdinium lunula Schiitt. 6. Gyrodinium falcatum (Schutt). After Schiitt (1895, pi. 25, fig. 

 8I3). 7. Gyrodinium ovum sp. nov. Formation of three successive cysts. X 250. 



Sex.^ — The question of sex and sexual phenomena in the dinoflagellates is one 

 about which, thus far, but little is definitely known. In only one member of the 

 Gymnodinioidae, Noctilucn scintillans, has conjugation been described (Ishi- 

 kawa, 1891). This process is stated to take place by the cytoplasmic fusion of 

 two isogamic, motile individuals, apparently of the normal type, whose nuclei 

 become apposed but remain distinct. Both the nuclei and the cytoplasm then 

 divide and the syngamic fusion takes place between the two nuclei in each of 

 the daughter cells thus produced. This is undoubtedly an incorrect interpre- 

 tation of the normal mitosis in which the large j^ai-adesmose .so indents the 

 nucleus as to give the false appearance of two separated but apposed nuclei. 



