CHAPTER II 



GENERAL MORPHOLOGY: NUCLEI, PUSULES, OCELLI, 



NEMATOCYSTS 



Nuclei. — The Diuoflagellata are definitely marked off from the remainder 

 of the Flagellata by certain features which are distinct and i^eculiar to the 

 group. Two of these features, the furrows and the two types of fiagella, have 

 already been discussed. Another organelle no less peculiar is the nucleus with 

 its moniliform chromatin threads, one of the most characteristic structures 

 found within the group, and, in some respects, the most constant feature of its 

 organization. It retains its typical appearance during encysted stages when 

 tlie flagella are lost and the furrows have become obliterated (fig. P) and forms 

 the only distinguishing mark of the organism. 



Its appearance in the living organism is usually remarkably clear and dis- 

 tinct. Schiitt, in his monograi^h on the dinoflagellates (1895), clearly illustrates 

 its structure in very many memljers of the group. It is usually relatively large, 

 varying from spheroidal to ellipsoidal in shape, sometunes greatly elongate as 

 in Torodirimm (fig. II), and slightly curved to conform with the contour of 

 the body (pi. 10, fig. ]15). The latter type is more frequently found in the 

 thecate forms, ])ut in both cases is proba]:)ly only a predivision stage. The 

 relative size may vary considerably in individuals of the same species. 



The nucleus is surrounded by a distinct membrane which is often double- 

 contoured. In a few species it is surrounded by a wide zone of clear, homo- 

 genous appearance (pi. 6, fig. 63), which in others, as in Gjjrodinimn coralUniim 

 (pi. 10, fig. 117), may be filled with large, fairly regular alveoli. In the living 

 organism its chromatic contents present an organization of moniliform threads, 

 the constituent granules of which are rather coarse and closel}' pressed together 

 in linear rows (fig. X). The chromatin threads may l)e variously arranged, 

 coiled, or in parallel rows, and fill the entire nucleus. The threads vary in 

 length with the size of the nucleus and usually follow its longer axis in more or 

 less of a spiral with the ends of the threads sometimes apparent at the poles of 

 the nucleus. Nucleoli may be present, lying imbedded in the mass of chromatin 

 threads (fig. U, 10). 



The structure of the nucleus varies but little in the different groups. In 

 Erijthropsis the chromatin network is rarely visible in the living form, the 

 nucleus presenting that glaucous appearance noticeable in the entire organism 

 (pL12). 



The position of the nucleus is generally near the center of the body, though 

 this may vary greatly throughout the different groTips. Its position may also 

 be changed by the presence of ingested food bodies within the cytoplasm. 



Nuclear division in the dinoflagellates has received comparatively little 

 attention, yet they possess a distinct type of mitosis closely correlated with the 



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