268 BIOLOGY OF THE PROTOZOA 



lated phase, or the Pha^ocapsida? as Cryptomonads for similar 

 reasons. 



The majority of Dinoflagellida are covered by distinct shells 

 composed of a cellulose-like substance; yet some of them (Gymno- 

 dinioida?) have none; the majority also bear characteristic cross 

 and longitudinal furrows ; yet some of them (Adinina) have no trace 

 of furrows. The surface-dwelling forms for the most part have 

 distinct chromatophores; but a number of these, and all of the depth- 

 dwelling types, are devoid of chromatophores. The peculiarities 

 of the group in regard to these structures are so characteristic that 

 if only one is present and the others absent, it is sufficient to classify 

 the organism. 



The Dinoflagellida are sometimes associated in classification 

 with the diatoms and desmids (Zygophyta). Many zoologists still 

 regard them as representing a major group of flagellates (Biitschli 

 for example, divided the Mastigophora into Flagellata, Dinoflagel- 

 lata, and Cystoflagellata) but with increasing knowledge of life 

 histories and finer structures the prevailing opinion at present is 

 that the Dinoflagellida are more closely related to the Crj^^tomona- 

 dida than to any other group. 



While many of the Dinoflagellida are naked, the majority are 

 entirely covered by a definite refractile membrane of cellulose-like 

 substance which is often impregnated with inorganic, frequently 

 calcareous, deposits. In some forms a limited region, the so-called 

 rhombic area (Kofoid), remains unprotected. The test is simple 

 and apparently made up of one piece in Glenodinium and Heini(B- 

 dinimn, or of two valves as in the Adinina and the Dinophysidae, 

 while in the family Peridinidae they are composed of plates of definite 

 form and arrangement which are frequently areolated and character- 

 istic of the species in each case. These plates are often drawn out 

 into typical horns and processes {e. g., Acanthodinium Kof.) Avhile 

 in the Dinophysidae fin-like ridges and wing-like processes often 

 give rise to fantastic shapes (DinoijJiysis, Ornithocercvs, etc.). 



Furrows on the surface of the cell, foreshadowed in forms like 

 Nephroselmis or Cryytochrysis of the Cryptomonadida, are highly 

 characteristic of the Dinoflagellates. One of these the girdle, or 

 annulus, is annular, running around the body near the center or 

 excentrically, or following a spiral course. In Polykrikos there are 

 several such transverse furrows (Fig. 132). In the corticate forms 

 there is usually a separate girdle plate which follows the contour 

 of the groove while its free edges are often drawn out into ledge-like 

 outgrowths. In some remarkable t}^es the transverse groove is at 

 the apical extremity of the body (Amphisolenia, DinojjJiysis, 

 Amijhidinwm, Fig. 70) and a series including these forms with 

 Prorncenirum and E.ruricBUa (Fig. 129), suggests the possibility 

 that the Adinina are extreme representatives of a series in wdiich 



