KOFOID AND SWEZY: UNARMORED DINOFLAGELLATA 73 



Eneystment in the dinoflagellates, at least in the marine forms, is probably 

 due to one of two causes. The first of these is the need for protection during 

 the non-motile stage of the jieriod of multijile and binary fission. Binary fission 

 may occasionally take place in non-encysted forms, but is by far more frequently 

 seen in encysted ones. Multiple fission has never been observed in the non- 

 encysted state in our material nor is it illustrated in the literature. 



The second cause of eneystment is correlated with a holozoic type of 

 nutrition. It is apparently induced by the ingestion of large food bodies, and 

 indicates a need for a quiescent period for its assimilation. There are several 

 reasons for this conclusion. One of these is the prevalence of food bodies 

 within encysted individuals. These may give indications of having been re- 

 cently ingested with few signs of digestive changes having taken place (pi. 8, 

 fig. 85), or they may represent partly digested bodies in what are evidently 

 older, larger cysts (pi. 8, fig. 87) , or the final remains towards the end of the 

 period of digestion (pi. 8, fig. 89). When these encysted forms are observed 

 under tbe microscope a difference may usually be detected in their activities. 

 A recently encysted individual with a large food mass within its body is seldom 

 active and has never been observed to })reak out of the cj'st. On the other 

 hand, tliose individuals which give indications of having neared the end of the 

 period of digestion within the cyst are active, rotating within the cyst and often, 

 while under observation, escaping from it altogether. 



Another fact pointing towards the correlation of eneystment with holozoic 

 nutrition is the relative infrequency of eneystment in members of the group 

 having chromatophores, such as Ampliidiniiim and the lower species of Gymno- 

 dinium, and its f requeue}^ among the higher genera of the group, such as Cochlo- 

 dinium and Pouchetia. Other causes leading to cyst formation are probably 

 operative on all groups alike, hence the greater frequency of its formation 

 among holozoic types would lead to the conclusion that a correlation exists 

 between these two factors named. 



The cyst is first formed as a thin membrane secreted by the body wall and 

 closely investing it (fig. W, 1). The flagella are apparently lost at the begin- 

 ning of the process and the meml)raue is closely adherent to the outlines of 

 both girdle and sulcus. The flagella are probably absorbed, though no obser- 

 vations on this point can be presented except the occurrence of individuals with 

 short stout flagella. Very soon after the formation of the membrane the cyst 

 begins to enlarge as a i-esult of the incoming fluids which fill the spaces ])etween 

 the cyst wall and the body. With this distension tlie cyst gradually loses the 

 outlines of the body and becomes nearly symmetrical, either ellipsoidal or 

 spheroidal, varying in different species, but fairly constant in shape within the 

 species. The body also may lose its own characteristic outlines and furrows 

 and become rounded up (fig. V). The retention of tlie bodily features is the 

 condition more conmionly found. 



