PERIDINIEJ5 



(Peridiniales or Dinoflagellata) 



THE Peridiniese form a very natural group of purely aquatic organisms 

 embracing a large number of motile unicells furnished with two flagella and 

 a cell-wall, which in the majority of forms is composed of a definite system 

 of articulated plates. They are very largely organisms of the marine and 

 freshwater plankton, and next to the diatoms they are the most important 

 'producers' of organic substance in the sea. Peridinians rarely become 

 absolutely dominant in the freshwater plankton, but in the sea, and especially 

 in the warmer oceans, they occur in vast numbers, and along with associated 

 diatoms, constitute the principal food-supply of countless numbers of the 

 smaller marine animals. 



The marine genera and species are much more numerous than the fresh- 

 water ones, and they exhibit a much greater diversity of form, attaining a 

 larger size and a more elaborate external configuration. The marine and 

 freshwater species are quite distinct, but a few of the smaller marine forms 

 occur in brackish waters. 



The PeridinieaB are brown organisms, possessing as a rule a number of 

 yellow-brown chromatophores. For the most part their nutrition is holo- 

 phytic, starch and a fatty oil being stored as food-reserves. There are also 

 a number of colourless, saprophytic forms and a few in which holozoic 

 nutrition may sometimes occur. The cell-wall consists of cellulose, or more 

 rarely has become converted into a wide gelatinous envelope. A few forms 

 are naked that is, they are without a cell-wall. 



One of the leading features of the group is the presence of a transverse 

 furrow, which divides each cell into a front apical half and a back antapical 

 half. Crossing the transverse furrow is a much less evident longitudinal 

 furrow of very variable extent. 



The vast majority of the forms are motile during their active vegetative 

 existence, swimming with some rapidity by means of two flagella, one of 

 which lies and vibrates in the transverse furrow, while the other, which is 

 responsible for the propulsion of the organism, passes from the longitudinal 

 furrow obliquely outwards into the surrounding water. The transverse fla- 

 gellum is in some forms a ribbon-like band of protoplasm encircling the 

 body, and by its undulating movements causes the body to rotate upon its 



