The Prorocentrales are a small order of dinoflagellates. They are distinguished by having their two flagella

Flagellum

A flagellum is a tail-like projection that protrudes from the cell body of certain prokaryotic and eukaryotic cells, and plays the dual role of locomotion and sense organ, being sensitive to chemicals and temperatures outside the cell. There are some notable differences between prokaryotic and...

 inserted apically, rather than ventrally as in other groups. One flagellum extends forward and the other circles its base, and there are no flagellar grooves. This arrangement is called desmokont, in contrast to the dinokont arrangement found in other groups. Accordingly, the Prorocentrales may be called desmoflagellates, and in some classifications were treated as a separate class Desmophyceae.

All members have chloroplast

Chloroplast

Chloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis.Chloroplasts are green...

s and a theca, which is composed of two large plates joined by a sagittal suture. This structure is shared with the Dinophysiales, and they are probably sister groups. This does not show up on rRNA trees, which also show two separate groups of Prorocentrum, but they leave their relationships mostly unresolved.