There is a widespread presumption, as here, that the terms 'stramenopile' and 'heterokont' are synonyms. They are not because they are defined differently and despite compositional overlap, most applications of the names imply differing compositions. Morphology Motile cells Many heterokonts are unicellular flagellates, and most others produce flagellated cells at some point in their lifecycles, for instance as gametes or zoospores. The name heterokont now refers to the characteristic form of these cells, which typically have two unequal flagella. The anterior flagellum is covered with one or two rows of lateral hairs or mastigonemes, which are tripartite (i.e. with a flexible basal insertion, a stiff hollow component, and tipped with fine delicate hairs), while the posterior flagellum is smooth, and usually shorter, or sometimes reduced to a basal body. The flagella are inserted subapically or laterally, and are usually supported by four microtubule roots in a distinctive pattern. Opalines have many rows of flagella that do not have flagellar hairs. They are closely related to endosymbiotic proteromonad flagellates some of which have tripartite hairs extending from the body surface. Mastigonemes are manufactured from glycoproteins in the endoplasmic reticulum before being transported to the anterior flagellar surface. When the hairy flagellum beats, the stiff mastigonemes are forced backwards and this creates a retrograde current, pulling the cell through the water or bringing in food. The term mastigonemes refers to various types of flagellar hairs, but those of stramenopiles have a distinctive structure. It was treated as the evolutionary innovation that defined the stramenopiles, and although not initially so, it is increasingly treated as the defining characteristic of the heterokonts. Mastigonemes have been lost in a few heterkont lines, most notably the diatoms, opalines, and actinphryid heliozoa. Chloroplasts Many heterokonts are algae with chloroplasts surrounded by four membranes, which are counted from the outermost to the innermost membrane. The first membrane is continuous with the host's chloroplast endoplasmic reticulum, or cER. The second membrane presents a barrier between the lumen of the cER and the primary endosymbiont or chloroplast, which represents the next two membranes, within which the thylakoid membranes are found. This arrangement of membranes has led to the hypothesis that heterokont chloroplasts were obtained from the reduction of a symbiotic red algal eukaryote, which had arisen by evolutionary divergence from the monophyletic primary endosymbiotic ancestor that is thought to have given rise to all eukaryotic photoautotrophs. The chloroplasts characteristically contain