Diagnosis_Genus: Oodinium Chatton 1912. External parasite of tunicates, chaetognaths and possibly polychaetes. The trophont is sac-like and is attached to the host by a stout peduncle with a very large, flat attachment surface closely adhering to but not penetrating into the epithelial cells of the hosts. The cell membrane of the attachment surface has numerous thin and long invaginations serving for increased absorption of the nutrients by osmotrophy. There are no chloroplasts, the trophont's theca has plates filling its alveoli and constituting together the dinoflagellate armour of the cell. There are no starch grains. The nucleus contains stout coiled chromosomes only in young individuals. The dinospore is a gymnospore. (Lom 1981).
Diagnosis_Species: Oodinium jordani McLean & Nielsen 1989. Trophozoites oval, l= 125 to 394 µm, w= 47 to 243 µm. Nucleus large, oval, chromosomes not visible; cytoplasm contains lipid droplets and paraglycogen-like granules; pedoncular bulb constricted centrally, densely provided with membraneous channels and limited distally by an osmiophilic ring; short peduncular cortex provided with longitudinal ridges; peduncular medulla penetrates host and terminates in many vase-like structures formed from 2 membranes and rarely joined with membraneous channels. Cleavage of the sporocyte is longitudinal; after 9 divisions, a stage where the formation of dinokaryotic chromosomes is visible. Trichocysts present. Host, Sagitta elegans Verill, 1873 (phylum Chaetognatha).
Body_trophont_length: 125-394 µm
Body_trophont_wide: 47-243 µm
Pigment: No