Collar Cells
Historical document, translated for reference. It reflects medical knowledge of the 1920s–30s and is not medical advice.
Summary
Collar cells (choanocytes) are specialized cells found in certain protozoa and all sponges. In sponges, they line the internal cavity or specialized chambers, capturing and digesting food, and also play a role in fertilization by transporting sperm to egg cells.
Encyclopedia entry (1928–1936)
Collar cells (choanocytes), occur among protozoa only in flagellates, and among multicellular organisms only in sponges. One group, Mastigophora, received the name collar cells (Choanoflagellata), because the base of the single flagellum of these protozoa is surrounded by a high plasma collar, which facilitates the capture of food particles by the animal. In sponges, the entire endoderm consists of collar cells, which line either the internal cavity of the sponge or special flagellated chambers in the mesoglea of the sponge. The collar cells of sponges capture and digest food. According to new data, they also play a role in fertilization; collar cells capture sperm that penetrate into the sponge's body, retreat into the mesoglea, and deliver sperm cells to the egg cells located there.
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“Collar Cells.” Soviet Medical Encyclopedia. English translation of Bolshaya Meditsinskaya Entsiklopediya, 1st ed. (Moscow, 1928–1936), ed. N. A. Semashko. https://sovietmedicalencyclopedia.pages.dev/article/collar-cells/