“Journey to the Microcosmos: Gastrotrichs--Four Day Old Grandmothers” explores gastrotrichs, tiny multicellular aquatic animals in the phylum Gastrotricha. Using microscopy, the video explains anatomy and classification, including cilia, cuticle, scales, adhesive tubes, freshwater chaetonids, and mostly marine macrodasyids. It highlights their ecological role as meiofauna that transfer nutrients through food webs and their diet of bacteria, algae, and small particles. A major focus is reproduction: rapid direct development, parthenogenesis, hermaphroditism, and specialized opsiblastic eggs adapted for harsh conditions. Biology topics include microbiology, zoology, ecology, invertebrates, microscopy, life cycles, and animal reproduction. The content strongly supports life science curricula focused on organism structure, function, classification, ecosystems, and inheritance. Part of the "Journey to the Microcosmos" series.