Consider the slime molds, which have no brain, specifically Physarum polycephalum and Dictyostelium discoideum (you'll forgive me for conflating them a bit in this brief comment):

It can solve mazes. [0]

"They remember, anticipate and decide." [0]

They practice primitive agriculture: "they carry, seed and prudently harvest their food" (though no cultivation - what dummies!). [1]

And here's the kicker: "P. polycephalum ... spends most of its life as a single cell containing millions of nuclei, small sacs of DNA, enzymes and proteins." [0], but "When prey bacteria become scarce, Dictyostelium discoideum amoebae aggregate by the tens of thousands and produce a multicellular migratory slug that becomes a fruiting body in which about 20% of cells die to form a sterile stalk. The stalk aids the dispersal of the remaining cells, which differentiate into spores in a spherical structure called the sorus ..." [1]

[0] https://www.nature.com/articles/nature.2012.11811

[1] https://www.nature.com/articles/nature09668