During gastrulation, morphogenetic movements establish the three embryonic cell layers. In sea urchins, secondary mesenchyme cells (SMC) at the vegetal pole trigger the formation of the archenteron, the central cavity of the gastrula that later develops into the primitive gut. On p. 547, Croce and co-workers report that a newly identified sea urchin Wnt receptor –Frizzled (Fz) 5/8 – is required in SMCs to control archenteron invagination. They show that Fz5/8 is expressed only in the animal domain and in the SMCs during sea urchin embryogenesis. Loss-of-function analyses indicate that Fz5/8 is not involved in the early specification of embryonic cell types but is required to control the primary invagination of the archenteron, which it does through the non-canonical planar cell polarity pathway of Wnt signalling. The researchers suggest that Fz5/8 modulates the morphogenetic movements that initiate gastrulation by controlling SMC adhesion, shape and polarity, a mechanism that is likely to be conserved in vertebrates.