Fig. 1.

Invaginating projections in simple animals. a, b In the sponge, Tethya lyncurium, elongate processes with many fine filaments, from neuron-like cells (a network of elongate cells with thin, often branched processes that may interconnect among the cells), project “boutons” (asterisks) that can make invaginated contacts with other cells, including scleroblasts (a; sc) and flagellated endopinacocytes (b; ep). The processes contacting the latter kind of cell are 100–250 nm in diameter, and the intercellular space at the contact is about 20 nm (Pavans de Ceccatty 1966). c “Trophic prolongation” from a parenchymal cell that extends into an adhesive gland cell in the flatworm, Temnocephala novaezealandiae; the swollen end forms a desmosome attachment (arrow) near the gland cell nucleus (nucleus not shown; Williams 1994). All drawings in Figs. 1, 3, 4, 7, 8, and 9 are original and are based on micrographs and illustrations in the original publications