Figure 2.

(a) Regulatory genes (rectangular boxes) and (external) environmental influences (round-edged boxes) controlling the two major morphogenetic biomechanical processes, viz. the deposition of the skeleton and the formation of exhalant pores and the aquiferous system in a sponge through removal of material. Triangular arrows indicate activation of a gene or process, while blocked arrows indicate suppression of a gene or process. Only the shaded components in the diagram have been included in the model. (b) Small detail of the accretive growth model showing the construction of a new layer (layer i+1) on top of the previous layer (layer i). The thickness l_i+1 of the new layer and the distance between the vertices V_i and V_i+1 are determined by the growth function in equation (2.2). (c) Basic idea of the coupled simulation model. The simulated form consisting of triangulated growth layers is mapped onto a discrete lattice. The discrete representation is used for computing the diffusion model and the flux of nutrients into the sponge surface. Simulated silicate is exclusively distributed by diffusion, and the top plane is the source of silicate, while all silicate is absorbed at the surface of the growth form and periodic boundary conditions are applied at the vertical faces of the simulation box.