Figure 12

Homology of the sea urchin gastric caecum based on its integration into the haemal system of the digestive tract as a primary criterion. Schematic representations of the digestive tract haemal system in Cidaridae (A), Echinidae (B), and Spatangidae (C, D). The sea urchin stomach is accompanied by an inner (im) as well as an outer (om) marginal haemal duct. The outer marginal haemal duct sends out branches towards the dorso-ventral mesentery (dm) which is connected to the axial complex (ac) (A). The gastric caecum (gc) - if present (B-D) - is well-integrated into the haemal system. The black arrow in (D) depicts the conspicuous side branch of the inner marginal haemal duct present in presumably all spatangoids and potentially further irregular sea urchin taxa. AB = aboral view, LA = lateral view. Not to scale.