The former separates it from the branchial cavity; the latter from the fifth sac, to be described by-and-by.

Qu quadrate bone, Psph parasphenoid, PtP pterygopalatinum, Vo vomer, d teeth, na nostrils, Br branchial cavity, C first rib.

It is also called the branchial cavity and the cloaca, because it receives the excrements and sexual products as well as the respiratory water.

The prorenal duct of the latter (Figure 2.218 C) corresponds to the branchial cavity or atrium of the former (Figure 2.217 C).