Section 16. the Coelom of the Dog-Fish Is Peculiar-- Among the Types

we treat of-- in the possession of two direct communications with the exterior, in addition to the customary indirect way through the oviduct. These are the abdominal pores (a.p.) on either side of the cloaca in either sex. They can always be readily demonstrated by probing out from the body cavity, in the direction indicated by the arrow (a.p.) in Figure 1, Sheet 15. They probably serve to equalize the internal and external pressure of the fish as it changes its depth in the water, just as the Eustachian tubes equalize the pressure on either side of the mammal's tympanic membrane.