A selachian, the common skate (Raja ocellata) or dog-fish (Acanthias vulgaris).
The comparison of this system with that of a selachian is very simple.
Of the archipallium, the portion which constitutes the olfactory lobe is well formed in the selachian fish.
The end result is an amphicoelous or biconcave centrum in general appearance much like that of the selachian.
The abdominal vein represents the lateral vein of the selachian and corresponds to the umbilical vein of the higher vertebrates.
The selachian portal vein represents the united vitelline veins, into which the veins of the digestive tract open.
The further points of comparison between the selachian and the bird are very simple.
The hepatic portal system differs in two respects from the selachian type.
At this stage the Amphibian kidneys are nearly in the same condition as the selachian, in the stage represented in Fig. 2.
In Osseous fishes we probably have an embryonic condition of the selachian kidneys retained permanently through life.