Crossword-Solution: ACRANIA 7 letters, 1 clue 🏆 scrabble score: 9

The brain and nervous system display the same progressive ascent from the brainless acrania, up through the fishes, batrachia, reptiles, and birds to the top in mammals.

The chief difference between the coelomation of the acrania (amphioxus) and the other vertebrates (with skulls--craniotes) is that the two coelom-folds of the primitive gut in the former are from the first hollow vesicles, filled with fluid, but in the latter are empty pouches, the layers of which (inner and outer) close with each other.

When they are not so filled (for instance, when the primitive gut of the gastrula is filled with yelk, or when the walls of the empty coelom-pouches are pressed together), these vesicles must not be cavities any longer, but "solid structures." The accumulation of food-yelk in the ventral wall of the primitive gut (Figures 1.85 and 1.86) is the simple cause that converts the sac-shaped coelom-pouches of the acrania into the leaf-shaped coelom-streaks of the craniotes.

Nearly all these glands are wanting in the acrania; probably there were merely a couple of simple hepatic tubes (Figures 1.98 and 1.100 l) in the vertebrate stem-form.

The second chief section of the vertebrate-gut, the trunk or liver-gut, which accomplishes digestion, is of very simple construction in the acrania.