Key points: 1. (1) 1.5 million backbones (2) (the one from beginning to end) (3) land, water and air (4) fish, coelenterates, mollusks and crustaceans 2. Tail fin and gill.

Practice ABABDA 7. A-d B-b C-c D-a 8。 Simulation experiment 9. Chest, abdomen, back, buttocks and tail 12.c 13. A large number of capillaries are distributed in red. The oxygen in the water increased by 15. (1) The difference between normal pectoral fin and ventral fin is as follows.