V E Deuterostomes W N LAMPREY T T T N ARCHAEOP TERYX TUNI CATES E E ○ Ꭰ ○ B R C ○ R T R H T A RADIAL H Y E R S R E F M C D R EPT I LE S M U M L N S M CARTI LA GE I A N P M A I C COELACANTH M R L ○ Ꭰ L I A T L I L U S B IRDS S B X U LOBE F INNED TETRAPODS www.CrosswordWeaver.com ACROSS 4 agnathan with circular, sucking disk 6 earliest known species of bird 8 sessile, marine chordates often mistaken for sponges 10 adult echinoderms have symmetry 11 body covered with hard, dry, horny scales 15 the sharks' skeleton is composed of 17 a lobe-finned fish 19 bones contain air spaces; no teeth 20 the fish are thought to be the ancestors of the tetrapods 21 amphibians, reptiles, and mammals are vertebrate classes that are DOWN 1 one of four distinguishing characteristics of chordates, dorsal, tubular 2 The system and are unique to echinoderms feet 3 one of four distinguishing characteristics of chordates 5 mammals that lay eggs 7 the only the mammals animals are the birds and 9 modern fish are thought to have descended from the fish. 12 scales characteristic of sharks 13 a cephalochordate 14 have amnion, hair, four-chambered heart 16 the operculum covers the 18 in sharks, the receives digestive wastes, urine, and gametes