. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. io Biology of the Vertebrates tract. In some chordates this structure persists throughout life; in others it is partially or completely replaced by a skull and a "backbone" made up of separate bony elements, or vertebrae, as the name "vertebrate" indicates. Essentially the notochord consists of a tough connective tissue sheath in which soft cells are packed so tightly that the whole structure possesses a certain turgor, somewhat like th
. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. io Biology of the Vertebrates tract. In some chordates this structure persists throughout life; in others it is partially or completely replaced by a skull and a "backbone" made up of separate bony elements, or vertebrae, as the name "vertebrate" indicates. Essentially the notochord consists of a tough connective tissue sheath in which soft cells are packed so tightly that the whole structure possesses a certain turgor, somewhat like that when sausage meat is crowded into a casing (Fig- 98). 2. Dorsal, Hollow, Central Nervous System The chordate nervous system develops on Sgg^^^gjsS the dorsal side of the body by a process known as invagination (Fig. 4). In this structure, even sft\ /?&. SassHssS in adult animals, there is a cavity which is con- w^ tinuous from near the anterior end of the brain gs^^rjs to the posterior end of the nerve cord. The cen- tral nervous system of non-chordates, on the SaS-g-? other hand, is formed on the ventral side of the body and is solid. In the vertebrate members of _. „ . the chordate phylum, the anterior end of the Fig. 4. Successive stages in ' . the migration of outside tis- central nervous system is much enlarged into a sues to the inside, a, by in- brain with which there are associated three pairs vagination; b, by delamina- 0f major sense organs: olfactory (nose); optic tion. , i • / \ (eyes); and otic (ears). 3. Pharyngeal Breathing Device Fishes have several porthole-like passage-ways, or gill slits, penetrating through the lateral walls of the food tube on either side of its anterior end. Within these gill slits in water-dwelling chordates hang feathery tufts of capillaries, or gills, which rob the circulating water of some of its dissolved air, thus accomplishing the function of breathing. Gill slits, or traces of them, are present, at least in embryonic lif
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