BIO2231 Chapter Notes -Dorsal Nerve Cord, Agnatha, Endostyle
Document Summary
The notochord is a rod-like, semi-rigid body of fluid-engorged cells enclosed by a fibrous sheath, which extends, in most cases, the length of the body just ventral to the central nervous system. Thus, the notochord is a hydrostatic organ, similar to the hydrostatic skeletons of nematodes. Its primary purpose is to stiffen the body, providing skeletal scaffolding for the attachment of swimming muscles. The structural plan of chordates shares features of many non-chordate invertebrates, such as bilateral symmetry, anteroposterior axis coelom, tube within a tube arrangement, metamerism, and cephalization. However, the exact phylogenetic position of chordates within the animal kingdom is unclear. Five characteristics that, taken together, set chordates apart from all other phyla are notochord, dorsal tubular nerve cord, pharyngeal pouches or slits, endostyle, and postanal tail. These characteristics are always found at some embryonic stage, although they may change or disappear in later stages of the life cycle.
