Practice question

Cephalochordates exemplified by Branchiostoma lanceolatum or amphioxus retain notochord as persistent axial skeleton extending from anterior tip beyond cerebral vesicle to posterior tip of tail throughout entire life, without replacement by cartilage or bone. Notochord consists of stacked vacuolated myofibril-containing lamellar cells capable of slow contraction. Dorsal nerve cord lies above, segmental myotomes alongside. Lifelong retention illustrates primitive chordate condition before vertebral evolution and contrasts sharply with tunicates where notochord exists only in larval tail and vertebrates where it is transient embryonic structure replaced during skeletogenesis, making it model for chordate ancestry.