Craniates, one of the three subdivisions of chordates, have distinct skulls-including hagfish, which have no vertebrae. Michael J. Benton comments, craniates are characterized by their heads, just as chordates, or possibly all deuterostomes, are by their tails.t has also proved difficult to produce a detailed classification within the living chordates. Attempts to produce evolutionary  family trees  give results that differ from traditional classes because several of those classes are not monophyletic. As a result, vertebrate classification is in a state of flux.
The skeletal muscle fibers contract when the sarcomere in the myofibrils contract. The contraction of the sarcomeres is explained by the sliding filament theory. (The word theory is used here in its scientific sense as meaning generally accepted laws and principles, as in the theory of evolution.).he plasma membrane of the skeletal muscle fiber is called a sarcolemma. The muscle fiber contains long cylindrical structures, the myofibrils. The myofibrils almost entirely fill the cell and push the nuclei to the outer edges of the cell under the sarcolemma.
Near the end the collagen merges to form the tendons, which attach the muscle to the bone. It is through this connective tissue that the force generated by the individual cells is transmitted to the bone. A group of muscle cells are bundled together by collagen to form a fascicle.Since muscle cells are elongated and cylindrical, each muscle cell is usually called a muscle fiber.In skeletal muscle, the muscle fibers are very large, multinucleated, and up to several millimeters in length. group of muscle cells are bundled together by collagen to form a fascicle. Since muscle cells are elongated and cylindrical, each muscle cell is usually called a muscle fiber.