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.
The length of the sarcomeres dictates the overall length of a muscle fibre. Graph 1. Length-tension relationship of sarcomeres presented in a graphical form. At 1 on Graph 1, the sarcomere is overly contracted at rest.There is a high degree of overlap between the thin and thick filaments. Muscle contraction causes actin filaments to slide over one another and the ends of myosin filaments.t is the pulling of the actins by myosin heads towards each other that exerts this tension. The magnitude of the tension depends on the frequency of the stimulation and the initial resting length of muscle fibres, of which will be discussed now. Keep in mind that muscle fibres are composed of many sarcomere units.