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.
Every fibre of the muscle is approximately forty millimeter in length and is made up of a number of very small strands which are known as fibrils.These fibres are extremely slender, so much so that one square centimeter of a muscle contains in it up to one million fibres.Each fibre is commanded by a nerve which makes it to contract. The number of fibres, contracting, determine the strength of muscles work.uscle is mainly composed of muscle cells. Within the cells are myofibrils; myofibrils contain sarcomeres, which are composed of actin and myosin. Individual muscle fibres are surrounded by endomysium.