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.
Excitation Contraction Coupling Process. The sequence of events that converts action potentials in a muscle fiber to a contraction is know as excitation contraction coupling. In order for a skeletal muscle fiber to contract, it has to get a signal from the nervous system. The part of the nervous system that it gets a signal from is called a motor neuron.