Groupings within these types of muscle are based on embryonic origin. For example, the visceral branchiomeric musculature is formed from hypomere cells in the region of the branchial arches. All other muscles discussed here are somatic and formed from myotomes. The group names correlate with the region of the body where those muscles are found. Many muscle names are indicative of the origin and insertion points of that muscle.
Extrinsic muscles originate on the axial skeleton and insert on a girdle or limb.
These muscles are generally more primitive. Intrinsic muscles originate on a girdle or proximal part of the skeleton, and insert on more distal parts. The mudpuppy Necturus represents an early tetrapod. The general trends are toward the differentiation of muscle masses into more numerous individual muscles, the elongation of muscles in various groups, the expansion of broad muscles, more complexity of movement associated with more joints, and the spiraling of muscles in the limbs as appendages move underneath the body. It is important to remember that Necturus also represents an exception in vertebrate evolution due to neoteny, or the retention of larval characteristics into adulthood.