Muscles are almost the definition of the animal kingdom. The larger the animal the larger its proportion of muscle. It is a consequence of the strength of a muscle being proportional to its cross-sectional area which increases in amount with the square of lineal dimensions. The strength needed to move its mass increases with the cube of the lineal dimensions. Thus muscles scale up in size with increasing animal size faster than do most animal organs.
Two types of muscle are found in vertebrates and most other advanced groups of animals. Striated muscle, often referred to as skeletal muscle in vertebrates, is usually the most abundant. Intestines and blood vessels are the major locations of smooth muscle. The sliding filaments of actin and myosin are aligned in the fibrils of the muscle fiber so striations are evident, keeping their orientation as they contract or relax and lengthen as the biochemical reactions powering the movement is regulated by control by voluntary nerves. Smooth muscle is the other type of muscle and is controlled mostly by nerve fibers of the autonomic nervous system. Smooth muscle is the muscle type responsible for contraction of veins and intestines. Like skeletal muscle it works only by contracting.
A special type of striated muscle, not associated with the skeleton, is found in the heart and called cardiac muscle. Cardiac muscle fibers branch and fuse forming a network. The network probably is needed to prevent blood from separating fibers and pushing through the wall of the heart. The fibers of cardiac muscle are partitioned at intervals by intercalated disks. Intercalated disks are also found in some protostome hearts thus providing some evidence supporting the annelid ancestry of advanced protostomes and deuterostomes.
MUSCLE ATTACHMENTS
Bone
Skeletal muscles, which do their work by contracting as do all muscles, have an origin on a bone at one end and an insertion at the other end on an adjacent bone that is moved when the muscle contracts. To move the bone back to its original position and opposing muscle must contract on the other side of the bones and joint.
Soft tissue
So how do we stick out our tongue. We do it by contracting other muscles within the tongue oriented in directions other than the length of the tongue in which muscles are relaxing. If they relax more on one side the tongue will curve more to the other side.
Hydrostatic or hydraulic skeletons
The earthworm provides a good example of this type of skeleton. Under the cuticle and epidermis, the outer layer of muscle is circular muscle. The soft tissue and fluid in the coelom is forced by contractions of the circular muscle into lengthening the worm as it gets thinner in the segments containing the contracting muscle. Interior to the circular muscles are longitudinal muscles that lengthen. If all the segments lengthen, the worm gets long and thin, the total worm volume remains the same. Conversely, when the longitudinal muscles contract the worm shortens and gets much thinker, but total volume remains the same.
The leech also is dependent on the muscular system using the transmission of fluid and flexible soft tissue moved by the opposing contractions of outer circular and inner longitudinal muscles. But the body is flattened by transverse muscles connected to dorsal and ventral surface tissues. Thus movement involves less lengthening and shortening but more up and down bending to either swim by undulations of the body or move along surfaces my bringing the posterior sucker up close to the attached anterior sucker which is then released as the body straightens its bend to attach it further forward.
Hydras do much the same, but the interior of the hydra is fluid filled and the body has the circular muscle fibrils of the gastrodermal cells inside the longitudinal fibrils of the epidermal cell where the fibrils are in more of a network. The fibrils of both layers of the layers are in close association with the thin layer of connective tissue separating the two cell layers.
Resolving the evolutionary switch from outer longitudinal to outer circular muscle
It is evident that the cnidarian medusa to planarian flatworm suggested in earlier posts and illustrated in [ http://evolutioninsights.blogspot.com/2015/03/evolution-quiet-pre-cambrian-genes.html ] provides a plausible mechanism for the transition.
I gained some insight into the functioning of hydra's hydrostatic skeleton when I had students study the feeding reaction of hydra. When a weak solution of glutathione is introduced into the water near a hungry hydra it will start moving its tentacles toward its mouth in the same manner it would do if a small crustacean had been captured by a tentacle. I was amazed to see that the mouth was wide open. I was thinking it was impossible for it to maintain its posture if the fluid in the gastrovascular cavity was not prevented from exiting the open mouth. Examination of hydra sections, in particular the tentacles, showed the gastrodermal cells were very large, almost balloon like, and the cavities in the tentacles were almost non-existent.
It made sense that each gastrodermal cell had a single circular muscle fibril so that contraction would be most directly applied to the cell and not slip between cells. And the thick gasterodermis near the mouth allowed longitudinal fibers of the epidermis to open the mouth more efficiently.
Although the bloated gastrodermal cells may be a more important part of hydra's hydrostatic skeleton, large anthozoan anemones do utililize the fluid filling the gastrovascular cavity to a much greater extent. Many have one or two siphonoglyphs which are ciliated grooves of the gastrodermis extending from near the mouth to near the basal disk. Siphonoglyphs enable the anemone to remain inflated and/or reinflate itself after major contractions reducing size for self-protection.
Nematode muscles and evolution
The muscular system of large nematodes such as Ascaris is unique in that the muscles of the body wall are all longitudinal muscles. The fibrils are enclosed in cells that are long tapering tubes with the nuclei containing part projecting into the pseudocoel. From the bulbous part containing the nucleus, a slender tubular extension runs to one of two major nerves extending the length of the worm in dorsal or ventral positions; the synaptic connection to the nerve is made at the nerve by the muscle cell rather than at the muscle as in most organisms. The muscular pharynx is a characteristic of nematodes important in maintaining fluid volume of the worm enabling its hydrostatic skeleton to function. The cuticle is strong enough to keep the fluid retained for its function as a skeleton. As a result, when the muscles of dorsal and ventral halves of the worm alternate contractions, the nematode bends or undulates up and down.
The peculiarity of the muscular system of nematodes and their lack of motile cilia are reason enough to reject the Ecdysozoa as a legitimate phylogenetic group. The molecular basis for its formation is unsustainable if the evidence presented in [ http://evolutioninsights.blogspot.com/2013/05/science-screw-up-no-1.html ] is known.
Joseph G. Engemann Kalamazoo, Michigan January 28, 2016
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