What is Metamerism?
Metamerism is the repetition of homologous body segments. This type of development can be seen in the Annelids, which are earthworms, leeches, tube worms, and their relatives. It is also seen in a more advanced form in the Arthropods or crustaceans, insects, and their relatives.
Metamerism results from teloblastic development, in which cells divide asymmetrically and form rings of small cells around the embryo. These cells multiply in the many segments of worms and can be homonomous, meaning very similar, or they can be differentiated into a variety of functions.
These segments are called heteronomous. Metamerism has resulted in a wide variety of Annelids, Arthropods, and other segmented animals in the world. The simply segmented state of annelids allows them to exist in any setting from the deepest parts of the ocean to the bottom of some of the highest mountains.
Metamerism in animals
In animals, zoologists define metamerism as a mesodermal event that results in a serial repetition of the subdivisions of ectoderm and mesoderm products. Endoderm is not involved in metamerism.
Segmentation is not the same concept as metamerism: segmentation can only be limited to ectodermally derived tissue, e.g. in the Cestoda tapeworms. Metamerism is far more important biologically as it leads to metamers – also called somites – which play a crucial role in advanced locomotion.
Metamerism can be divided into two main categories:
1). Homonomous Metamery
Homonomous metamerism is a strict serial sequence of metamers. It can be broken down into two more classifications known as pseudometamerism and true metamerism.
An example of pseudometamerism is the class Cestoda. The tapeworm is made up of many repeating segments – mainly for reproduction and basic nutrient exchange. Each segment acts independently of the others, which is why it is not considered a true metamerism.
Another worm, the earthworm in the Annelida tribe, can illustrate a real metamerism. A repetition of organs and muscle tissue is found in each segment of the worm. What sets the annelids apart from Cestoda is that the segments in the earthworm all work together for the entire organism.
The segmentation is believed to have evolved for many reasons, including a higher degree of motion. Take the earthworm, for example: by segmenting the muscle tissue, the worm can move in a centimeter pattern.
The circular muscles allow the segments to lengthen one at a time, and the longitudinal muscles then work to shorten the elongated segments. This pattern continues the length of the worm, allowing it to move along a surface. Each segment is allowed to work independently, but in the direction of the movement of the entire worm.
2). Heteronomous Metamery
Heteronomous metamerism is the condition under which metamers have come together to perform similar tasks. The extreme example of this is the insect head (5 metamers), the thorax (3 metamers) and the abdomen (11 metamers, not all of which can be seen in all insects).
The process that results in the grouping of metamers is called “tagmatisation” and each grouping is called a tagma (plural: tagmata). In organisms with highly derived tagmata such as insects, a large part of the metamerism within a tagma may not be trivially distinguishable.
Structures that do not necessarily reflect clustered metameric function (e.g., the ladder nervous system or some do not reflect the unitary structure of a thorax) may need to be sought.
In addition, an animal may be classified as “pseudometameric”, meaning that it has clear internal metamerism but no corresponding external metamerism – as is seen, for example, in Monoplacophora.
Humans and other chord dates are striking examples of organisms whose metamers are closely grouped in tagmata. In the chordata, the metamers of each tagma are so fused that only a few repeating features are directly visible. Intensive research is required to identify the metamerism in the tagmata of such organisms. Examples of detectable evidence of residual metameric structures are branch arches and cranial nerves.
Some schemes consider the concept of metamerism to be one of the four principles of construction of the human body common to many animals, along with general bilateral symmetry (or zygomorphism), pachymerism (or tubulation), and stratification. Newer schemes also include three other concepts: segmentation (other than metamerism), polarity and endocrinosity.
A metamer is one of several segments that are involved in the development of a drive or into which a drive can conceptually (at least) be resolved. In the metameristic model, a plant consists of a series of “phytons” or phytomeres, each of which consists of an internode and its upper node with the attached leaf.
The branch, or simple stem itself, is manifestly an assemblage of similar parts, placed one above another in a continuous series, developed one from another in successive generations. Each one of these joints of stem, bearing its leaf at the apex, is a plant element; or as we term it a phyton,—a potential plant, having all the organs of vegetation, namely, stem, leaf, and in its downward development even a root, or its equivalent. This view of the composition of the plant, though by no means a new one, has not been duly appreciated. I deem it essential to a correct philosophical understanding of the plant.
Some plants, especially grasses, have fairly clear metameric construction, but many others either lack discrete modules or their presence is more questionable. The Phyton theory has been criticized as an ingenious, academic conception that has little to do with reality.
In 1961 Eames concluded that “concepts of the shoot, which consist of a number of structural units, have been obscured by the dominance of stem and leaf theory. Anatomical units like this do not exist: the shoot is the basic unit. “
Nonetheless, others still consider the comparative study along the length of the metameric organism to be a fundamental aspect of plant morphology.
Metameric concepts generally segment the vegetative axis into repeating units along its length, but constructs based on other subdivisions are possible. The pipe model theory provides that the plant (especially trees) consists of standard pipes (“metamers”), each of which carries a unit amount of photosynthetic tissue.
Vertical metamers are also suggested in some desert shrubs where the stem is converted into isolated xylem strips, each with continuity from root to shoot. This can allow the plant to remove a large portion of its shoot system in response to drought without damaging the remaining portion.
In vascular plants, the shoot system differs fundamentally from the root system in that the former shows a metameric construction (repeated units of organs; stem, leaf, and inflorescence), while the latter does not. The plant embryo represents the first metamer of the shoot in spermatophytes or seed plants.
Plants (especially trees) have a “modular structure” where a module is an axis in which the entire sequence of air differentiation from the beginning of the meristem to the onset of sexuality (e.g. flower or cone development) completes its development from. These modules are viewed as development units that are not necessarily structural.