Metameric segmentation

[zenith_beth]

i understand that metameric segmentation involves the organism being made up of similar segments.. but what evolutionary significance does it have?

[Morris]

What is metameric segmentation??

[zenith_beth]

Annelid worms eg. earthworms, exhibit metameric segmentation. Their bodies are made up of segments, and the segments are similar to each other - that is metameric segmentation. What i cannot understand is what exactly is its evolutionary significance!!!

[February Beetle]

Are the segments in Annelids the same or related to the segments in Arthropods? I don't know if that is the difference in "metameric segmentation" and other segmentation. The way I learned it Arthropods like insects, spiders, crustations all are made of segments that have been modifed to have a better defined head, apendages, tail, organs, etc... so maybe if the types of segmentation are related that would be a big evolutionary signifigance. Also, with Annelids their segments are very good because a lot of them can be injured or cut in half and they grow into two seperate worms! I don't believe Arthropods can do that, so who has the greater advantage?

Sorry I misread the last reply. But, if the segments have some type of relation on how they form it might be interesting to look at the differences of Arthropods and Annelids. Maybe they came from some type of common ansestor?

[iri_black]

Annelids are considered ancestors for Arthropods. because of their common segmented structure. In their case, some segments have joined to form new structures - different regions of the body (head, chest, abdomen)

[zenith_beth]

Thanks! So I guess, the evolutionary significance would be that the Annelids were the "first" to exhibit a form of segmentation, even though, in their case, it doesn't make much difference since all their segments are similar (ie. metameric). This segmentation then developed even further in the Arthropods to the point that the animal has a head, abdomen and thorax, each of which are specilaised and different. So the significance lies in the fact that the annelid is segmented and not that it is metameric - am I getting it right

[Beetle]

Metameric segmentation means serial repetativ position of body parts. It can be complete when all body parts/systems are repeated in every segment (like in Annelids or in Platodes; nervous, vascular, excretory, gonads, digestiv, muscular, coelom) or incomplete when only some systems are repeated (like in Arthropods; nervous, vascular, tracheas, or in Chordata; sceletal, muscular, nervous). Term segmentation is just shorter version of this and it can be used for any kind of repeating parts. The prime function of segmentation is intesification of function of repeated organs. 10 serial pairs of metanefridias will excrete better than just one pair. The other function is specialization of body part. If Centipede (Chilopoda) have many pair of legs it can modify first pair into poisonous fangs without endangering locomotory function. This on garter scale can lead to differentition of whole groups of segments into body regions (like in Insects head, thorax and abdomen). If you look through phylogenetic tree of animals you can see that in many groups there is a gradual increase of segments, than their specialization and than again gradual decrease of number. As for the similarity between Annelid and Arthropods segmentation it is still unsertain how the evolution went by since their is no adequate middforms between them and as it is in hole evolutionary biology while some morphological features confirm our theory others totaly shatter it. Also Annelids are not first to "discover" segmentation. It also occurs in flat worms (Plathelmintes) and some say in some of the Protozoas, offcourse in the terms of organells metameria.

[zenith_beth]

Thanks! that is very helpful!!!