Topic—Metamerism
Some definitions-
Benthic environment—the environment at the bottom of a lake or ocean.
Some subdivisions
Epibenthic or epifaunal—creatures that live on top of the substrate
Infaunal—creatures that live in the substrate.
Before the evolution of metameric creatures, most organisms lived on top of the substrate. After the invention of metamerism, creatures were able to move into the substrate. What enabled them to do so was the fact that no longer did the organism have to contract it whole body for movement. Individual segments could be contracted and expanded. This allowed for regional patterns of body movement. Typicaly in an animal such as an earth worm a wave of contraction passes down the body. In as much as each segment has bristles that anchor that segment, this —peristalsis
Nematodes, acanthocephalans, arthropods
Chitin in the outer body wall—complex mucopolysacaride
For growth to occur, the outer layer of chitin has to be shed—ecdysis
Homeobox genes, rRNA
Acoels—they’re not platyhelminthes
Whether things shed or don’t shed
Ecdysozoa—arthropods, aschelminthes, etc.
Changes in environments
Cambrian—organisms that live in aqueous environments
Movement into freshwater
Ionic balance and osmotic balance
Movement on to land