Worms do not comprise a single taxonomic category, but rather include members of quite different phyla such as Annelida, Platyhelminthes and Nematoda. Almost any invertebrate animal that is long and thin is called a "worm". Biologists have even referred to ancestral organisms more resembling planula as "worms".
Even caterpillars and beetle larvae are sometimes given that name. Annelida is a group commonly referred to as segmented worms, descendents from ancestors existing perhaps well before the Cambrian. They are found worldwide from the deepest marine sediments to the soils in our city yards. There are about 13, species of annelids.
They vary in size from Annelids asexual reproduction video a few millimeters in length to the giant earthworms of Australia that may be three meters long.
Since most children in the US are familiar with earthworms, this is the group most are referring to when they talk about free living "worms" in general. Interestingly, parasitic worms generally invoke images of forms and species that are found among the Platyhelminthes, a group you have already encountered or the nematodes, a group we will examine later.
General introduction to the Annelids http: Through most of the 20th century Annelida was split into three major groups; Polychaeta, Oligochaeta earthworms etc.
Earthworms and leeches are the familiar annelids for most people, but polychaetes comprise the bulk of the diversity of Annelida and are found in nearly every marine habitat. It is now recognized that Oligochaeta and Hirudinea, comprised of several thousand species, form a clade and should be referred to the Clitellata. Polychaetes are less related to either living members of this group, although they are the oldest line of still existing Annelids. It has long been recognized that there are 3 main branches of animal descent, once we start dealing with coeloms or cavities and three layers.
Deuterostomia is a line that includes Annelids asexual reproduction video and other vertebrates as well as some invertebrate Chordates.
The Lophotrochozoa is named after the feeding habits or larvae associated with some groups. This line of descent includes the annelids and another major group, the Molluscs which we will examine shortly. Finally we have the or animals that Annelids asexual reproduction video. We have already looked at Arthropods, but will examine other Ecdysozoa at the end of this course.
Examine the following website to see that other groups are located in the http: To find the Cnidaria or Porifera, you will have to go back a page by choosing the back arrow in the middle of the diagram. The annelid body plan is essentially a tube within a tube. This segmentation is both internal and external, and each internal segment of the body is separated by a tissue called septa, which partially divides the coelom. Segmentation is also called metamerism. Segments each contain elements of such body systems as circulatory, nervous, and excretory tracts.
Much discussion surrounds the origins and homology of segmentation, which is a characteristic shared not only by Annelida but also by Arthropoda.