In the evolution of metazoans, the stage of development after that of the simple three-layered structure of the flatworms seems to be the appearance of a coelom, or fluid-filled cavity between the body wall and the gut. Another feature that appears at this stage is metamerism, or segmentation, displayed particularly in the annelids. Ail of these animals share some features but are too different to be placed under one phylum, so each has its own.
Ribbon worms
This group, known as nemerteans, do not have a true coelom. Instead, a solid tissue called a parenchyma fills the cavity between the gut and the body wall. In this respect nemerteans resemble the flatworms. But nemerteans are more highly specialized, with more complex nervous and circulatory systems, and an alimentary canal with a mouth and anus.
Roundworms
This group, the nematodes, found in most environments, and millions may occur in only a couple of acres of soil. Scientists have discovered between 10,000 and 20,000 species of nemotodes, and some estimate that tens of thousands more remain undiscovered.
Annelids
Annelids, members of the phylum Annelida, are worms whose body is divided into many segments. As such, they are a little more advanced than the unsegmented nemerteans and nemotodes. The phylum Annelida contains about 12,000 species divided into three classes: Polychaeta, Oligochaeta, and Hirudinea. In the polychaete ragworm each segment of the body—apart from the head and the last segment—is identical, and the external and internal organs are repeated in each segment. In the oligochaetes, such as the earth worms, and the Hirundinea, or leeches, the segments are not all identical and some are specialized for particular functions. Annelids have a true coelom.
Read More Nematodes and annelids
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