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Bilateria: Increasing Complexity

The evolutionary relationships of the Bilateria are still not fully elucidated.
Three major, putatively monophyletic groups are:

I. Lophotrochozoa

The most familiar Lophotrochozoan taxa are the Molluscs and Annelids.
But there are several other, more obscure groups.

Lophotrochozoan Bauplan:



Annelida - The Segmented Worms

Annelids are commonly known as the Segmented Worms because of their distinct metamerism. They have undergone a great deal of cladogenesis, and their phylogeny is still being investigated and constantly updated.

The Annelid Bauplan

Annelids all have

Annelid Diversity

Annelida has undergone perhaps more drastic systematic revision than any other major phylum. Once considered to consist only of three major groups--Polychaeta (Marine Segmented Worms), Oligochaeta (Earthworms) and Hirudinea (Leeches)--the taxon has since been divided, regrouped, and re-divided many times. Molecular data have provided a clearer picture of Annelid phylogeny, at least until we get a new picture.

(Read the paragraph on this site regarding Annelid phylogenetic reorganization to get an idea of just how work-intensive this can be! And note that Dr. Krempels's professor for Invertebrate Zoology at U.S.C. was Dr. Kristian Fauchald, now of the Smithsonian Institute, who is one of the authors responsible for much of the currently accepted phylogeny.)

  • polychaetes - (poly = "many"; chaet = "bristle") Marine segmented worms

  • oligochaetes - (oligo = "few"; chaet = "bristle") - Earthworms & their allies (whatever that means)
  • "Hirudinea" - (hirudo = "leech") Leeches

    Deep Sea Polychaetes

    Let's meet some adorable annelids.


    Mollusca - The Mollusks

    These are the soft-bodied coelomates that secrete a shell.

    The Mollusk Bauplan
    The Hypothetical Ancestral Mollusk (H.A.M.) had all the major mollusk characters in their most primitive form.

    All Mollusks have

    Mollusk Diversity

    More marvelous mollusks.


    Note that both Annelids and Mollusks share the trochophore larva at some stage of devlopment:

    ...which is further testament to their likely evolutionary relationship. (Remember what we said about similar embryonic development in related groups.)