As before, these are the animals with bilateral body symmetry. Today, we enter the realm of the coelomates. Until we leave again.
The Coelomate Bauplan:
- true plane of symmetry: bilateral
- three germ layers leading to true tissues
- ectoderm (becomes the epidermis in the adult)
- endoderm (becomes the gastrodermis in the adult)
- mesoderm (derived from the endoderm)
- true organs and organ systems
- integumentary system - YES
- digestive system - YES
- nervous system - YES
- muscular system - YES
- reproductive system - YES
- excretory system - YES
- circulatory system - YES
- respiratory system - YES, IN MOST PHYLA
The Lophotrochozoa - A Wealth of "Worms"
This clade includes both familiar and unfamiliar phyla, all of which are
protostome coelomates.
The Lophophorates - morphologically linked by the lophophore feeding
apparatus
The Trochozoans - linked by the trochophore larva
- Mollusca - The Mollusks
- Annelida - The Segmented Worms
- Nemertea - The Ribbon Worms
- Sipuncula - The Peanut Worms
- Entoprocta - many weird and wormlike creatures
- Pogonophora - Deep Sea Tube Worms
For now, we'll concentrate on only two of the most familiar, coelomates in
the Annelida (Segmented Worms) and Mollusca (Molluscs).
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
- typical protostome coelomate developmental characters
- metamerism reflected in external and internal anatomy
- coelom serving as large, fluid-filled hydrostatic skeleton
A hydrostatic skeleton consists of closed or partly closed, fluid-filled chambers with flexible, muscular walls. Internal pressures generated by muscle contractions allow movement, with the muscles working against the fluid pressure as vertebrate muscles work against bone. The fluid-filled chambers also maintain the shape of the animal. Common examples of animals with a hydrostatic skeleton are earthworms, roundworms, etc.
- well-developed nervous system with a cephalic ganglion & ventral,
ganglionated nerve cord
- a dorsal, closed circulatory system with several hearts
- appendages present on each body segment (even if simply bristles)
- metanephridia or protonephridia serve as excretory system
- larva a characteristic form called a trochophore.
Diversity of Annelids
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. You are one degree removed from an Annelid Rock Star. I don't know if he knows Kevin Bacon.)
For tradition's sake, we'll quickly examine the most familiar annelids. Taxonomic names in "quotes" indicate names that are no longer in use because it was discovered that they described polyphyletic or paraphyletic groups.
"Polychaeta" - (poly = "many"; chaet = "bristle") Marine segmented worms
Oligochaeta - (oligo = "few"; chaet =
"bristle") - Earthworms & their allies (whatever that means)
and the Oligochaete group, "Hirudinea" - (hirudo = "leech") Leeches
- now considered to be monophyletic with the Oligochaetes
- free-living predators/ectoparasites
- coelom very reduced; body wall very muscular
- salivary secretion "hirudin" is still sometimes used as an
anticoagulant
- many leeches are species specific, and many won't feed on a human
- form a clade with the earthworms
Let's meet some adorable annelids.
Phylum 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
- typical protostome coelomate characters
- coelom reduced to a vestigial gonocoel (surrounding reproductive
organs & a few other structures
- a haemocoel as the primary body cavity
- an open circulatory system
- viscera (guts) concentrated in a visceral mass
- a thick, epidermal-cuticular layer of skin called the mantle
which secretes the shell
- large, well-defined, muscular foot
- buccal (mouth) opening with a rasplike radula feeding structure
- large, complex metanephridia serving as excretory system
- primitive forms have a terminal trochophore larva
- in more derived forms,
the trochophore develops into a more complex veliger larva before
metamorphosing into the adult form
Diversity of Mollusks
- Polyplacophora - (poly = "many"; plac = "plate";
phor = "to bear") - The Chitons
- all marine (i.e., living in the ocean)
- all benthic (i.e., living on the bottom substrate, such as rocks, coral, mud, etc.)
- characterized by eight separate dorsal plates instead of a single shell
- intertidal forms extremely muscular foot keeps them anchored in spite of wave action. (Just try to pry a chiton off a rock.)
- of the forms we will consider, the chitons are probably the most physically similar to H.A.M.
- Gastropoda - (gastr = "stomach"; pod =
"foot") - Slugs & snails
- free-living
- marine, freshwater and terrestrial species
- high degree of cephalization with well-developed sense organs
- torsion
- single shell, often spiral or conical
- Bivalvia - (bi = "two";valve = "shell"; ) - clams, oysters, mussels, etc.
- marine and freshwater species
- characterized by hinged shell enclosing the body entirely
- gill used for both respiration and feeding
- all are suspension filter-feeders
- When is a bivalve not a bivalve? When it's a brachiopod! (Convergent evolution at its best.)
- Cephalopoda - (cephalon = "head";pod = "foot"; ) - The
Chambered Nautilus, Squids, Octopus
- exclusively marine
- fast-swimming predators
- camera eye is analogous to the vertebrate eye, and forms complex,
color images like our own
- comparatively intelligent; Octopus are able to solve problems!
- Cute octopus stories for your entertainment
Let's look at some 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.)
In many molluscs, the trochophore goes one step further and becomes a veliger:
...before it undergoes further development to become whatever type of
mollusk is its fate. Once again we see that more complex ontogeny results in more derived animal forms.
