ANSC 420 Lecture Notes - Lecture 3: Limnognathia, Lophophore, Acanthocephala
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The Platyzoa include the phylum Platyhelminthes, the flatworms. These were originally considered some
of the most primitive Bilateria, but it now appears they developed from more complex ancestors. A
number of parasites are included in this group, such as the flukes and tapeworms. Flatworms
are acoelomates, lacking a body cavity, as are their closest relatives, the microscopicGastrotricha
The other platyzoan phyla are mostly microscopic and pseudocoelomate. The most prominent are
the Rotifera or rotifers, which are common in aqueous environments. They also include
the Acanthocephala or spiny-headed worms, theGnathostomulida, Micrognathozoa, and possibly
the Cycliophora These groups share the presence of complex jaws, from which they are called
The Lophotrochozoa include two of the most successful animal phyla, the Molluscaand Annelida. The
former, which is the second-largest animal phylum by number of described species, includes animals
such as snails, clams, and squids, and the latter comprises the segmented worms, such
as earthworms and leeches. These two groups have long been considered close relatives because of the
common presence of trochophore larvae, but the annelids were considered closer to the arthropods
because they are both segmented. Now, this is generally considered convergent evolution, owing to
many morphological and genetic differences between the two phyla.
The Lophotrochozoa also include the Nemertea or ribbon worms, the Sipuncula, and several phyla that
have a ring of ciliated tentacles around the mouth, called a lophophore. These were traditionally grouped
together as the lophophorates. but it now appears that the lophophorate group may be paraphyletic, with
some closer to the nemerteans and some to the molluscs and annelids. They include the Brachiopoda or
lamp shells, which are prominent in the fossil record, the Entoprocta, the Phoronida, and possibly
the Bryozoa or moss animals.
Because of the great diversity found in animals, it is more economical for scientists to study a small
number of chosen species so that connections can be drawn from their work and conclusions
extrapolated about how animals function in general. Because they are easy to keep and breed, the fruit
fly Drosophila melanogaster and the nematode Caenorhabditis elegans have long been the most
intensively studied metazoan model organisms, and were among the first life-forms to be genetically
sequenced. This was facilitated by the severely reduced state of their genomes, but as
many genes, introns, and linkages lost, these ecdysozoans can teach us little about the origins of animals
in general. The extent of this type of evolution within the superphylum will be revealed by the crustacean,
annelid, and molluscan genome projects currently in progress. Analysis of thestarlet sea
anemone genome has emphasised the importance of sponges, placozoans, and choanoflagellates, also
being sequenced, in explaining the arrival of 1500 ancestral genes unique to the Eumetazoa.
An analysis of the homoscleromorph sponge Oscarella carmela also suggests that the last common
ancestor of sponges and the eumetazoan animals was more complex than previously assumed.
Other model organisms belonging to the animal kingdom include the house mouse (Mus musculus) and
zebrafish (Danio rerio).
Aristotle divided the living world between animals and plants, and this was followed byCarolus
Linnaeus (Carl von Linné), in the first hierarchical classification. Since then biologists have begun
emphasizing evolutionary relationships, and so these groups have been restricted somewhat. For