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An Introduction to Zoology: Chapter 12
An Introduction to Zoology: Chapter 12
Published by The Lone Ranger
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An Introduction to Zoology

Chapter 12: The Bilateria: Smaller Lophotrochozoan Phyla:

Minor Lophotrochozoan Phyla:
You may recall that during the “Cambrian Explosion,” about 535 – 525 million years ago, the rate of evolution (as measured by the rate at which new species arose) appears to have accelerated by at least an order of magnitude. Prior to the Cambrian Explosion, almost all life on Earth consisted of single-celled organisms or colonies of cells. With the Cambrian Explosion, the situation changed dramatically. In only a few million years’ time, the diversity of animal life increased dramatically, and by the end of the Cambrian Explosion, most modern animal phyla had come into existence.

In fact, some have argued that there were more animal phyla extant during the Cambrian than there are today. It’s certainly true that, since the Cambrian, some animal phyla have flourished while others have not. Some phyla have proved very successful and have produced many species that survive into the present – phyla such as the annelids, mollusks, arthropods and chordates. Other phyla have been less successful. A few animal phyla have apparently become extinct; others have managed to hang on, but are represented by only a handful of surviving species. For the most part, these “minor phyla” have managed to hang on by living in very specialized habitats where few other species compete with them.

It is these “minor lophotrochozoan phyla” to which we will now turn our attention. These animals are poorly known by most people, and most of them have not been thoroughly studied. The relationships between these phyla of “evolutionary experiments” are poorly studied, and so are poorly understood.

Some of these minor lophotrochozoans are acoelomates, but most are pseudocoelomates. Most of these animals are quite small, and the pseudocoelom, when present, generally serves as a sort of simple circulatory system that helps to distribute materials throughout the animals’ bodies.

Four of the minor lophotrochozoan phyla appear to be closely-related; they form the clade Gnathifera. Most gnathiferans share the common characteristic of having complex jaws. The gnathiferans include the phyla Gnathostomulida, Micrognathozoa, Rotifera, and Acanthocephala. In addition to the gnathiferans, there are six other minor lophotrochozoan phyla. These are the phyla Cycliophora, Gastrotricha, Entoprocta, Ectoprocta, Brachiopoda and Phoronida.

The “minor lophotrochozoans.” Note that the Gnathifera form a distinct clade, and the
Syndermata are a smaller clade within the Gnathifera. The Brachiopoda and the Phoronida appear
to be fairly closely-related, but the relationships between the remaining four phyla are uncertain.


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