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Desert Diary
Biology/Serial Homology


Most educated people know that much of the basic structure of land vertebrates follows the same general pattern. The differences from one kind to another often are described as variations upon a theme. The classic example is the forelimb, where the same basic arrangement is seen in things as dissimilar as lizards, birds, and mammals. The major differences are in proportions and in fusions and losses of parts.

Not so generally understood is that in many invertebrates, the same basic plan is seen in the appendages from segment to segment. A good example is seen in the crayfish that occur along our Rio Grande and irrigation canals. Antennae, mouth parts, legs, structures involved in mating, and those used in the backwards swimming so characteristic of crayfish are all based on a common pattern. Two segments are attached to the body and, off of this, a pair of parallel structures of several segments each. Thus, from a series of similar appendages running down the length of the body, a set of tools for diverse functions has evolved.
pen and ink


Contributor: Arthur H. Harris, Laboratory for Environmental Biology, Centennial Museum, University of Texas at El Paso.

Desert Diary is a joint production of the Centennial Museum and KTEP National Public Radio at the University of Texas at El Paso.


A crayfish. Note the clearly segmented abdominal area and the series of appendages. Photograph by Eric Engbretson, courtesy of the U.S. Fish and Wildlife Service.