Evolution and systematics

The fossil record of the ophiuroids extends back some 500 million years to the early Ordovician period. They are the most diverse class of echinoderms, with some 250 described genera and 2,000 species. The most recent phy-logeny and upper-level taxonomy places extant ophiuroids into two subclasses: the Oegophiuridea with only one family, the Ophiocanopidae; and the Ophiuridea, which is divided into two distinct orders—the Euryalida, including the basket stars with branched arms, and the more familiar Ophiurida or brittle stars. The phylogenetic tree for ophi-uroids suggest that they underwent a major diversification in the Triassic and early Jurassic periods; that is, about 200-250 million years ago. The classification of the ophi-uroids, however, remains a subject of debate. The placement of the unusual Ophiocanops fugiens, thought to be a "living fossil," into the subclass Oegophiuridea, is especially interesting, since this subclass was thought to have become extinct after the late Carboniferous period. The families are as follows: Ophiocanopidae; Asteronychidae; Gorgono-cephalidae; Asteroschematidae; Euryalidae; Ophiomyxidae; Hemieuryalidae; Ophiuridae; Amphiuridae; Ophiothricidae; Ophiactidae; Ophionereididae; Ophiocomidae; Ophichi-tonidae; Ophiodermatidae; Ophiolepididae.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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