Evolution and systematics

As with most soft-bodied parasites, no fossil record of acan-thocephalans is known. However, prehistoric human copro-lites at archeological sites in the United States and Brazil have revealed infections by acanthocephalans. Additionally, some 9,000-year-old animal coprolites also from Brazil have been found to contain acanthocephalan eggs. The phylum Acan-thocephala is divided into 3 major taxonomic groups: the Archiacanthocephala, Eoacanthocephala, and Palaeacantho-cephala, which are considered by some to be classes and others to be orders. A fourth group, the Polyacanthocephala, has been proposed but its status is controversial. Taxonomic groups are based on morphological characters of the worms as well as their hosts' taxonomy and ecology, and such division is supported by molecular data. Molecular, morphological, and ultrastructural analysis of 18S ribosomal DNA sequences has revealed that acanthocephalans and their closest living relatives, members of the phylum Retifera, should be in one clade—referred to as the Syndermata. Acantho-cephalans include 22 families and about 1,000 species.

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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