Evolution and systematics

Nematoda, the phylum above the class Adenophorea, have left very few fossil remains. The earliest fossils that contained nematode remnants were found in Eocene strata (the era from about 55-38 million years ago). More authenticated fossils are of nematodes preserved in amber, such as those from fossilized shark muscles and mammals frozen in permafrost. The fossil record is too fragmented to explain much about nematode origins, and conclusions about nematode phylogeny have been mostly based on observations of living species. It is hypothesized that nematodes originated during the Precambrian era, what was the Proterozoic period (about one billion years ago).

As of 1994, about 20,000 species of nematodes have been described, with an estimate by various researchers of the total number of nematode species living on the planet at 80,000-1,000,000. This phylum is considered to have the lowest number of yet-to-be-described species of any animal. Hy-man divided nematodes into 17 orders, whereas Chitwood separated them into two main classes, Aphasmidia (now Adenophorea) and Phasmidia (now Secernentea). Controversies still exist, but for the most part, many scientists such as A. R. Maggenti, who helped to develop the classifications, treat nematodes as a separate phylum with two classes, Adenophorea and Secernentea, which were divided based on molecular and morphological characteristics. These two classes are primarily separated (along with other important criteria) with respect to whether they do not possess phasmids (as in Adenophorea) or do possess phasmids (as in Secernentea). Two subclasses are recognized: Enoplia and Chromadoria. In addition, there are

11 orders and approximately 96 families. The total number of species of adenophoreans is estimated at about 12,000 worldwide. Scientific surveys of seabed mud, along with other reliable evidence, suggest that a great number of species are yet-to-be discovered.

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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