Evolution and systematics

The fossil record of nemertines is extremely sparse, as would be expected from a soft-bodied animal. There is a trace fossil (genus Archisymplectes) from the Pennsylvania-age from central Illinois that may represent an anoplan nemertine. The anoplan palaeonemerteans have been regarded as the phylo-genetically basal nemerteans based on a simpler nervous system and cerebral organs, but recent molecular studies are inconclusive.

Class Anopla used to be divided in three subclasses, Archinemertea, Palaeonemertea, and Heteronemertea, but Archinemertea has been shown to be paraphyletic and the name is not used by most authors. Recent studies based on nucleotide sequences established that the paleonemertea is a non-monophyletic group but did not formally reclassify the order. The term palaeonemerteans and Paleonemertea thus refer to a presumed paraphyletic assemblage and are used for convenience and tradition without reflecting a monophyletic group. The Palaeonemertea includes those species with two (outer circular and inner longitudinal) or three (outer circular, middle longitudinal, and inner circular) body muscle layers. The subclass Heteronemertea includes those species with primarily three body wall muscles, although there exist species with an additional inner circular layer. The central nervous system is situated between the outer longitudinal and the middle circular muscular layers in heteronemerteans, and in the inner longitudinal muscle layer, or external to body wall muscles, in the palaeonemerteans.

The class is currently divided into 11 families and 93 genera comprising approximately 500 described species. The largest genus, Cerebratulus contains 116 species, and the second largest, Lineus, has 80 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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