Evolution and systematics

The phylum Chaetognatha is thought to be an old group with a long and separate path of development. It is not clear what the nearest relatives are, but evidence provided by developmental and molecular studies show that this group may be an early split-off of the metazoan phyla. It is a paradox that there are so few species, because the group has been so successful in increasing its number of individuals and maintaining widespread distribution. It is a soft-bodied group and several researchers (Owre and Bayer, 1962, and Conway-Morris, 1977) have raised doubts concerning fossils of certain species (e.g., Amiskwia sagittiformis) that have been recorded. A. sagittiformis has raised particular doubts because it has no substantive structures that are usually present in a chaetognath, such as hooks or teeth; more over it has a gut that runs to the end of the tail unlike any chaetognath.

Two orders are usually recognized, Phragmophora and Aphragmophora. The order Phragmophora contains three families (Heterokrohniidae [three genera], Spadellidae [four genera], Eukrohniidae [one genus]), and the order Aphrag-mophora also contains three families (Sagittidae [one genus], Pterosagittidae [one genus], Krohnittidae [one genus]). The genus Sagitta (Sagittidae) is split into 11 genera by some authors, but because of the uncertain nature of some of the groupings and the need for further molecular evidence to determine the true nature of these genera, the name Sagitta will be used in this chapter. There are about 100 species under discussion but there may be as many as 200 valid species, many of which may be benthic.

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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