Determination of phylogenetic relationships within the phylum Ctenophora and with other phyla is a difficult task. Although the Ctenophora have been closely aligned with Cnidaria, numerous alternative schemes exist. Ctenophores have been linked with several phyla (Cnidaria, Platy-helminthes, Porifera, and Echinodermata), but molecular evidence supports them as a unique phylum linked most closely to the Cnidarians (based on ribosomal RNA analysis). Ctenophores were once classified as coelenterates based on the presence of nematocysts in one species (Haeckelia rubra), which were later found to be kleptocnidae. There are currently two classes (Cydippida and Nuda) in this phylum, seven orders, including Lobata, Cydippida, Cestida, Thalassocaly-cida, Ganeshida, Platyctenida, and Beroida, 20 families, and 100-150 described species. There have been several new species described since 1995 and there are many more awaiting descriptions. There are two recognized fossil species found dating from the Devonian age (417-354 million years ago) that are recognizable as cydippids.
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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.