Arthropod

Their vision relies on various combinations of compound eyes and pigment-pit ocelli: in most species the ocelli can only detect the direction from which light is coming, and the compound eyes are the main source of information, but the main eyes of spiders are ocelli that can form images and, in a few cases, can swivel to track prey. Arthropods also have a wide range of chemical and mechanical sensors, mostly based on modifications of the many setae (bristles) that project through their cuticles. Arthropods' methods of reproduction and development are diverse; all terrestrial species use internal fertilization, but this is often by indirect transfer of the sperm via an appendage or the ground, rather than by direct injection. Aquatic species use either internal or external fertilization. Almost all arthropods lay eggs, but scorpions give birth to live young after the eggs have hatched inside the mother. Description[edit] Diversity[edit] Segmentation[edit] = Body = Coxa (base) = Gill branch x

595 Articulata The Articulata is a class of brachiopods which comprises those with hinged, calcareous shells that generally bear well developed teeth and sockets and a simple muscle system. The name was first applied to this major group of brachiopods by Huxley in 1869, ten years prior to Zittel choosing the same name in 1879 for modern crinoids. The Articulata has been replaced by the Subphylum Rhynchonelliformea which except for taxonomic rank and revision of some taxa within, is essentially synonymous. Moore, Lalicker, and Fischer, 1952, divided the Articulata into the following groups: PaleotremataOrthidaTerebratulidaPentameridaRhynconellidaStrophomenidaSpiriferida The earlier version of the Treatise, part H, 1965, is similar, except for the elimination of the Paleotremata and the order in which the remaining orders are presented. Inarticulata

Anomalocaris Anatomy[edit] Size comparison of Anomalocaris (red) and a human. For the time in which it lived, Anomalocaris was a truly gigantic creature, reaching lengths of up to two meters.[2] Discovery[edit] Anomalocaris "arm" from the Mt. Stephen Trilobite Beds, Middle Cambrian, near Field, British Columbia, Canada. In 2011, six fossils of compound eyes dated to the Cambrian period (515 million years ago) were recovered from an archaeological dig at Emu Bay on Kangaroo Island, Australia. Ecology[edit] Anomalocaris had a cosmopolitan distribution in Cambrian seas, and has been found from early to mid Cambrian deposits from Canada, China, Utah and Australia, to name but a few.[6][15][16][17] A long-standing view holds that Anomalocaris fed on hard-bodied animals, including trilobites. See also[edit] [edit]

Arandaspis Arandaspis prionotolepis is an extinct species of jawless fish that lived in the Ordovician period, about 480 to 470 million years ago. Its remains were found in Alice Springs, Australia in 1959, but it was not discovered that they were the oldest known vertebrates until the late 1960s. Arandaspis is named after a local Aboriginal tribe, the Aranda (currently called Arrernte). Description[edit] Restoration Arandaspis was about 15 cm (6 in) long, with a streamlined body covered in rows of knobbly armoured scutes. Although it had no jaws, Arandaspis might have had some moveable plates in its mouth, serving as lips, sucking in food particles. It had no fins; its only method of propulsion was its horizontally flattened tail. References[edit]

5. Mathematics. Natural Sciences. 55. Earth Sciences. Geological Sciences.