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The cephalon is typically proparian or gonatoparian. The preglabellar field is often very short or absent. If eyes are present they are schizochroal or holochroal . The thorax contains 8-19 segments, some furrowed, with the thoracic axis usually broad. Pygidiums in this order are typically micropygous, variable, lobed or spiny or smooth margined (see diagram below for exoskeleton nomenclature).
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Order Phacopida exhibit phacopid type enrollment. The Posterior margin of the pygidium fits into a furrow on the cephalon which runs closely parallel to the cephalic margin. The thoracic plates overlap each other, thus giving the trilobite protection against predators. The diagram below illustrates this enrollment type.
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The exoskeleton of the trilobites is hard and unyielding and must be shed in order for them to grow and increase in size. The sutures on the cephalon are thought to have played an important role in ecdysis by creating lines of weakness along which different pieces of the cephalon cleaved apart. The junction between the cephalon and the thorax was also very improtant in molting, particurlarly in trilobites with none functional cephalic sutures (like mosts Phacopids).
In Phacopid type molting the cast off exoskeleton (exuviae) consists of the three parts: the cephalon, thorax and pygidium. The main line of ecdysis seems to have been between the cephalon and the thorax. When these two pieces cleaved apart, the animal crawqled backwards and buckled upwards, forcing its head through the opening while shedding the old cephalon. As a result the cephalon ends up overturned and facing backwards. Once the cephalic region was free, the animal crawled forward, leaving the old thoracic and pygidial segments behind (see diagram below).
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Author: Sarah Vinnell
Last updated: 21/11/05
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