Agnostida

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The Order Agnostida are a group of mostly blind, tiny Cambrian arthropods. The Agnostids strongly resembled a pair of castanets, in that, the cephalon and the pygidium were grossly enlarged, and resembled each other, and were connected to each other by a thorax that only consisted of 2 to 3 segments. Some experts dispute the idea that the Agnostids should not be considered proper trilobites, on account of the discovery and examination of the limbs of a larval specimen of Agnostus pisiformis. The limbs were unlike the limbs of other trilobites whose legs have been described, and were more like those of crustaceans. However, given as how the limbs of other trilobite larvae have yet to be discovered and described, and how the limbs of adult agnostids have yet to be discovered and described, it is not known whether or not the limbs of trilobite larvae resembled those of crustaceans, and became modified as the larvae developed, or whether or not the limbs of adult agnostids were similar to those of other trilobites.

Agnostida is divided into two suborders, Agnostina, and Eodiscina. The Agnostina are the typical agnostids, and have no eyes, circular cephalons and pygidiums that are connected with thoraxes with 2 segments. They are often simply referred to as "agnostids," in fact. The Eodiscina links Agnostida to the rest of the trilobites, and tend to resemble other trilobites better than the Agnostina agnostids. Eodiscids may or may not have two eyes, and have semi-circular cephalons and pygidiums that are connected by a thorax of 3 segments. Some eodiscids have a tail-like spine on their pygidiums.

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