Trilobites belonged to the phylum Arthropodalike crustaceans, spiders and insects. They aredivided into three parts from head to tail (head, thorax and tail) and from side-to-side (the central axis and two lateral lobes). The word 'trilobite' refers to the side-to-side partition, not the head, thorax and tail.
The picture shows how trilobites from different periods lived in the sea.
In shallow waters amongst the bivalves 1, crinoids 2, algae 3 and gastropods 4, the faunas were large, but of low diversity, e.g. Flexicalymene 5.
Intermediate faunas were more diverse Selenopeltis 6, Geragnostus 7, Chasmops 8 and Remopleurides 9 lived alongside brachiopods 10 and nautiloids 11.
In the depths of the sea, the trilobites were small and blind with moderate diversity Ampyx 12, Tretaspis 13 and Shumardia 14, with Pricyclopyge 15 swimming above.
Trilobites have long attracted popular as well as scientific attention. The Pahvant Ute Indians of Utah wore specimens of the Middle Cambrian trilobite Elrathia kingii around their necks as lucky charms. It is reported that they were used to guard against sickness and being shot.