Arthropods Trilobites & Chelicerates

Eucoelomata in which:-1. The body is metamerically segmented. 2. There is more than one preoral segment, three being the typical number. 3. The gut is usually straight or nearly so, the anus terminal or sub-terminal. 4. The nervous system typically comprises preoral ganglia joined by circum-oesophageal commissures to a double ventral ganglionated nerve cord, but may be condensed in some groups. 5. Organs of special sense are typically present and they may be very elaborate. 6. The epidermis is covered by a stout chitinous cuticle. 7. Paired appendages are present on some or all of the segments and are typically jointed and may bear claws. 8. The muscles of the body wall occur as bundles or as separate fibres and are rarely in continuous layers. 9. The coelom is greatly reduced, the perivisceral cavity being a haemocoel. 10. Nephridia of the annelid type are absent; excretory organs are either coelomoducts or tubules associated with the hind gut. 11. The blood vascular system is open with few vessels, as they have been largely replaced by the perivisceral haemocoel. 12. The products of the gonads are shed through coelomoducts. 13. The sexes are almost always separate. 14. Larvae of various types occur, but they are never trochophores.