Arthropods Trilobites & Chelicerates

Abstract

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.