The circulatory system and its spatial relations to other major organ systems in Spelaeogriphacea and Mictacea (Malacostraca, Crustacea) – a three‐dimensional analysis

Spelaeogriphacea and Mictacea are enigmatic taxa within malacostracan crustaceans that play a pivotal role in peracarid phylogeny. Anatomical data on both taxa that are suitable for use in cladistic analyses are still scarce. Here, we provide for the first time detailed three‐dimensional information on the major organ systems of Spelaeogriphacea and Mictacea (the circulatory system, the digestive system, and the central nervous system) using semithin sections in combination with computer aided three‐dimensional reconstruction techniques. The digestive system in both Spelaeogriphus lepidops and Mictocaris halope is made up of a short oesophagus leading to a voluminous stomach chamber. Posteriorly, a pylorus is attached to the stomach chamber. An antechamber of the midgut glands is situated at the transition into the midgut, from which up to four tubular midgut glands emanate. The midgut is a straight tube running through the body terminating in a short hindgut. The central nervous system in the cephalothorax is made up of a brain and a suboesophageal ganglion. Both species show some reduction of the protocerebrum caused by the lack of eyes. The circulatory system is made up of a tubular heart that is situated in the thorax. It is equipped with two pairs of incurrent ostia in S. lepidops and one pair in M. halope . The only artery leading off the heart is the anterior aorta, which runs into the cephalothorax. A dilation is formed between the brain and the anterior stomach wall, into which oesophageal dilator muscles are internalized. The function of this so‐called ‘myoarterial formation a’ as an accessory pulsatile structure in the anterior cephalothorax of these animals is discussed. © 2007 The Linnean Society of London, Zoological Journal of the Linnean Society , 2007, 149 , 629–642.