Mictocaris halope, Bowman & Iliffe, 1985

Wirkner, Christian S. & Richter, Stefan, 2007, The circulatory system and its spatial relations to other major organ systems in Spelaeogriphacea and Mictacea (Malacostraca, Crustacea) - a three-dimensional analysis, Zoological Journal of the Linnean Society 149 (4), pp. 629-642 : 634

MICTOCARIS HALOPE

DIGESTIVE SYSTEM ( Figs 3A and 4A ): a short oesophagus connects the mouth to the stomach chamber. The transition is guarded by the cardiooesophageal valve (oe; Fig. 5G). The proventriculus is equipped with a complex filtering apparatus composed of filtering spines in combination with invaginations and grooves. The stomach chamber opens out in a short pyloric region, which is confluent with the midgut. Postero-ventrally of the pyloric region, the antechamber of the midgut glands is situated. Four pairs of midgut glands emanate from this region (mgg; Figs 4A and 5C) that show (also in vivo) a corrugated surface (mgg; Figs 3C and 4A ): two pairs dorsally and two pairs ventrally. Their wall is made up of longitudinal ridges projecting into the lumen, giving a typical shape in cross sections (mgg; Fig. 4F ). Two pairs of midgut glands are shorter and run in an anterior direction. The other two pairs run alongside the midgut above each other as far as the 4th and 7th thoracic segments, respectively. The position of the midgut glands seems not to be variable (mgg; compare Fig. 3C with Fig. 4A ). All midgut glands have approximately the same diameter (∼ 80–90 µm). The tubular midgut ( Fig. 3B ; diameter ∼70–90 µm) runs through the whole trunk and opens into a short hindgut. The anus lies in the anterior part of the telson.

CENTRAL NERVOUS SYSTEM ( Fig. 4B ): different parts of the proto-, deuto- and tritocerebrum can be recognized. Although M. halope has neither eyes nor visual elements, only the optic neuropils are missing in the protocerebrum. In the lateral protocerebrum, two different neuropils are discernible, probably corresponding to the hemielipsoid bodies (hb) and the terminal medullae ( Fig. 5A, B). In the median protocerebrum a protocerebral bridge and a central body are distinct (cb; Fig. 5A). No clear distinction of the anterior and posterior medial protocerebral neuropils could be made in the sections. The protocerebral bridge and the central body are surrounded by distinct neuropils. The anterior medial cells (am) are split by a deep sagittal groove ( Figs 4B and 5A) into two lobes that protrude in a dorsal direction. In the deutocerebrum, voluminous olfactory lobes are present (ol; Figs 4B and 5A, B). They contain the olfactory glomeruli (alg), which are clearly visible as areas of densely packed cone-shaped structures ( Fig. 5B). An olfactory globular tract (ogt; Fig. 5B) is present. The nerves for the first antennae emanate ventrally off the olfactory lobes and the deutocerebrum. In the tritocerebrum, the larger part is made up of the antenna II neuropil from which the nerves of the second antennae lead off. The brain is linked by the rather strong oesophageal connectives to the suboesophageal ganglion ( Fig. 4B ).

CIRCULATORY SYSTEM ( Figs 3A and 4C–E ): the tubular heart runs from the border between the cephalothorax and the 2nd thoracic segment to the 5th thoracic segment ( Fig. 5E). Its wall is thin and has only a few muscle fibres that are arranged spirally. In the 2nd thoracic segment its diameter is ∼80–90 µm, narrowing down to ∼40–50 µm in the 5th thoracic segment. It is equipped with a pair of incurrent ostia that lie in the 2nd thoracic segment ( Fig. 5D, E). The lips of the ostia are thin and ∼15 µm long ( Fig. 5D). Neither posterior aorta nor cardiac arteries were observed. The dorsal diaphragm is thin and runs through the whole thorax. In each thoracic segment a podopericardial sinus leads into each thoracic leg.

In the cephalothorax, the heart is continued by the anterior aorta. A valve, which is made up of two horizontal flaps that protrude into the lumen of the aorta, is located at the transition (va; Fig. 5D). The aorta runs through the dorsal anterior pair of midgut glands, and is laterally compressed. It is suspended by connective tissue strands from the dorsal cuticle of the cephalothorax. At the level of the dorsal stomach dilator muscles m116 (nomenclature after Scheloske, 1976), the aorta bends ventrally and then widens laterally forming a large dilation that directly lies against the stomach wall. This dilation has a vertical extension of ∼150 µm (∼ 30 µm deep, if not compressed, and ∼90 µm wide) and fills the space between the anterior stomach and oesophagus wall and the posterior front of the brain. Oesophageal dilator muscles run through this dilation (oed; Fig. 6 ; see the Discussion). In the centre of the deutocerebrum a median unpaired vessel emanates from the dilation. It runs into the brain and splits into four branches that curve back, thus supplying the brain with haemolymph (herein called brain artery, ba; Figs 4C–E and 5A, F). The ventral part of the aorta dilation is a complex vascular structure made up of channels and vessels that run around the brain ( Fig. 4C–E ). Just ventral to the brain artery a broad vessel branches off running underneath the deutocerebrum in an anterior direction (arrow; Fig. 4D ). At the base of the first antennae, it splits to supply these appendages ( Fig. 5G). At the level of the brain artery a pair of arteries branches off the dilation laterally (arrows; Fig. 4E ). They run through a groove between the lateral part of the deutocerebrum and the medial parts of the olfactory lobes and, together with the second antennal nerves, pass into the second antennae ( Figs 4C–E and 5B). From these vessels, at the base of the second antennae, a vessel emanates in a medio-posterior direction (arrows; Fig. 4C ), which fuses with the labral funnel (lv; Fig. 4C, D ). A direct supply to mouthparts could not be observed.

Figures

Figure 3. Major organ systems in Mictocaris halope. A, Schematic diagram of the haemolymph vascular system (HVS) and digestive system in M. halope. Arteries running into the antennae are depicted by broken lines. B, C, photographs of live animals showing the digestive system and the gonads (courtesy of Peter Parks, Image Quest 3D). B, dorsal aspect. C, lateral aspect. See the Appendix for a list of abbreviations used.

Figure 4. Three-dimensional reconstructions of a semithin cross section series of Mictocaris halope. A–E, surface renderings of the main organ systems in the cephalothorax. A, three aspects of the digestive system. B, three aspects of the central nervous system. C, D, The haemolymph vascular system (HVS) in the cephalothorax. C, frontal aspect. D, lateral aspect. E, antero-dorsal aspect. F, three virtual sections of the whole image stack. Same colour codes as in the surface renderings. See the Appendix for a list of abbreviations used.

Figure 6. Schematic diagrams of the pumping function of the ‘myoarterial formation a’ in Mictocaris halope. A, diastole. B, systole. Arrows depict flow directions of the haemolymph. See the Appendix for a list of abbreviations used.