Microscopic anatomy of the axial complex and associated structures in the sea cucumber Chiridota laevis Fabricius, 1780 (Echinodermata, Holothuroidea)

Abstract

The absence of axial and pericardial coeloms and the axial organ, presence of a mesocoelomic tentacular apparatus, correspondence of the larval sagittal plane with the madreporic plane of the adult, and some other peculiarities set holothurians apart from all other echinoderms. We studied the holothuroid Chiridota laevis with histological methods and constructed a 3D-model. The bilateral symmetry of the water-vascular system is clear in spite of a peculiar coelomic connection with one of the tentacles in radius D. The somatocoelomic and axocoelomic perihaemal coeloms are absent. There are perioral and water-vascular circumoral coelomic rings. The hyponeural ring is absent. Each radial complex includes a water-vascular canal, a hyponeural canal, two nerve bands (ectoneural and hyponeural), and the non-coelomic epineural canal. The stone canal starts from the water ring on the aboral side in the CD interradius and ends with a madreporic ampulla, from which a few pores open into the perivisceral coelom, and one pore canal opens into the environment. This organization seems to be the intermediate state between most echinoderms, in which the madreporic pores open into the environment, and the typical organization of holothurians, in which the madreporic pores open into the perivisceral coelom. The histological structure of the oral haemal ring resembles the axial organ of other Eleutherozoa. The haemocoel lacune of the stone canal communicates with the haemal system of the gonad, the oral haemal ring, and the gut haemocoel lacunae. The genital coelom is represented by narrow spaces surrounding the “genital rachis” of the gonad.