Table 3 Definition of technical terms assigned to the axial complex of echinoids used in this manuscript.
Structure | Type of body cavity involved | Developmental origin of body cavity | Topography |
|---|---|---|---|
Axial complex | Primary and secondary body cavities | Left larval axocoel Right larval axocoel Left larval hydrocoel Right larval somatocoel Left larval somatocoel Haemal structure | Located between madreporic plate/hydropore and ring canal, within interradius CD (sensu Lovén) below genital plate 2, lodged within the dorso-ventral mesentery, surrounded by oral and aboral somatocoel |
Madreporic ampulla | Secondary body cavity | Left larval axocoel | Small space beneath the madreporic plate, connected to the exterior by madreporic pore canals, adorally gives rise to stone canal and axial coelom |
Stone canal | Secondary body cavity | Left larval hydrocoel | Single tube arising from lower end of madreporic ampulla, descending towards ring canal, can be calcified, in some irregular taxa divided into three distinct parts, can be pulsatile |
Dorsal sac | Secondary body cavity | Right larval axocoel | Closed cavity enclosing the head process, with muscularized epithelium, lateral to madreporic ampulla |
Head process | Primary body cavity & connective tissue matrix | Haemal structure | Aboral extension of the axial organ, surrounded by the dorsal sac, pulsatile, perforated by canaliculi and haemal lacunae |
Axial coelom | Secondary body cavity | Left larval axocoel | Orally oriented blindly-ending part of the axocoel, partly enwraps stone canal as well as axial organ, connects adapically to madreporic ampulla, podocyte lining |
Axial organ | Primary body cavity & connective tissue matrix | Haemal structure | Parallel to axial coelom, partially surrounded by somatocoel, connected to dorso-ventral mesentery, crossed by numerous canaliculi and haemal lacunae |
Pulsating vessel | Primary body cavity | Haemal structure | (Pulsatile) haemal structure running along the outside of the axial organ, protruding into the axial coelom, can be lined by muscularized epithelium, adoral extension of head process |
Canaliculus | Secondary body cavity | Left larval axocoel Right larval somatococel Left larval somatocoel | Randomly distributed small invagination formed through infolding of coelomic epithelium, ending blindly, found inside the matrix of axial organ and head process |
Haemal lacuna | Primary body cavity | Haemal structure | Randomly distributed anastomosing compartments within axial organ, head process and mesenteries, not lined by epithelium, part of the haemal system |