Morphology and function of the tube feet of Florometra serratissima (Echinodermata: Crinoidea)

Nervous system characterization during the development of a basal echinoderm, the feather star Antedon mediterranea

Neural development of echinoderms has always been difficult to interpret, as larval neurons degenerate at metamorphosis and a tripartite nervous system differentiates in the adult. Despite their key phylogenetic position as basal echinoderms, crinoids have been scarcely studied in developmental research. However, since they are the only extant echinoderms retaining the ancestral body plan of the group, crinoids are extremely valuable models to clarify neural evolution in deuterostomes. Antedon mediterranea is a feather star, endemic to the Mediterranean Sea. Its development includes a swimming lecithotrophic larva, the doliolaria, with basiepithelial nerve plexus, and a sessile filter-feeding juvenile, the pentacrinoid, whose nervous system has never been described in detail. Thus, we characterized the nervous system of both these developmental stages by means of immunohistochemistry and, for the first time, in situ hybridization techniques. The results confirmed previous descriptions of doliolaria morphology and revealed that the larval apical organ contains two bilateral clusters of serotonergic cells while GABAergic neurons are localized under the adhesive pit. This suggested that different larval activities (e.g., attachment and metamorphosis) are under the control of different neural populations. In pentacrinoids, the analysis showed the presence of a cholinergic entoneural system while the ectoneural plexus appeared more composite, displaying different neural populations. The expression of three neural-related microRNAs was described for the first time, suggesting that these are evolutionarily conserved also in basal echinoderms. Overall, our results set the stage for future investigations that will reveal new information on echinoderm evo-devo neurobiology.

Comparative studies on receptor structure in the brittlestar Ophiura ophiura

The morphology of cells incorporating modified cilia in the epithelium of the spines, tube-feet and general surface of the ophiuroid echinoderm Ophiura ophiura are described using transmission and scanning electron microscopy and compared to those on the tube-feet of the echinoid Echinus esculentus. Some receptor cells have cilia showing a normal 9 + 2 microtubular arrangement and these project from the cuticular surface. There are also three basic cell types in which the cilia do not show a continuous normal 9 + 2 microtubular arrangement. In one class, a ring of microvilli form a tube which protrudes from the cuticular surface and which contains a cilium. This type is present on both spines and tube-feet and is identical in structure to the stäbchen receptors described previously. The second type, termed the non-projecting receptor, is not surrounded by specialized microvilli, and the cilium which projects from the receptor cell ends flush with the cuticle which may be 3 or 4 micron above the epithelial surface; these are only present on the tube-feet. The third type of receptor occurs on the general surface of the arm, and from its position and some aspects of its structure may be a general photic receptor. It consists of a highly modified cilium characterized by a very short ciliary root. The cilium projects very little above the receptor cell surface and terminates among the extracellular fibrillar material that packs the gap of variable width between the epithelial cellular surface and the non-cellular cuticle. The distribution of these presumptive receptor cells is discussed in the light of recent sensory physiological findings and previous descriptions by other authors of receptor structures in the whole phylum.