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Model organisms are frequently used in science teaching as they provide good exemplars for students that want to understand basic biological phenomena. The common sea urchin Paracentrotus lividus is a familiar animal that students can frequently observe in nature and represents a good didactic model for teaching various biological concepts in schools. Its simple body structure and anatomy allow easily recognition of the main external and internal organs, including spines, tube feet, feeding apparatus (Aristotle's lantern), digestive tract and reproductive organs, providing several practical cues for interactive discussions on animal physiology and ecology. Additionally, this organism is an optimal model for in vitro observation of major reproductive processes, including release of gametes, fertilization and recognition of various embryonic stages. In fact, this species produces millions of spermatozoa and thousands of large, transparent eggs which can be easily collected; these can be used for in vitro fertilization and observation of basic biological processes such as mitosis, cell migration and differentiation and of early developmental stages.

Echinoderms have been used often as experimental models in developmental biology and evolutionary biology studies. Numerous data on echinoid and asteroid development are available, whereas little is known about crinoid larval biology. This contribution focussed on the life cycle of the Mediterranean feather star Antedon mediterranea.Light and electron microscopy were used to characterize, in detail, the morphology and behaviour of the main larval stages. Similarities and differences with respect to what is already known for other crinoids, and echinoderm species, were explored. In view of the importance of serotonin during settlement and morphogenesis, analyses of the distribution of this molecule were carried out on swimming larvae. Immunolabelling results suggested a role for serotonin in A. mediterraneadevelopment, underlining the ancestral importance of this conserved neurotransmitter in deuterostome evolution.