Barnacles (Crustacea: Cirripedia)

Cirripedes (barnacles) are known for their sessility – it’s their defining characteristic. They count as one of the first model organisms of evolutionary biology, having been comparatively studied by Darwin for over 8 years (one would think they’re his favourite animals – although this letter says otherwise!). The 4 resulting monographs (two for Recent, two for fossil) are still some of the best examples of systematic biology done right (cf. Newman, 1987), and his experience working on them was extremely influential in setting his ideas on adaptation straight; among other things, it led to him stressing the importance of embryology for identification of possible homologies, laying the foundation for Haeckel’s expansion of that idea.