Arbacioida
Arbacioida is an order of sea urchins within the class Echinoidea. They are characterized by their solid, robust tests (shells), typically with short, stout spines. This order includes some of the most familiar sea urchins, such as the heart urchins and the sand dollars. Arbacioida is a diverse group, with approximately 400 known species found in a wide range of marine environments.
Common Names
Members of the Arbacioida are commonly known by a variety of names, including:
- Heart urchins
- Sand dollars
- Sea biscuits
- Keyhole urchins
- Cake urchins
Characteristics
Arbacioida are characterized by the following features:
- Solid, robust tests: Arbacioida have thick, solid tests, often with a rough, bumpy texture.
- Short, stout spines: The spines of Arbacioida are typically short and thick, providing protection and helping with movement.
- Five-part radial symmetry: Like all echinoids, Arbacioida exhibit five-part radial symmetry, with their bodies divided into five equal sections.
- Tube feet: Arbacioida possess tube feet, which are used for locomotion and feeding.
- Aristotle's lantern: This complex feeding apparatus, found in all regular sea urchins, is present in Arbacioida. It consists of five teeth that are used to scrape algae and other food off of rocks and surfaces.
Environments
Arbacioida are found in a wide range of marine environments, including:
- Intertidal zones: Some species, like sand dollars, live in the intertidal zone, where they are exposed to air and waves.
- Subtidal zones: Many species live in the subtidal zone, below the low tide mark.
- Deep sea: Some Arbacioida species, like the heart urchins, inhabit deep-sea environments.
- Rocky shores: Many species can be found in rocky habitats, where they graze on algae.
- Sandy bottoms: Sand dollars and other flattened species often burrow in sandy bottoms.
References
For more information on Arbacioida, refer to the following reliable resources: