Echinozoa

Echinozoa is a subphylum within the phylum Echinodermata, which includes marine invertebrates known for their spiny skin. This subphylum encompasses animals with a fundamentally globoid body shape that is secondarily cylindrical or discoid, and they lack external arms or brachioles.1

The living classes within Echinozoa are:
* Holothuroidea (sea cucumbers): These are cylindrical, elongated animals with a mouth at one end and an anus at the other, surrounded by feeding tentacles. Their skeletons are typically reduced to microscopic spicules.1
* Echinoidea (sea urchins, heart urchins, sand dollars): These have a globular, discoid, or oval shape with a complete skeleton (test) bearing movable spines and pedicellariae. They have a downward-facing mouth and an anus.1

Echinozoans are exclusively marine and can be found in various habitats, from rocky shores to deep ocean trenches, where they can be the dominant organisms. They play important ecological roles, such as scavenging, preying on other organisms, and altering seafloor sediments.23