Zoology · Ch 2 — Kingdom Animalia
Phylum Echinodermata
Phylum Echinodermata
Echinodermata (Greek echinos, spiny; dermos, skin) are exclusively marine animals, and the name refers to their mesodermal endoskeleton of calcareous ossicles, which often bears spines. Adult echinoderms are radially symmetrical (typically pentamerous — five-part), but their larvae are bilaterally symmetrical, which reveals the phylum's true position among the triploblastic, deuterostome Bilateria despite its radially symmetrical adult body plan.
The single most distinctive feature of the phylum is the water vascular (ambulacral) system, a network of fluid-filled canals ending in tube feet (podia) that together handle locomotion, food capture and transport, and respiration. The digestive system is complete, with the mouth on the ventral side and the anus on the dorsal side. Echinoderms have no dedicated excretory organs, and their nervous system and sense organs remain poorly developed. Circulation is of the open type, and notably there is no heart or blood vessels at all. …
What this figure shows. Four echinoderm body forms, each showing five-part (pentamerous) radial symmetry to some degree: Starfish, a flattened five-armed body with visible tube feet along grooves on the underside of each arm; Sea cucumber, an elongated soft cylindrical body with a leathery skin and a ring of tentacles around the mouth, the pentamerous symmetry far less obvious externally; Sea urchin, a spherical body densely covered in long movable calcareous spines; and Brittle star, a small central disc with five long, thin, …