Echinodermata: Characteristics, Classes, and Phylogeny
Echinodermata: Unique Marine Animals
Echinoderms are easily distinguished from other animals due to their unique characteristics:
- Tube feet
- Hard & spiny skin
- Water vascular system
- Endoskeleton consisting of ossicles
Echinoderms exhibit radial symmetry as adults, although their larvae are bilateral. This transformation involves a unique metamorphosis that reorients the body axis by 90 degrees.
Water Vascular System
The water vascular system allows echinoderms to move, exchange gases, capture food, and excrete wastes. It operates using hydraulic pressure. Water enters through the madreporite on the aboral surface, which filters out large particles. This system enables complex and intricate movements by moving water through a system of tubes.
Tube feet and ampullae are connected to the water vascular system. Each moves independently, allowing the animal to move and attach itself to prey or the substrate. The thin walls of the tube feet also facilitate gas exchange through diffusion.
Feeding & Digestion
Some echinoderms are predators (e.g., sea stars), pushing their stomach out of their mouth onto food for external digestion before pulling the stomach back in. Others are grazers of algae (most urchins), while brittle stars, sea lilies, and sea cucumbers feed on detritus on the sea floor. All echinoderms have a complete digestive system (mouth, stomach, intestines, anus).
Nervous Control
Echinoderms lack a head or brain but possess a central nerve ring surrounding the mouth. Nerves extend into the rays, branching into a nerve net.
Eyespots at the end of sea star rays sense light.
Diversity of Echinodermata
Class Asteroidea: Sea Stars
Many sea stars have five rays (pentaradial symmetry), but some have more.
Class Ophiuroidea: Brittle Stars
Brittle stars are very fragile, and their rays easily break off but can be regenerated in a few weeks.
Tube feet are used for feeding but not movement, which is achieved through a snake-like motion.
Class Crinoidea: Sea Lilies & Feather Stars
Many resemble plants or corals. Sea lilies are filter-feeders and the only sessile echinoderms.
Feather stars swim from place to place using their thin, feathery arms.
Class Holothuroidea: Sea Cucumbers
Their name comes from their vegetable-like appearance. A leathery covering makes them more flexible than other echinoderms. They use tube feet and tentacles to move along the ocean bottom.
When threatened, they can expel organs through the anus, which are later regenerated.
Class Echinoidea: Sea Urchins & Sand Dollars
These are globe or disk-shaped animals covered with spines and lack rays.
Sand dollars have a circular, flat skeleton with a 5-petal pattern on the upper surface. Tube feet on the upper surface, modified into gills, are located on the petal pattern.
Tube feet on the lower surface move food towards the mouth and facilitate movement. Sea urchins’ bodies are composed of long, pointed spines that protect them from predators and, in some species, contain venom. Spines also aid in locomotion and burrowing, creating a depression in the rock to which they attach themselves.
Phylogeny of Echinodermata
There are two major hypotheses regarding echinoderm phylogeny:
- Early ancestors had bilateral symmetry as adults and were attached to a substrate (sessile).
- Early ancestors were bilateral and free-swimming.