- Most echinoderms are sessile or sedentary marine forms with radial symmetry as adults. Their larvae contain bilateral symmetry.
- A thin skin is a covering for a hard calcium plate-like exoskeleton.
- Most contain bumps and spines which serve various functions.
- They contain a water vascular system. This system is a network of hydraulic canals which branch into extensions called tube feet. These tube feet function in movement, feeding and respiration.
- These organisms are true coelomates.
- Simple nervous system with no brain
- No specialized excretory organs.
- They are deuterstomes. The digestive tract begins with the anus first.
- They contain separate sexes and carry out external fertilization.
- There are six classes of echinoderms ( Asteroidea - sea stars ) is the most common.
- This class contain sea stars which have 5 or more arms extending from acentral disc.
- The tube feet on the under surface of the arms are swollen when fluid is pushed into them. Suction cups at the end of each tube foot attaches to the ground while the muscles contract to shorten the foot. coordination of extension, attaching, contraction and release allow slow movement and attachment to prey.
- These organisms eat by attaching their tube feet to the shells of the clam and wrap around it,holding it tightly. The clam's muscles fatigue allowing the shell to be pulled open. The sea star everts its stomach into the clam and secrete digestive juices onto its soft tissues.
- Sea stars have a limited ability to regenerate. A single arm cannot regenerate an entire body.
- Class Ophiuroidea:
- Brittle stars differ from the sea stars in many ways. They also typically have five arms, but these arms are slender and distinctly set off from the central disc.
- The tube feet have no suckers, and are of limited use in locomotion; the brittle stars use their entire arms to aid in movement.
- They do not have an anus: indigestible material is expelled from the mouth.
- Example: Brittle stars
- Class Echinoidea:
- Echinoids have a compact body encased in test, or shell. The dermal ossicles, which are modified to be close-fitting plates, make up the test.
- Echinoids lack arms, but their pentaradial structure is evident in the arrangement of the ambulacral areas, appearing as grooves on the test that run from the mouth to the aboral side, ending at the area around the anus.
- "Regular" sea urchins are the familiar hemispherical shape with medium to long spines.
- They move by means of their tube feet, with some assistance from their spines. "Irregular" urchins, the sand dollars, have become secondarily bilateral: their spines are usually very short and their bodies are more flattened than those of regular urchins.
- They move chiefly by their spines.
- Examples: Sea urchins, sand dollars
- Class Holothuroidea :
- Sea cucumbers are greatly elongated along the oral-aboral axis compared to the other echinoderms, and the ossicles are much reduced in most, so the animals are soft-bodied.
- They too have become secondarily bilateral, though in a different way from the irregular urchins: their tube feet are well developed along only one ambulacral groove.
- Example: Sea cucumbers
- Class Crinoidea:
- In crinoids, the body disc has a leathery skin containing calcareous plates.
- Five flexible arms branch to form many more arms, each with many lateral pinnules arranged like barbs on a feather.
- Their ambulacral grooves are open and ciliated, and serve to catch small organisms and carry them to the mouth.
- Examples: Sea lilies and feather stars
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