Echinoderms

Those spiny skinned critters

Intro video

Phylum EchinodermataThe phylum consists of several classes. Some are:

– 1. Asteroidea: Sea stars– 2. Echinoidea: Sand dollars and sea urchins– 3. Holothuroidea: Sea cucumbers – 4. Ophiuroidea: Brittle sea stars

• We will use the sea star as our main study subject• Asteroidea:

– Sea stars also wrongly called starfish (they are not fish)– Have a star shaped body (most with five arms) LINK– Radial symmetry– Flexible skeleton made of calcareous plates (endoskeleton: in the body

like ours but not bone)– Predators and scavengers

– The sea star has mouth located in the very center of the body on the oral surface ( the side with the groove running down the arm).

– There is no head and the animal moves in the direction of any arm.

– They use hydraulic pressure to operate the tube feet so they can move. It is called the water vascular system.

• The water vascular system consists of the following parts: – Madreportite (sieve plate) which is the small disc on the

aboral(top) surface of the sea star.– Water enters and exits the sieve plate.– The stone canal connects the sieve plate to the ring canal (it forms

a ring around the mouth)– The ring canal connects to the radial canals on each arm.– The radial canal connects to the ampulla (a small bulb like a

pipette top) that connects to the tube feet.

• Water is used to extend and retract the tube feet • Tube feet video

The nervous system

• The nervous system is simple – No brain

– The nervous system is similar to the water vascular system in that it has a nerve ring around the mouth and radial nerves down each arm.

– They also have a nerve net over the surface of the animal that directs local movements

– The end of each arm has sensory tentacles that respond to chemicals. They also have a pigmented eye spot that is sensitive to light.

The digestive system

• They have a stomach that they can stick out of their mouth and therefore have external digestion.

• The digested material is transferred by cilia tracks to the digestive glands found in each arm. Each arm has one pair.

• Eating video

Circulatory System

• Hemal vessels run between the radial canal and the radial nerve.

• The fluid is pushed by a pulsing heart like sac found under the sieve plate.

Respiratory System

• Gas exchange takes place at skin gills. And through the thin membranes on the tube feet.

Reproduction

• Most sea stars have separate sexes. The gonads are located at the base of each arm.

• Fertilization is external.

• Mass breeding to ensure success.