Echinoderms and Deuterostomes

Early Development

• Zygote Formation:

  • Starts with an egg and sperm. The DNA combines to form a single fertilized cell called a zygote.

  • The zygote divides into two cells within a couple of hours.

• Urchin Development Model:

  • Urchins are important developmental models due to the ease of obtaining sperm and eggs.

  • Injecting urchins with salt solution induces them to release sperm (white) and eggs (yellow).

  • Allows observation of live development under a microscope.

• Embryonic Development:

  • After the two-cell stage, the embryo progresses to four cells and then eight cells.

  • It forms a ball of cells.

  • The ball of cells develops a hole, which becomes either the mouth or the anus.

Anus First vs. Mouth First

• Deuterostomes:

  • Echinoderms and chordates (including humans) are deuterostomes.

  • Deuterostomes develop the anus first.

  • The mouth develops second.

  • The term "deuterostome" means "second mouth."

• Protostomes:

  • Other animal groups develop the mouth first and are called protostomes.

• Digestive Systems:

  • Periphera (sponges) lack a mouth.

  • Cnidarians have a single opening that serves as both mouth and anus.

  • Deuterostomes and Protostomes develop a digestive system that goes all the way through during development

Echinoderm Characteristics

• Echinoderm Meaning:

  • "Echino" means spiny, and "derm" means skin; thus, echinoderms have spiny skin.

• Examples of Echinoderms:

  • Urchins.

  • Sand dollars.

  • Sea stars (starfish).

  • Brittle stars.

  • Sea cucumbers.

  • Sea lilies and sea feathers (crinoids).

  • Sun stars.

• Pentaradial Symmetry:

  • Echinoderms exhibit pentaradial symmetry, meaning they have five-fold symmetry.

  • This is evident in the five-leaf pattern on sand dollars.

  • Sunstars typically have arms in multiples of five (e.g., twenty arms).

• Feeding Strategies:

  • Sea Feathers and Brittle Stars: Filter feeders; they passively filter food from the water using their arms.

  • Sea Stars and Sun Stars: Predatory; they use their arms to open prey like mussels and oysters.

  • Sea Cucumbers: Detritivores; they feed on detritus (decomposing organic matter).

Echinoderm Anatomy and Physiology

• Nervous System:

  • Octopuses have many nerves in their arms, allowing movement even after death.

• Pentaradial Symmetry Visualization:

  • A sand dollar can be visualized as a flattened urchin.

  • A sea star can be visualized as five arms wrapped into a ball, with the spaces solidified.

• Water Vascular System:

  • Unique to echinoderms.

  • Hydraulic system using fluid-filled tubes and reservoirs.

  • Water enters through an opening and is distributed through a ring canal.

  • Tube feet operate via hydraulic pressure, acting like suction cups.

  • Strong enough for sea stars to open mussels and oysters by flexing muscles for hours.

  • Sea stars can evert their stomach into prey to digest them externally.

  • The braking system in cars operate on hydraulic pressure.

Additional Features

• Pedicellariae:

  • Small pincers on the surface of sea stars used to remove debris and organisms.

• Ossicles:

  • Bony plates that provide rigidity in echinoderms.

  • Reduced in sea cucumbers, making them soft.

• Crinoids:

  • Ancient group of echinoderms, including sea lilies and feather stars.

  • Sea lilies resemble fans in the deep sea.

• Larval Forms:

  • Urchin larvae have a unique larval form.

  • Brittle stars have articulated feet instead of tube feet.

• Sea Cucumbers and Recycling:

  • Sea cucumbers recycle materials on the ocean floor.

• Cloaca: An orifice for breathing, pooping, and eating unique to sea cucumbers.