Nematodes, Echinoderms

Phylum 7: Nematodes (roundworms)

  • not segmented

  • tapered at both ends

  • most are smaller than earthworms (1mm or 2)

Round worms are the most common animals in the top soil

  • they act as helpful detritivores (break down and recycle dead bodies)

  • many others are parasitic

    • Ex: pin worms live in human intestines

      • female crawls down the intestine and out, lays eggs on the skin

There are over 100,000 different species of roundworms

  • one of the most diverse phyla

  • they are pseudocoelomate

    • muscles of roundworms are simpler because the muscles only go one way

  • they only have longitudinal muscles

    • causes thrashing movement

Not all of the worms are a monophyletic group, they are different on the inside

  • roundworms go through ecdysis (periodically molt their cuticle [non cellular outer covering made of collagen])

  • nematodes are placed within a larger clade called “ecdysozoa”

Phylum 8: Echinoderms

  • sea stars (star fish)

  • sand dollars

  • sea urchins

  • sea cucumbers

First phylum of deuterostomes (second opening that forms during development is mouth)

Echinoderm means “spiny” or “bumpy” skin

  • They are fairly sedentary

    • they do move a little bit by the use of tube feet (kind of like hundreds of little legs)

      • used for slow motion gliding over rocks

  • They have radial symmetry

    • they evolved from bilateral ancestors, they’re said to have secondarily radial symmetry

    • sea star larvae are not radial

One difference between echinoderms and other radial animals is that their radial symmetry is often pentamerous (5 sided symmetry)

Echinoderms have a water vascular system

  • series of tubes that run through their body like veins, except instead of filling them with blood, they fill them with ocean water

    • tube feet are part of the water vascular system

      • they are hydraulically powered

Tube feet are also used for feeding

  • sea stars often eat bivalves

  • sea urchins are herbivores (eat algae)

  • sand dollars are scavengers, both them and cucumbers are detritivores

Echinoderms have an internal skeleton (endoskeleton)

  • just underneath the skin