Laboratory Notes for BIO 1003, BIO 1016, & BIO 3001

© 11 February 2017, John H. Wahlert, Mary Jean Holland, & Krista Dobi

Domain Eukarya
Kingdom Animalia


This laboratory exercise and the next are an introduction to the Phylum Echinodermata and their nearest relatives, the Phylum Chordata. Fish, amphibians, reptiles, birds, and mammals are familiar chordates. Echinoderms and chordates share unique features of embryonic development that set them apart from the rest of the Animalia; these are described below.

[Nick and Ilir]The word echinoderm means spiny skin. These are marine organisms with five-fold radial symmetry, commonly called starfish (Class Asteroidea), sand dollars and sea urchins (Class Echinoidea), sea cucumbers (Class Holothuroidea), and sea lilies (Class Crinoidea). They are coelomate. Just under the skin, they have an endoskeleton that is made of calcium carbonate, the substance of corals and clam shells. The skeleton arises from mesodermal tissue, an origin unlike that of bivalve shells, which are secreted by the epidermal sheet of mantle tissue. Sexes are separate in most echinoderms and fertilization is external; there is a larval stage with bilateral symmetry. A unique derived feature of the group is its water-vascular system; seawater is pumped through a series of ducts to work the tube feet and the suction cups at their tips.

Class Asteroidea

The Asteroidea are sea stars or starfish. On the dorsal side you can see the central disk and radiating arms. The central disk has a small sieve plate or madreporite (off center) where external ocean water moves into the water vascular system. Small fleshy extensions among the spines are the soft, hollow skin gills for respiration; they communicate with the coelom. On the ventral side of the arms there are ambulacral grooves that are filled with the hollow, fleshy tube feet. Tube feet and skin gills are both used for gas exchange and excretion of nitrogenous waste. The mouth is in the center of the ventral side. Echinoderms move by alternating the suction and release of tube feet. Asteroids can regenerate lost or damaged arms. If a live starfish is cut, as long as a ring canal and arm remain, the animal can regenerate missing parts. An isolated arm soon dies (there is an exception, known in one genus where an arm can regenerate the rest).

[dissected starfish arm]Dissection: Using scissors, cut the tip end off of one arm (it’s best not to use the arm next to the madreporite). From the removed tip, cut along the side of the arm down its length to the center of the starfish. Now cut down the other side of the arm and then along the center of the starfish so you can remove the entire dorsal piece of skin and expose the internal structure of the arm. Next, cut around the center of the starfish, avoiding the madreporite, which is left intact. After you have removed the center disk of skin, use your forceps to gently remove the parachute-like stomach from the center. The organs fill the internal space of the coelom. With the stomach removed you can see the ring canal, which is connected to the madreporite via the stone canal. In the water vascular system, the water enters via the madreporite, moves through the stone canal to the ring canal, then out to each arm via a radial canal, and out into the tube feet. Using a probe, you can feel the bumpy ampullae (singular: amuplla) along the radial canal. Extending from the central stomach, a pair of many-chambered digestive glands runs the length of each arm. In feeding, most sea stars evert their stomachs over the food source, secrete enzymes, and absorb the digested food. The stomach is so thin walled, it can be slipped between the nearly closed valves of large prey such as clams. Gonads are located in each arm towards the center of the starfish ventral to the digestive glands.

Other Echinoderm groups include:

  • Class Echinoidea: Sea urchins and sand dollars
  • Class Ophiuroidea: Brittle stars and basket stars
  • Class Holothuroidea: Sea cucumbers
  • Class Crinoidea: Sea lilies and feather stars

Echinoderms and chordates share important derived features in the embryos. Both are called deuterostomes (other mouth): the pore leading into the forming digestive system in the gastrula becomes the anus and not the mouth as in invertebrates (protostomes = first mouth); the mouth is another opening (deutero-stome) that forms later in echinoderms and chordates. The zygote (fertilized egg) of echinoderms and chordates undergoes mitosis but the new cells are not yet committed to forming a particular tissue; each, if separated from the others can form an entire individual. This is called indeterminate cleavage and can produce identical twins. In the invertebrates cleavage is determinate and the first two cells are already committed to forming specific parts of the organism; identical twins do not occur.

Click here on starfish development if the subject is included in your lab exercise.

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Last updated 23 December 2019 (JHW)