Biology 3B Laboratory Invertebrates II: Annelida, Nematoda ...

OBSERVATION OF ECHINODERMATA
CLASS ASTEROIDEA
Sea stars are found in relatively shallow waters, and range in size from less than an inch to nearly
three feet in diameter. They feed primarily on bivalves, prying the shell to open with their tube feet,
everting their stomach into the victim's body cavity, and digesting it. The larvae are known as
bipinnaria and have bilateral symmetry, whereas the adult form is star shaped with arms not sharply
marked off from the central disk. Sea stars can perform autotomy (self-amputation) of their arms.
However, if a small portion of the central disc remains attached to it, the amputated arm can then
regenerate and form a new individual (a clone).
o Examine live, preserved or dehydrated sea stars (figure 15). Identify the oral and aboral surfaces.
Radiating from the central disk are the five arms, noting their spiny texture (from which they get
the name echinoderm - spiny skin). At the tip of each arm is the eyespot. Note the calcareous
spines, dermal branchiae (skin gills - little sac-like structures on the skin) and pedicellariae
(claws - tiny pincer-like structures on some living sea stars that can aid in food capture or keep the
sea star clean of debris).
o The madreporite (a light colored, circular, slightly raised structure located on the aboral surface
near the base of two arms) is the opening, or intake, of the water vascular system. The anus is
seen as a minute opening at the center of the aboral surface. Ambulacral grooves are the deep
grooves that extend from the oral surface along the midline of each arm. The tube feet are seen
as double rows of soft tubular "feet" on each arm, lying along and just inside the ambulacral
groove. On the dehydrated specimen, the tube feet may or may not be present.
o Examine the living sea star, if available, observe the madreporite plate, eyespot, sensory
tentacles (located at the tip of each arm/ray), ambulacral groove, tube feet and pedicellariae
(if present).
Figure 15: Dorsal and ventral sea star surfaces
Biology 3B Laboratory Invertebrates II Page 14 of 17