THE ECHINODERMS
Sea stars (starfishes), sea urchins and sand dollars, brittle stars, sea cucumbers, and sea lilies are all members of Phylum Echinoderrnata. This large phylum of marine organisms are well-preserved in the fossil record and represented today by five distinct classes. Many adult echinoderins are easily recognized by their 5-part (pentamerous) radial symmetry. Be aware that their symmetry does not ally the echinoderms with the cnidarians! In cnidarians, radial symmetry is present in each stage of the life cycle. This is called primary radial symmetry. In comparison, the larva of echinoderms are actively swinuning, bilaterally symmetrical animals. The radial symmetry of the adult echinodenns is thought to have been added later in the evolutionary history of this group, as a modification for a slow moving fife on the ocean floor. This type of specialization is called secondary radial symmmetry. Adult symmetry alone, then, is not a good character to use in determining phylogeny of the echinoderms. Interestingly, there is strong embryological evidence that suggests a close evolutionary relationship between the echinoderms and the chordates. Both of these groups, unlike any of the others we have studied, are deuterostome coelomates. In deuterostomes:
Class Asteroidea, the sea stars, are typically 5-armed, but some species may have as many as 50 arms. The endoskeleton, just below the epidennis, is made of separate plates which allows the sea star considerable flexibflity. The tube feet form rows along the ventral surface of each arm. Most are predaceous.
Class Echinoidea includes the sea urchins, heart urchins and sand dollars. In this group, the calcareous plates are fused, forming a rigid skeleton (test). The test is globular in urchins and flattened in sand dollars and in both, the body is covered with movable spines. The tube feet form 5 symmetrically placed rows along the body. The Echinoidea are p y herbiverous.
The soft-bodied sea cucumbers belong to Class Holothuroidea. Their skeleton is composed of many tiny plates embedded in the body wall. The body is elongated and lacks spines and arms. Sea cucumbers do have 5 or more longitudinal rows of tube feet and the tentacles surrounding the mouth are modified tube feet. They are filter feeders, trapping plankton with the mucus covered tentacles, or grazers, collecting food materials from the bottom sediments.
Class Ophiuroidea (brittle stars and basket stars) are similar to sea stars, but the arms are very slender and distinct from the central disk. The tube feet, lacking suckers, have a sensory function and are not used for locomotion.
The stalked sea lilies and the trees ng feather stars
belong to Class Crinoidea. They typically have 5 branching
arms with tube feet that are used for filter feeding and, unlike other
echinoderms, the mouth is on the upper surface.
THE CHORDATES
The chordate link with echinoderms has already been noted. In addition to being deuterostome coelomates, all chordates, at some point in their cycle, possess 5 basic structures:
Subphylum Cephalochordata, the lancelets,though
diminutive, are somewhat eel-like in appearance and have well-developed
chordate-characteristics. The immature lancelets are smaller than,
but otherwise identical to the adult. Both of these groups are on
display in lab this week.
Next week we will discuss Subphylum Vertebrata.
In this group, the nerve cord enlarges anteriorly to form a brain and
vertebrae surround or replace the notocord. Take special note that
the taxonomy of the vertebrates has been revised from seven classes to
twelve clades (groups) based on phylogenetic systematics or cladistics.
As you know from your work this quarter, cladistics is an approach
to classification that relfects recent origin from a common ancestor. The
grouops are defined and their relatedness is determined by the number of
shared derived characteristics. Your textbook uses the traditional classical
evolutionary taxonomy (seven vertebrae classes), but the cladistic approach
is widely accepted because it is more rigorous and less arbitrary in defining
group relationships. In this course we have time for only a brief overview
of vertebrate taxonomy. In the Bios 303 course, Comparative Vertebrate
Anatomy, you will have the opportunity to further explore this topic.
KINGDOM ANIMALIA: Phylum Echinodermata
Class Asteroidea (sea stars)