Download Echinoderms and Chordates: Anatomy and Symmetry and more Lab Reports Biology in PDF only on Docsity!
Station 1
The Echinoderms
The echinoderms (phylum Echinodermata ) are coelomate deuterostomes. All echinoderms are marine and are much different that the phyla studied recently; as adults, they have abandoned the evolutionary advantage of being bilaterally symmetrical. The adult echinoderms exhibit pentaradial symmetry. This pentaradial symmetry develops secondarily as the larvae of echinoderms are clearly bilaterally symmetrical and metamorphose into the pentaradial adults. The pentraradial symmetry of the adults results in a reduced degree of cephalization and the nervous system is simple and consists of a ring of nerves with radial nerves branching laterally; there is no centralized brain. Generally speaking, echinoderms are slow moving organisms.
Echinoderms possess a supportive endoskeleton of calcareous plates, spines, and ossicles. Many of these calcified structures protrude through the epidermis and give echinoderms a “spiny” appearance (hence the name of the phylum, echinos = spiny).
Perhaps the most distinguishing feature of an echinoderm is the water-vascular system. The water- vascular-system is used for locomotion, foraging, gas-exchange and the removal of metabolic wastes. The water-vascular system consists of a series of tubular ducts which can be used to move seawater. The movements of the seawater create hydrostatic pressure differences on the distal ends of the flexible tube feet and create suction.
Echinoderms also have the ability to regenerate lost body parts and many echinoderms can control the physical release of body parts to avoid predation. This may be due in part to the fact that echinoderms have mutable connective tissue. They have the unique ability to control the hardening and softening of their connective tissues at a moments notice. The presence of this mutable connective tissue has made echinoderms important research subjects in understanding and possibly treating disorders like arthritis.
The Echinoderms and Chordates
Laboratory
Bilateral symmetry in sea star larvae, 40x. Lateral view left; dorsal view, right
Examples of Echinoderms
Sea urchin crawling on coral
Sand dollar. Lateral view left; endoskeleton, right
Sea cucumber with oral tentacles extended
Sea urchins Sea urchins have pronounced spines protruding from their rounded, disc-shaped bodies. They truly resemble living pincushions. The spines are moveable and are used for defense and for anchoring the urchin in a burrow or rock crevice. Sea urchins use their elongated tube feet for locomotion and move about slowly to feed on algae.
Sand dollars Sand dollars lack the elongated spines of their urchin relatives. Their flattened bodies are covered with tiny tube feet which gives them a “velvety” appearance. The movements of the tube feet help the sand dollar to burrow into the sand or silt where they feed on detritus which they filter from the water.
Sea cucumbers Sea cucumbers often lack visible elements of the calcareous endoskeleton. Their pentamerous, cylindrical bodies appear soft. Rows of tube feet are found on the ventral side of the body and provide for locomotion. The tube feet on the dorsal sides of the body are quite reduced and fewer in number. Surrounding the mouth on the anterior end are clusters of retractable oral tentacles. When the sea cucumber is feeding, the oral tentacles are extended to aid in food collection. Sea cucumbers may eviscerate (forcefully eject their organs) themselves when disturbed by a predator. These internal organs are regenerated later.
4
Station 2A
Sea Star Anatomy
Sea Stars are some of the most recognizable echinoderms. Pentaradial symmetry is evident as five arms radiate out from a central disk. The sea star’s body is compressed forming the oral side and the aboral side (note- not dorsal and ventral). The mouth and tube feet are located on the oral side while the anus is located on the aboral side. Numerous short, endoskeletal spines protrude through the epidermis on the aboral side and give the sea star a rough texture.
Each tube foot is connected to an ampulla which arise from a radial canal present in each arm. Each radial canal connects to the ring canal located in the central disk which leads to the external opening called the madreporite. Sea water is added or removed from these ducts through the madreporite located on the aboral surface.
Sea stars are dioecious but are not sexually dimorphic. During the breeding season, the gonads are evident within coelomic cavity of the arms where the arm joins the central disk.
Sea stars feed primarily on bivalves. Small bivalves may be ingested whole, but larger bivales are consumed by a unique adaptive strategy. Sea stars actively hunt for bivalves by gliding along on their numerous tube feet. Once suitable prey is located, the sea star first positions its body over the two valves and rotates the bivalve with its tube feet so that the bivalve’s ventral side is positioned near the sea star’s mouth. Using the mutable connective tissue the sea star’s body becomes very rigid to support the contractions of the body wall muscles. The tube feet anchor the sea star to its prey and the muscle contractions open the bivalve’s shell slightly.
The sea star’s stomach is divided into two regions; the pyloric region located near the anus and the cardiac portion positioned near the mouth. After the bivalve’s shell is opened slightly, the cardiac portion of the stomach is everted through the mouth and into the bivalve. Digestive juices from large digestive glands found in each arm flow through the pyloric stomach, cardiac stomach and into the bivalve; the bivalve is digested within its own shell! Digested nutrients are absorbed in the pyloric stomach and waste is eliminated through the anus.
Use the models and laminated sheets to label the illustration below.
Sea star, aboral cut-a-way view. Label arm, central disk, gonad, anus, madreporite, ring canal, radial canal, ampulla, tube foot, digestive gland, pyloric stomach, cardiac stomach, spines
Review Questions
- Name the large opening to the water-vascular system found on the aboral surface of a sea star.
- Name the primary prey item for sea stars.
- Which portion of the sea star digestive system is everted through the mouth as it feeds on larger prey?
- Explain how a sea star moves in the space below.
- Explain how you could identify the sex of a sea star using a microscope slide prepared from a sample of gonad tissue.
Station 3A
The Chordates
The chordates ( phylum Chordata ) are coelomate deuterostomes with closed circulatory
systems, complete digestive systems with some degree of metamerism. Like the arthropods,
there is a tremendous amount of diversity within members of the phylum; from very simple
animals like the tunicates to very complex animals like humans.
All chordates possess at some point in their life cycle four main characteristics (the “big 4”).
1. Notochord – a slender supportive rod of cartilage which runs longitudinally through
most of the length of the animal.
2. Pharyngeal clefts (or slits) – series of pairs slits or folds in the pharynx.
3. Dorsal hollow nerve cord – portion of the central nervous system located dorsal to
the notochord and runs longitudinally through length of the animal. Unlike the
longitudinal nerves in previous animal phyla, it is hollow and filled with fluid.
4. Post-anal tail – an extension of body beyond the anus.
In many chordates, these four characteristics are only observed in the larval stages while
others retain them into adulthood.
The phylum Chordata is subdivided into three main groups. The urochordates, the
cephalochordates and the vertebrates. This week’s lab will examine the urochordates and
the cephalochordates; vertebrates will be examined in exercise 13.
Generalized chordate. Label the notochord, dorsal hollow nerve cord, pharyngeal slits, post anal tail
Examples of Chordates
The urochordates – tunicates or sea squirts
The tunicates are strange looking animals. At first glance, they superficially resemble a lower animal like a cnidarian or mollusk. However, upon closer examination their chordate heritage is evident. Only the larval stages will possess the “Big 4” characteristics. The larvae are free- swimming and use the pharyngeal clefts to filter plankton from the water column. An eyespot is located at the anterior of the dorsal hollow nerve cord and is used for photoreception. At the end of the larval stage, the tunicate larva attaches itself to a suitable substrate (such as a piece of coral) using its adhesive papillae located on the anterior end. Once attached, the tunicate will undergo metamorphosis into the adult form. The adults have highly reduced cephalization and of the “Big 4”, only retain the pharyngeal slits.
A translucent tunic , a heavy coat of tissue that surrounds the body, covers the body of the adult tunicate Usually visible through the tunic are the numerous pharyngeal slits used for filter-feeding plankton. The tunic is folded to form the incurrent and excurrent siphons. As the tunic contracts and relaxes, water is drawn into the body through the incurrent siphon and routed to the mouth where the plankton is filtered. Once filtered, the water is expelled through the excurrent siphon along with any digestive wastes eliminated from the anus.
Review Questions
- What do tunicates feed on?
- How does the larva of a tunicate differ from the adult?
- Which of the four main characteristics of chordates is found in the adult form of a tunicate?
- What structures are used by the tunicate larva to attach itself to the substrate?
Tunicate life cycle. Adult left and center (cut-a-way view); larva right (not to scale).
Review Questions
- Describe the feeding habits of a lancelet.
- Name the opening by which water enters the body of a lancelet.
- Name the opening by which water exits the lancelet’s body.
- Name the “big 4” characteristics seen in an adult lancelet.
- Describe the shape of the muscle segments seen externally in a lancelet.
- How did this group of chordates become known as the “Cephalochordates”?
