Anatomy of Animals

Echinoderms

The echinoderms include only marine animals -- starfishes, sea urchins, sea cucumbers, and sand dollars (see Figure 17). The starfish will be studied as the representative for this group. Their unique feature is the water vascular system, which is used as a means of locomotion. They also have a carbonaceous endoskeleton, whose projecting spines give the phylum its name - "spiny skin" in Greek. The echinoderms seem the most unpromising of all as potential ancestors of the vertebrates.

They are radially symmetrical, in contrast to vertebrates; they have no internal skeleton, no trace of any of the three major chordate characters of notochord, nerve cord, or gill slits, and they have many peculiar and complicated organs of their own. But the embryology sheds an unexpected gleam of light. The early embryo of the echinoderm is a tiny creature, which floats freely in the sea water. Unlike the adult, the larva is bilaterally symmetrical, suggesting that the radial symmetry of the starfish is a secondary affair, assumed when the ancestors of these forms look up a sedentary existence. Then, too, the type of development of certain of the body cavities is identical with that found in the embryos of some primitive vertebrates. It is believed that the bilateral larva developed types which retained the original symmetry, and gradually evolved into the chordates and, finally, the true vertebrates.

Figure 17 Echinoderms
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Circulatory - Coelomic fluid, circulated by ciliary action, performs many of the normal functions of a circulatory system.
Digestive - Starfishs feed on mollusks. When a starfish attacks a clam, it arches its body over the shell, and by the concerted action of the tube feet, forces the clam to open. Then it everts a portion of its stomach to digest the contents of the clam. The mouth of a starfish opens into a narrow esophagus, which in turn leads to an expanded stomach. The stomach has two portions: the saclike cardiac, which can be everted as described, and the narrower pyloric, which is connected to a short intestine. The anus opens on the aboral or upper side of the animal.
Exocrine - Each of the five arms contains a well-developed coelom, a pair of large digestive glands that secrete powerful enzymes into the pyloric portion of the stomach, and gonads.
Excretory - Starfish has no excretory organs.
Immune - Sea urchins are long lived, normally healthy animals that display remarkable abilities to heal wounds and combat major infections. From an external point of view, their immune systems obviously work very well. Thus, their cellular defense systems are extremely sensitive, and they respond rapidly to minor perturbations, all without any specific adaptive capabilities. These systems probably function through the transduction of signals conveying information on injury and infection, just as do the equivalent systems that underlie and back up the human immune systems, and that provide the initial series of defenses against pathogenic invasions.
Musculo-skeletal - The water vascular system is purely for locomotion. Water enters this system through a structure on the aboral side called the sieve plate, or madreporite. From there, it passes through a short canal called the stone canal, to a ring canal, which surrounds the mouth. From the ring canal, five radial canals extend into the arms. From the radial canals, many lateral canals extend into the tube feet. One lateral canal goes to each tube foot, where it ends in the ampulla. When the ampulla contracts, the water is forced into the tube foot, expanding it and giving it suction. By alternating the expansion and the contraction of the tube feet, the starfish moves along slowly. Typically, echinoderms have an endoskeleton (internal skeleton) consisting of hard calcite ossicles embedded in the body wall and often bearing protruding spines or tubercles.
Nervous and Sensory - The nervous system consists of a central nerve ring that supplies radial nerves to each arm. A light-sensitive eyespot is at the tip of each arm.

Reproductive - The starfish usually becomes sexually mature at about 12-14 months. Mating mainly takes place between May and June (water temp. about 8 °C) when the whole population are at the same depth and like an epidemic. The lifecycle of most starfishes starts by shedding their eggs and sperm freely into the water, so fertilization is externally. The very small chance of fertilization is compensated by the enormous amounts of eggs and sperm cells. A female starfish sheds in two hours several millions of eggs into the water, with a mean diameter of 0.16-0.19 mm . After fertilization, a hollow ball develops, called the blastula. The cells of the blastula possess cilia on the outside for swimming. After one day a deep groove develops, leading to the gastrula. The gastrula's of all types of echinoderms are very similar. But then differentiation starts. The common starfish develops a so-called bipinnaria larva, with ciliated bands running about the periphery. After several weeks the bipinnaria larva takes on a more

elaborate form, with longer projecting arms and after some more weeks, a brachiolaria larva is formed. The larvae have their own gut, with inside cilia to inhale and transport food particles. They feed themselves with diatoms and other organisms in the plankton. The stomach is large and round and situated at the backside. After this phase a large part of the larva degenerates and at the rear side a rudimentary formed juvenile starfish develops. The organs of the young starfish are formed anew.

Respiratory - There are skin gills, which project from the coelomic cavity, serve the function of respiratory exchange.