Organismal Biology #25
DEUTEROSTOMES

ONLINE CLASSROOM VERSION

Deuterostomes are animals that share such embryological similarities as radial, indeterminate cleavage and a blastopore that becomes the tail end.
Echinoderms (starfishes, crinoids, sea urchins, and their relatives) are often radially symmetrical as adults, but their embryonic stages show similarities to the chordates.
The Chordata include animals with a notochord, a dorsal hollow nerve cord, gill slits, and many embryological similarities linking them with echinoderms. Chordates include tunicates, sea lancets, fishes, amphibians, reptiles, birds, and mammals.

 Deuterostome characteristics: Embryological similarities shared by chordates, hemichordates, and echinoderms:
  • Radial cleavage: The 8-celled stage has 2 tiers of 4 cells each, with each cell directly below or above another.
  • Indeterminate cleavage: Cells separated in early embryonic stages can develop into an entire embryo.
  • Deuterostome condition: The embryo's blastopore becomes the posterior (tail) end. (Remember that in molluscs, arthropods, and other protostome phyla, the blastopore becomes the mouth.)
Phylum Echinodermata:
Animals with a unique water-vascular system, using sea water as a circulatory fluid. Water in this system connects to the outside through a pore called the madreporite, always in the center line (the original axis of bilateral syummetry). Several embryonic similarities to chordates, including a true coelom, which develops as an enterocoel. Change of symmetry in many cases, from a bilateral larva to a radial adult, typically in a 5-fold pattern. Protective plates or shells frequently made of calcium carbonate and armed with bumps or spines. High ability to regenerate lost parts.

Sessile (attached) echinoderms (Homalozoa and Crinozoa): Echinoderms that grow attached include crinoids (sea lilies) and a variety of extinct groups (blastoids, cystoids, carpoids, etc.). Many grow on stalks attached to the bottom. Body cup-shaped, open toward the top, with a mouth in the center of the top surface. Arm-like rays, in multiples of five, grow out and upward from the margins of the mouth. Each ray has a ciliated groove (the ambulacrum) that traps food particles and brings them to the mouth.
Feeding in crinoids and other attached echinoderms is by means of ciliated tentables that secrete a sticky mucus. Tiny food particles are trapped on this mucus, and conveyed by ciliary action into the ambulacrum. Within the ambulacrum, cilia convey the mucus and trapped food into the mouth.
The earliest fossil forms were irregular and lacked symmetry, but radial symmetry developed early, generally in a 5-fold pattern. Biologists believe that echinoderm ancestors were bilaterally symmetrical and that filter-feeding (filtering small particles of food from the water) made radial symmetry selectively advantageous. Attached echinoderms flourished mainly during Paleozoic times. Today, only a few crinoids remain; other attached echinoderms are extinct.

Free-moving echinoderms (Echinozoa and Asterozoa): Mostly bottom-feeding scavengers and predators that attack other invertebrates. The mouth, on the lower surface, faces downward. Branches of the water-vascular system may form foot-like podia, used in locomotion. Each of these podia has a suction-cup extension that can hold on and a water-filled bulb (the ampulla) that controls water pressure (or suction).
  • Asterozoa: Body star-shaped, with protruding arms. Includes starfishes and brittle stars.
    • Asteroidea (sea stars = starfishes): Arms wide at base, 2-3 times as long as diameter of central disk, inhabit shallow depths.
    • Ophiuroidea (brittle stars): Arms narrow throughout, more flexible, many times as long as diameter of central disk, inhabit deeper waters.
    • Starfish feeding habits: Starfish are usually predators (occasionally scavengers); their favorite food is clams. A starfish will find a clam and wrap is arms around both sides of the clamshell and then exert suction to pull the shell open. At first, the clam resists by tightening its adductor muscles and pulling shut. But the starfish is pulling with reduced water pressure (suction) that requires no muscular effort, while the clam muscles will eventually fatigue and weaken. Once the clam's shell is open a tiny bit, the starfish turns its stomach inside out and pushes it out of its mouth into the clam shell, and the clam is digested right inside its shell! When all the soft tissue is digested and absorbed, the starfish abandons the empty shell and roams around to look for another.
      Clam fishermen hate the starfish for preying upon the clams. When they find a starfish clinging to one of the clams that they dredge up, they are often tempted to take a knife or meat cleaver and angrily cut the starfish in half before throwing it overboard. Big mistake! With high ability to regenerate, each half of the starfish can regrow its missing half, so now there are twice as many starfish as there were before!
  • Echinozoa: Body globe-shaped, with no protruding arms. Includes sea cucumbers, sea urchins, and sand dollars.
    • Echinoidea (sea urchins and sand dollars): Body generally ovoid and compact; outer covering rigid and often spiny.
    • Holothuroidea (sea cucumbers): Body more soft and cucumber-shaped.
    Deuterostomes

Chordata and Hemichordata:
Notochord: A stiff, flexible rod, forming the body axis. When muscles contract, it allows bending but prevents the body from collapsing like an accordion. In embryos, it induces the nervous system to form above it.

Gill slits: Openings from pharynx to either side, just behind mouth.

Hemichordata: Acorn worms and their relatives. All of them filter feed. Some use gill slits; others use tentacle-like feeding structures. Related to Chordata, but now usually treated as a separate phylum.
  • Pterobranchs and Graptolites: Pterobranchs may be solitary or colonial. They filter-feed using ciliated tentacles that move sticky mucus and trapped food to the mouth along a ciliated groove similar to the ambulacrum of stalked Echinoderms. Their extinct relatives, graptolites, lived in floating colonies during the first half of the Paleozoic era (roughly 530-320 million years ago). The floating colonies were distributed worldwide and are therefore useful stratigraphic indicators in geologic dating.
  • Acorn worms are free-living hemichordates that have switched from feathery tentacles to seive-like gill slits as a filter-feeding mechanism.


Phylum Chordata: Animals with a notochord, a series of gill slits, and a dorsal, hollow nerve cord developing from a neural tube. These traits may occur in larval stages, not always in adults.
Swimming motions are usually made by side-to-side undulations of the body (mostly the tail end), made possible by alternating waves of muscle contraction on the left side, then on the right side, then on the left again. Each wave passes from the head end to the tail in rhythmic fashion, controlled by the central nervous system (derived from the neural tube). The notochord (or the backbone that replaces it in vertebrate animals) allow sideways flexion but not shortening along the body axis.

Urochordata (tunicates or "sea squirts"): An actively swimming larva with well-developed notochord and nerve cord undergoes metamorphosis into a filter-feeding adult. The adult usually passes large amounts of water through a large gill basket. Imagine an adult tunicate, attached to a rocky bottom surface. Its eggs develop into a motile larva that settles on a new surface. Natural selection favors motility in this larva (permitting it to move around in search of a permanent home) and also a nervous system capable of choosing a good location. A short larval stage gives the larva only a few opportunities to find a good location, so natural selection favors a longer and longer larval stage. If the larva can feed while swimming around, its larval stage can last even longer, so natural selection favors the earlier and earlier development of the pharynx and gill basket that permit feeding. Finally, if the larva can hurry the development of its sex organs and reproduce while it can still swim around, then it can do away with the attached adult stage entirely. Thus, the larval stage, originally a brief dispersal stage, becomes the entire life span, and the attached adult stage is lost. The earliest vertebrates and the sea lancets are both filter-feeding animals that swim around for their entire life.

Cephalochordata (sea lancets or amphioxus): Small, thin animals that filter feed by passing water through many gill slits. A notochord extends the entire length of the animal, including the head. A lancet-shaped tail is used to wedge the tail end downward into sediment with wiggling motions. Once the animal is mostly buried, it sits with its mouth exposed and filter-feeds.

Vertebrata (vertebrates): Animals with a vertebral column or backbone that functionally replaces the notochord in adults, and a braincase that encloses and protects the brain. Examples: fishes, amphibians, reptiles, birds, and mammals.

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