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Deuterostome Animals. 34. Key Concepts. Echinoderms are radially symmetric as adults and have an endoskeleton and water vascular system. They are among the most important predators and herbivores in marine environments.
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Key Concepts • Echinoderms are radially symmetric as adults and have an endoskeleton and water vascular system. They are among the most important predators and herbivores in marine environments. • All vertebrates have a skull and an extensive endoskeleton made of cartilage or bone; their diversification was driven in part by the evolution of the jaw and limbs. Vertebrates are the most important large-bodied predators and herbivores in marine and terrestrial environments.
Key Concepts • Humans are a tiny twig on the tree of life. Chimpanzees and humans diverged from a common ancestor that lived in Africa 6–7 mya. Since then, at least 14 humanlike species have existed.
Introduction • The deuterostomes include the largest-bodied and some of the most morphologically complex of all animals. • Deuterostomes contain four phyla: • Echinodermata—includes sea stars and sea urchins. • Hemichordata—burrowing, deposit- or suspension-feeding acorn worms. • Xenoturbellida—two recently discovered wormlike species. • Chordata—includes the vertebrates (animals with backbones). • Deuterostomes all share important features of embryonic development, but have widely variable morphology and behavior.
What Is an Echinoderm? • Echinoderms(“spiny-skins”) are named for the spines or spikes observed in many species. • All echinoderms are marine animals. • About 7000 species of echinoderm have been cataloged thus far. • This phylum is defined by several adaptations.
The Echinoderm Body Plan • Larvae are bilaterally symmetric, as are other deuterostomes • However, adults have pentaradial symmetry (five-sided radial symmetry). • Echinoderms have an endoskeleton—a hard protective and supportive structure inside the body. • Echinoderms are also defined by their watervascularsystem, a series of branching, fluid-filled tubes and chambers that forms a hydrostatic skeleton.
The Echinoderm Body Plan • An important part of the water vascular system is tube feet, which are elongated, fluid-filled structures. • Podia are sections of the tube feet that project outside the body and are involved in motion along a substrate. • Radial symmetry in adults, an endoskeleton of calcium carbonate, and the water vascular system are all synapomorphies—traits that identify echinoderms as a monophyletic group.
How Do Echinoderms Feed? • Echinoderms utilize most methods of feeding. • Podia play a key role in obtaining food: • Predatory species use podia to pry apart bivalve shells. They then extrude their stomach through the opening and secrete digestive enzymes and absorb the resulting molecules. • Suspension feeders use podia to flick food to the cilia, which sweep the food into the mouth. • Deposit feeders use podia to secrete mucus which sops up the food, and then to move the food into the mouth.
Key Lineages: Echinodermata • To summarize their characteristics, echinoderms have bilaterally symmetric larvae but are radially symmetric adults. All have a water vascular system with tube feet, and all produce an internal endoskeleton. • There are five major lineages of echinoderms living today: • Crinoidea—feather stars and sea lilies. • Asteroidea—sea stars. • Ophiuroidea—brittle stars and basket stars. • Echinoidea—sea urchins and sand dollars. • Holothuroidea—sea cucumbers.
Crinoidea, Ophiuroidea, and Holothuroidea • Most feather stars and sea lilies are sessile suspension feeders. • Brittle stars and basket stars have five or more long arms that radiate out from a small central disk. • They use their arms to suspension feed, deposit feed by sopping up material with mucus, or capture small prey animals. • Sea cucumbers are sausage-shaped animals. • They suspension feed or deposit feed using modified tube feet called tentacles that are arranged in a whorl around their mouths.
Echinodermata: Asteroidea • The 1700 known species of sea stars are found along the coasts of all the world’s oceans. • They have five or more long arms radiating from a central region that contains the mouth, stomach, and anus. • Sea stars are predators or scavengers, and crawl along a substrate using their tube feet. • Sexes are separate, and sexual reproduction predominates. • Reproductive organs are located in one or more of the arms. • Some species are capable of asexual regeneration.
Echinodermata: Echinoidea • Most of the 800 species of echinoids living today are sea urchins, which have globe-shaped bodies and long spines and crawl along substrates, or sand dollars, which are flattened and disk-shaped, have short spines, and burrow in soft sediments. • Most sea urchins are herbivores, while sand dollars are suspension feeders. • Sea urchins crawl, and sand dollars burrow, using their spines. • Sexual reproduction predominates in echinoids; sexes are separate.
What Is a Chordate? • The phylum Chordata is defined by the presence of four morphological features: • Openings into the throat called pharyngeal gill slits. • A dorsal hollow nerve cord that runs the length of the body, comprised of projections from neurons. • A supportive but flexible rod, called the notochord, that runs the length of the body. • A muscular post-anal tail.
Three “Subphyla” • The phylum Chordata is made up of three major lineages: • Cephalochordates. • Urochordates. • Vertebrates. • All four defining characteristics of chordates are found in these species at some stage in their life cycle.
Chordate Body Plans • Cephalochordates (lancelets or amphioxus) are small, mobile suspension feeders that resemble fish. Their notochord functions as an endoskeleton in adults. • Urochordates (tunicates or sea squirts) have pharyngeal gill slits in both larvae and adults, but the notochord, dorsal hollow nerve cord, and tail occur only in the larvae. • In vertebrates, the dorsal hollow nerve cord is elaborated into the spinal cord. The pharyngeal pouches present in embryos develop into gills in aquatic species, but not in terrestrial species.
Chordata: Cephalochordata (Lancelets) • About two dozen species of lancelet have been described to date, all of them found in marine sands. • While lancelets resemble fish, they actually have several characteristics that are intermediate between invertebrates and vertebrates. • For example, their notochord is retained in adults; it functions as an endoskeleton and aids in movement. • Lancelets are suspension feeders and efficient swimmers. • Lancelets are only known to reproduce sexually, via external fertilization. The sexes are separate.
Chordata: Urochordata (Tunicates) • All known species of tunicates are marine. Sea squirts live on the ocean floor and salps live in the open ocean. • The distinguishing characters of tunicates include an exoskeleton-like coat of polysaccharide called a tunic, a U-shaped gut, and two siphons. • Tunicates are suspension feeders. • Larvae swim with the aid of the notochord; adults float or are sessile. • Reproduction is both sexual and asexual. Most species produce sperm and eggs, and fertilization can be either external or internal.
What Is a Vertebrate? • The vertebrates are a monophyletic group distinguished by two synapomorphies: • A column of cartilaginous or bony structures called vertebrae, which form a column along the dorsal sides of most species, protecting the spinal cord. • The cranium (skull) is a bony, cartilaginous, or fibrous case that encloses and protects the brain.
What Is a Vertebrate? • The coordinated movements of vertebrates are possible in part because vertebrates have large brains that are divided into three distinct regions: • A forebrain, housing the sense of smell. • Part of the forebrain is elaborated into a large structure called the cerebrum. • A midbrain, associated with vision. • A hindbrain, responsible for balance and hearing. • In the jawed vertebrates, or gnathostomes, the hindbrain consists of enlarged regions called the cerebellum and medulla oblongata.
An Overview of Vertebrate Evolution • Vertebrates have been the focus of intense research for over 300 years in part because they are large and conspicuous and in part because they include the humans.
The Vertebrate Fossil Record • Fossils of both echinoderms and vertebrates are present in the Burgess Shale deposits that formed during the Cambrian explosion. • The earliest vertebrates lived in the ocean about 540 mya, had streamlined, fishlike bodies, and seem to have had endoskeletons or at least skulls made of cartilage, a strong but flexible tissue. • A cartilaginous endoskeleton is a basic vertebrate feature; only in the bony fishes and their descendants, the tetrapods, does the skeleton become composed of bone.
Key Innovations in the Vertebrate Lineage • Following the appearance of vertebrates, the fossil record documents a series of key innovations that occurred as this lineage diversified: • Bony exoskeleton. • Jaws. • Bony endoskeleton. • Limbs capable of moving on land. • Amniotic egg.
Key Innovations in the Vertebrate Lineage • The first fossils to contain bone are from about 480 mya, and contained a series of scalelike plates in the form of a protective bony exoskeleton. • The first bony fishes with jaws appear about 440 mya. Jaws opened up a new method of feeding; with jaws and teeth, vertebrates became armed and dangerous. • Unlike the dermal bone that had evolved earlier, the bony endoskeleton functioned to support movement—rapid swimming.
Key Innovations in the Vertebrate Lineage • The first animals that had limbs capable of moving on land date to about 365 mya. These were the first of the tetrapods—animals with four limbs. • The first animals with watertight eggs, called amniotic eggs, appeared about 345 mya. Amniotic eggs have membranes that help nourish and protect the embryo, allowing for larger, better-developed young. All tetrapods other than amphibians are amniotes.
Evaluating Molecular Phylogenies • A phylogenetic tree based on morphology and DNA agrees with the fossil record. • The sister groups to vertebrates lack the skull and vertebral column that define the vertebrates, and the sister groups to jawed vertebrates lack jaws and bony skeletons.
