A. Sponges -New data suggests they were alive 635 million years ago! • 1. Phylum Porifera: Sponges are sessile with porous bodies and choanocytes. Sponges (phylum Porifera) are so sedentary that they were mistaken for plants by the early Greeks. Living in freshwater and marine environments, sponges are suspension feeders. The body of a simple sponge resembles a sac perforated with holes. Unlike eumetazoa, sponges lack true tissues, groups of similar cells that form a functional unit. The sponge body does contain different cell types. Flagellated choanocytes, or collar cells, lining the spongocoel (internal water chambers) create a flow of water through the sponge with their flagella and trap food with their collars. Most sponges are sequential hermaphrodites, with each individual producing both sperm and eggs in sequence. • The zygotes develop into flagellated, swimming larvae that disperse from the parent. • When a larva finds a suitable substratum, it develops into a sessile adult. Sponges produce a variety of antibiotics and other defensive compounds.
B. Eumetazoa All animals except sponges belong to the Eumetazoa, the animals with true tissues. • 1. Phylum Cnidaria: Cnidarians have radial symmetry, a gastrovascular cavity, and cnidocytes. The cnidarians (hydras, jellies, sea anemones, and coral animals) have a relatively simple body construction. They are a diverse group with more than 10,000 living species, most of which are marine. • They exhibit a relatively simple, diploblastic body plan that arose 570 million years ago. The basic cnidarian body plan is a sac with a central digestive compartment, the gastrovascular cavity. A single opening to this cavity functions as both mouth and anus. This basic body plan has two variations: the sessile polyp and the floating medusa. The cylindrical polyps, such as hydras and sea anemones, adhere to the substratum by the aboral end and extend their tentacles, waiting for prey. • Medusas (also called jellies) are flattened, mouth-down versions of polyps that move by drifting passively and by contracting their bell-shaped bodies.
Cnidarians are carnivores that use tentacles arranged in a ring around the mouth to capture prey and push the food into the gastrovascular chamber for digestion. Batteries of cnidocytes on the tentacles defend the animal or capture prey. Cnidae called nematocysts are stinging capsules. Muscles and nerves exist in their simplest forms in cnidarians. When the animal closes its mouth, the gastrovascular cavity acts as a hydrostatic skeleton against which the contractile cells can work. Coral animals live as solitary or colonial forms and secrete a hard external skeleton of calcium carbonate. Each polyp generation builds on the skeletal remains of earlier generations to form skeletons that we call coral.
C. Bilateria The vast majority of animal species belong to the clade Bilateria, which consists of animals with bilateral symmetry and triploblastic development. Most bilaterians are also coelomates.
1. Phylum Platyhelminthes: Flatworms are acoelomates with gastrovascular cavities. Flatworms live in marine, freshwater, and damp terrestrial habitats. They also include many parasitic species, such as the flukes and tapeworms. Flatworms have thin bodies, ranging in size from nearly microscopic to tapeworms more than 20 m long. Flatworms and other bilaterians are triploblastic, with a middle embryonic tissue layer, a mesoderm, which contributes to more complex organs and organ systems and to true muscle tissue. Like cnidarians, flatworms have a gastrovascular cavity with only one opening (and tapeworms lack a digestive system entirely and absorb nutrients across their body surface). The flat shape of a flatworm places all cells close to the surrounding water, enabling gas exchange and the elimination of nitrogenous wastes (ammonia) by diffusion across the body surface.
2. Phylum Rotifera: Rotifers are pseudocoelomates with jaws, crowns of cilia, and complete digestive tracts. Rotifers are tiny animals (5 µm to 2 mm), most of which live in freshwater. Rotifers are smaller than many protists but are truly multicellular, with specialized organ systems. Rotifers have an alimentary canal, a digestive tract with a separate mouth and anus. Internal organs lie in the pseudocoelom, a body cavity that is not completely lined with mesoderm. The fluid in the pseudocoelom serves as a hydrostatic skeleton. The word rotifer, “wheel-bearer,” refers to the crown of cilia that draws a vortex of water into the mouth.
Some rotifers exist only as females that produce more females from unfertilized eggs, a type of parthenogenesis. The vast majority of animals and plants reproduce sexually at least some of the time, and sexual reproduction has certain advantages over asexual reproduction. For example, species that reproduce asexually tend to accumulate harmful mutations in their genomes faster than sexually reproducing species. As a result, asexual species experience higher rates of extinction and lower rates of speciation.
3. The lophophorate phyla: ectoprocts, phoronids, and brachiopods are coelomates with ciliated tentacles around their mouths. Bilaterians in three phyla—Ectoprocta, Phoronida, and Brachiopoda—are traditionally called lophophorate animals because they all have a lophophore. In contrast to flatworms, which lack a body cavity, and rotifers, which have a pseudocoelom, lophophorates have true coeloms completely lined with mesoderm. Ectoprocts are colonial animals that superficially resemble plants. Phoronids are tube-dwelling marine worms ranging from 1 mm to 50 cm in length. Brachiopods, or lampshells, superficially resemble clams and other bivalve molluscs.
4. Phylum Nemertea: Proboscis worms are named for their prey-capturing apparatus. The members of the Phylum Nemertea, proboscis worms or ribbon worms, have bodies much like those of flatworms. However, they have a small fluid-filled sac that may be a reduced version of a true coelom. Nemerteans and flatworms have similar excretory, sensory, and nervous systems. However, nemerteans have an alimentary canal and a closed circulatory system in which the blood is contained in vessels.
5. Phylum Mollusca: Molluscs have a muscular foot, a visceral mass, and a mantle. The phylum Mollusca includes many diverse forms, including snails and slugs, oysters and clams, and octopuses and squids. Slugs, squids, and octopuses have reduced or lost their shells completely during their evolution. Despite their apparent differences, all molluscs have a similar body plan with a muscular foot (typically for locomotion), a visceral mass with most of the internal organs, and a mantle. The mantle, which secretes the shell, drapes over the visceral mass and creates a water-filled chamber, the mantle cavity, with gills, anus, and excretory pores. Many molluscs feed by using a straplike rasping organ, a radula, to scrape up food. Most molluscs have separate sexes, with gonads located in the visceral mass. However, many snails are hermaphrodites. The basic molluscan body plan has evolved in various ways in the eight classes of the phylum. The four most prominent are the Polyplacophora (chitons), Gastropoda (snails and slugs), Bivalvia (clams, oysters, and other bivalves), and Cephalopoda (squids, octopuses, cuttlefish, and chambered nautiluses).
6. Phylum Annelida: Annelids are segmented worms. All annelids (“little rings”) have segmented bodies. The phylum Annelida is divided into three classes: Oligochaeta (earthworms), Polychaeta (polychaetes), and Hirudinea (leeches). Earthworms eat their way through soil, extracting nutrients as the soil passes through the alimentary canal. Earthworms are cross-fertilizing hermaphrodites. Two earthworms exchange sperm and then separate. Many leeches feed on other invertebrates, but some blood-sucking parasites feed by attaching temporarily to other animals, including humans.
7. Phylum Nematoda: Roundworms are nonsegmented pseudocoelomates covered by tough cuticles. Roundworms are found in most aquatic habitats, wet soil, moist tissues of plants, and the body fluids and tissues of animals. They have an alimentary tract and use the fluid in their pseudocoelom to transport nutrients since they lack a circulatory system. Nematodes usually reproduce sexually. The soil nematode, Caenorhabditis elegans, has become a model organism in developmental biology. Trichinella spiralis causes trichinosis when the nematode worms encyst in a variety of human organs, including skeletal muscle. They are acquired by eating undercooked meat that has juvenile worms encysted in the muscle tissue.
8. Arthropods are segmented coelomates with exoskeletons and jointed appendages. The world arthropod population has been estimated at a billion billion (1018) individuals. Two out of every three known species are arthropods. On the criteria of species diversity, distribution, and sheer numbers, arthropods must be regarded as the most successful animal phylum. The diversity and success of arthropods are largely due to three features: body segmentation, a hard exoskeleton, and jointed appendages. The body of an arthropod is completely covered by the cuticle, an exoskeleton constructed from layers of protein and chitin. In 2004, an amateur fossil hunter found a 428-million-year-old fossil of a millipede. Fossilized arthropod tracks date from 450 million years ago. Arthropods have an open circulatory system in which hemolymph fluid is propelled by a heart through short arteries into sinuses (the hemocoel) surrounding tissues and organs. • Open circulatory systems evolved convergently in arthropods and molluscs.
Most aquatic species have gills with thin, feathery extensions that have an extensive surface area in contact with water. For example, insects have tracheal systems, branched air ducts leading into the interior from pores in the cuticle. Evidence shows that arthropods diverged early in their history into four main evolutionary lineages: cheliceriformes (sea spiders, horseshoe crabs, scorpions, ticks, spiders), myriapods (centipedes and millipedes), hexapods (insects and their wingless, six-legged relatives), and crustaceans (crabs, lobsters, shrimps, barnacles, and many others). In most spiders, gas exchange is carried out by book lungs. The internal anatomy of an insect includes several complex organ systems. In the complete digestive system, there are regionally specialized organs with discrete functions.
Metabolic wastes are removed from the hemolymph by Malpighian tubules, outpockets of the digestive tract. Respiration is accomplished by a branched, chitin-lined tracheal system that carries O2 from the spiracles directly to the cells. The insect nervous system consists of a pair of ventral nerve cords with several segmental ganglia. The two cords meet in the head, where the ganglia from several anterior segments are fused into a cerebral ganglion (brain). This structure is close to the antennae, eyes, and other sense organs concentrated on the head. Metamorphosis is central to insect development. In incomplete metamorphosis (seen in grasshoppers and some other orders), the young resemble adults but are smaller and have different body proportions. In complete metamorphosis, larval stages specialized for eating and growing change morphology completely during the pupal stage and emerge as adults. Reproduction in insects is usually sexual, with separate male and female individuals.
D. Deuterostomia At first glance, sea stars and other echinoderms would seem to have little in common with the phylum Chordata, which includes the vertebrates. However, these animals share the deuterostome characteristics of radial cleavage, type of development of the coelom from the archenteron, and formation of the anus from the blastopore. • 1. Phylum Echinodermata: Echinoderms have a water vascular system and secondary radial symmetry. Sea stars and most other echinoderms are sessile or slow-moving marine animals. Sexual reproduction in echinoderms usually involves the release of gametes by separate males and females into the seawater. However, the radial anatomy of adult echinoderms is a secondary adaptation, as echinoderm larvae have bilateral symmetry. Living echinoderms are divided into six classes: Asteroidea (sea stars), Ophiuroidea (brittle stars), Echinoidea (sea urchins and sand dollars), Crinoidea (sea lilies and feather stars), Holothuroidea (sea cucumbers), and Concentricycloidea (sea daisies). Sea urchins and sand dollars have no arms, but they do have five rows of tube feet that are used for locomotion. Sea cucumbers do not look much like other echinoderms. They lack spines, the endoskeleton is much reduced, and the oral-aboral axis is elongated. However, they do have five rows of tube feet, like other echinoderms, and other shared features.
2. Phylum Chordata: The chordates include two invertebrate subphyla and all vertebrates. The phylum to which we belong consists of two subphyla of invertebrate animals plus the hagfishes and vertebrates.