Chapter 33
Given that I do not usually give the taxonomy lectures
(the material is addressed via projects and assignments) these notes will not
have any words removed.-Dr. G.
Ecdysozoans:
The Molting Animals
Ecdysozoans: The Molting Animals
Cuticles: Flexible, Unsegmented Exoskeletons
Arthropods and Their Relatives: Segmented External Skeletons
Crustaceans: Diverse and Abundant
Insects: Terrestrial Descendants of Marine Crustaceans
Arthropods with Two Body Regions
Themes in the Evolution of Protostomes
Introduction
An exoskeleton is a firm, nonliving covering that is difficult to penetrate and provides an animal with both protection and support.
Exoskeletons pose a huge problem: They cannot grow as the animal body inside them grows.
A solution evolved in the ancestors of the ecdysozoans.
They shed, or molt, the outgrown exoskeleton and expand and harden a new, larger one.
The presence of the exoskeleton required the evolution of new forms of movement and respiration.
Cuticles: Flexible, Unsegmented Exoskeletons
Some ecdysozoans have wormlike bodies covered by thick and flexible exoskeletons called cuticles.
The three phyla with thin cuticles include the Priapulida, Kinorhyncha, and Chaetognatha. These phyla contain very few species.
A thin cuticle allows the exchange of gases, minerals, and water across the body surface, but restricts the animal to moist habitats.
Their bodies are supported primarily by hydrostatic skeletons, not by their thin cuticles.
Cuticles: Flexible, Unsegmented Exoskeletons
The phylum Priapulida has 16 species of cylindrical, unsegmented, wormlike animals that burrow in fine marine sediments.
They capture prey with a toothed pharynx which is everted through the mouth.
Cuticles: Flexible, Unsegmented Exoskeletons
The phylum Kinorhyncha has about 150 described species.
All are less than 1 mm in length and live in marine sands or muds.
Their bodies are divided into 13 segments by a series of cuticular plates.
Cuticles: Flexible, Unsegmented Exoskeletons
The arrow worms (phylum Chaetognatha) have an uncertain phylogeny.
Recent evidence suggests that these animals may in fact belong among the deuterostomes.
Their body plan is based on a coelom that is divided into head, trunk, and tail compartments.
The arrow worms are small, and gas exchange and excretion occur by diffusion through the body surface.
Arrow worms lack a circulatory system; instead, wastes and nutrients are moved around the body in coelomic fluid that is propelled by cilia lining the coelom.
Cuticles: Flexible, Unsegmented Exoskeletons
Two phyla represent a lineage of ecdysozoans that developed tough external cuticles: Nematomorpha and Nematoda.
Cuticles: Flexible, Unsegmented Exoskeletons
The horsehair worms (phylum Nematomorpha) consist of about 320 species.
They are extremely thin and range in length from a few millimeters to up to a meter.
Most adults live among litter and algal mats near the edges of streams and ponds.
The larvae are internal parasites of terrestrial and aquatic insects and crabs.
The gut is much reduced with no mouth opening and is most likely nonfunctional.
They may feed only as larvae, absorbing nutrients from their hosts across their body wall.
Cuticles: Flexible, Unsegmented Exoskeletons
The roundworms (phylum Nematoda) have a thick, multilayered cuticle that gives their body its shape. A roundworm sheds its cuticle four times as it grows.
About 25,000 species of roundworms have been described, but the actual number of living species may be more than a million.
Roundworms exchange oxygen and nutrients with the environment through their cuticle and intestine.
Rhythmic contraction of the pharynx at the worms anterior moves materials through the gut.
Cuticles: Flexible, Unsegmented Exoskeletons
The roundworms are one of the most abundant and universally distributed animal groups.
There are predatory and parasitic species of roundworms.
Some parasitic roundworms cause serious diseases such as trichinosis.
The life cycles of many parasitic species have special stages that facilitate their transfer among hosts.
Arthropods and Their Relatives:
Segmented External Skeletons
During the Precambrian era, some wormlike ecdysozoans developed a layer of protective material called chitin, a strong, flexible, waterproof polysaccharide.
This change gave their rigid body covering both support and locomotory functions.
A rigid body does not allow wormlike movement; such animals require appendages that can be manipulated by muscles.
Appendages evolved several times late in the Precambrian era, leading to the phyla collectively called the arthropods.
Arthropods and Their Relatives:
Segmented External Skeletons
Recent molecular evidence links the onychophorans (phylum Onychophora) to arthropod lineages.
Onychophorans have a flexible cuticle that contains chitin, and use their fluid-filled body cavities as hydrostatic skeletons.
They have soft, fleshy, unjointed legs bearing claws that form from outgrowths of the body.
They are probably similar to ancestral arthropods.
Arthropods and Their Relatives:
Segmented External Skeletons
The water bears (phylum Tardigrada) also have fleshy, unjointed legs and use their fluid-filled body cavity as a hydrostatic skeleton.
Water bears are extremely small, and lack circulatory systems and gas exchange organs.
When dry conditions occur, the animal shrinks to a small barrel-shaped object that can survive for a least a decade.
Arthropods and Their Relatives:
Segmented External Skeletons
The trilobites (phylum Trilobita) were once the dominant line of arthropods, flourishing in the Cambrian and Ordovician seas before becoming extinct by the Paleozoic era.
They were heavily armored and had jointed appendages that showed the beginnings of specialization.
Arthropods and Their Relatives:
Segmented External Skeletons
Arthropod appendages have evolved an amazing variety of forms and functions, including walking and swimming, gas exchange, food capture, copulation, and sensory perception.
The similarities in segmentation patterns among arthropods arise from the actions of common developmental genes.
The arthropod body plan is characterized by a rigid exoskeleton with jointed appendages.
Arthropods and Their Relatives:
Segmented External Skeletons
This exoskeleton made aquatic arthropods excellent candidates for invading terrestrial environments, which they did several times.
There are four major arthropod phyla living today: the crustaceans, the hexapods, the myriapods, and the chelicerates.
The arthropods are the dominant animals on Earth, with about 1.5 million described species and an estimated 1018 individuals.
Crustaceans: Diverse and Abundant
The crustaceans (phylum Crustacea) are the dominant marine arthropods.
Members of the crustacean class Copepoda may be the most abundant of all animals.
Nearly all members have a body divided into three regions:
A head whose segments are fused together and which bears five pairs of appendages
A thorax with multiple segments that usually bear one pair of appendages each
An abdomen with multiple segments that also usually bear one pair of appendages each
Crustaceans: Diverse and Abundant
Many species have a fold of exoskeleton, called the carapace, that extends dorsally and laterally back from the head to protect other segments.
In most species, fertilized eggs are attached outside of the females body.
Some species release the young as larvae, while others are released as juveniles similar in form to adults.
The typical crustacean larva is called a nauplius.
The crustacean lineage may have been ancestral to all present-day arthropods.
Insects: Terrestrial Descendants
of Marine Crustaceans
Arthropods made the leap from aquatic to terrestrial environments during the Devonian, more than 400 million years ago.
The insects (phylum Hexapoda) are the most prominent of the several groups that successfully colonized the terrestrial habitat.
Insects are found in almost every freshwater and terrestrial environment.
About 1.4 million species of insects have been described.
These are believed to be only a small fraction of the total number living on Earth.
Insects: Terrestrial Descendants
of Marine Crustaceans
Insects have three basic body parts:
A head with a single pair of antennae attached
A thorax with three pairs of legs attached
An abdomen
To exchange gases with the environment, insects use air sacs and tubular channels called tracheae.
Insects: Terrestrial Descendants
of Marine Crustaceans
The class Apterygota consists of wingless insects that are probably most similar in form the ancestors of all insects.
Apterygota include firebrats, silverfish, and collembolans.
Members of the Apterygota have a simple life cycle, with hatchlings that look like small adults.
Insects: Terrestrial Descendants
of Marine Crustaceans
The winged insects are members of the class Pterygota and their life cycle is more complex.
Hatchlings undergo substantial changes at each molt in the process of growing larger.
The immature stages of insects between molts are called instars.
Metamorphosis is the substantial change that occurs between one developmental stage and another.
Insects: Terrestrial Descendants
of Marine Crustaceans
Insects that exhibit gradual changes between their instars are said to undergo incomplete metamorphosis.
Insects that exhibit dramatic changes between their instars are said to undergo complete metamorphosis.
The process in which a caterpillar changes into a butterfly is an example of complete metamorphosis.
In this process, a wormlike larva transforms itself during a specialized phase called the pupa.
Insects: Terrestrial Descendants
of Marine Crustaceans
The winged insects are divided into about 29 orders, with three major recognizable lineages:
Winged insects that cannot fold their wings back against the body
Winged insects that can fold their wings and that undergo incomplete metamorphosis
Winged insects that can fold their wings and that undergo complete metamorphosis
Insects: Terrestrial Descendants
of Marine Crustaceans
The orders Odonata (dragonflies and damselflies) and Ephemeroptera (mayflies) are the only surviving groups of the first lineage.
Members of these two orders have aquatic larvae that metamorphose into adults.
Dragonflies and damselflies are active predators as adults.
Adult mayflies lack functional digestive tracts and only live long enough to mate and lay eggs.
Insects: Terrestrial Descendants
of Marine Crustaceans
Members of the second lineage have hatchlings that are similar in form to adults and acquire adult organ systems gradually through their instars.
Orthoptera (grasshoppers, crickets, roaches, mantids)
Isoptera (termites)
Plecoptera (stone flies)
Dermaptera (earwigs)
Thysanoptera (thrips)
Hemiptera (true bugs)
Homoptera (aphids, cicadas, leafhoppers)
Insects: Terrestrial Descendants
of Marine Crustaceans
Insects in the third lineage have life stages specialized for living in different environments and using different food sources.
Many species have larval stages that are specialized for feeding and growing, and adult stages specialized for reproduction and dispersal.
Neuroptera (lacewings)
Coleoptera (beetles)
Trichoptera (caddisflies)
Lepidoptera (butterflies and moths)
Diptera (flies)
Hymenoptera (sawflies, bees, wasps, ants)
Insects: Terrestrial Descendants
of Marine Crustaceans
Winged insects that do not belong in the above three lineages include two parasitic orders: the Phthiraptera (lice) and Siphonaptera (fleas).
Insects in these orders are descended from flying ancestors but have lost the ability to fly.
Molecular data suggest that the lineage leading to insects separated from the lineage leading to modern crustaceans about 450 million years ago.
The remarkable success of insects is due in part to their wings, which arose only once during insect evolution.
Arthropods with Two Body Regions
Whereas insects and most crustaceans have tripartite body plans, two arthropod lineages evolved a body plan with two regions, a head and a trunk.
The phylum Myriapoda includes the centipedes and millipedes.
Centipedes have one pair of legs per trunk segment; millipedes have two pairs of legs per segment.
More than 3,000 centipede species and 10,000 millipede species have been described.
Arthropods with Two Body Regions
There are 63,000 described species in the phylum Chelicerata, collectively referred to as the chelicerates.
Their bodies are divided into two major regions
The anterior region bears two pairs of appendages, modified to form mouthparts.
Many chelicerates also have four pairs of walking legs.
Arthropods with Two Body Regions
Chelicerate species are usually placed in three classes: Pycnogonida, Merostomata, and Arachnida.
Members of the class Pycnogonida are also called the sea spiders or pycnogonids.
The horseshoe crabs comprise the single order within the class Merostomata.
Arthropods with Two Body Regions
The spiders, scorpions, mites, and ticks are the most species-rich and abundant members of the class Arachnida.
The 30,000 described species of mites and ticks live in a variety of environments, including as parasites of plants, vertebrates, and invertebrates.
They are vectors for wheat and rye mosaic viruses, and they cause mange in domestic animals and skin irritation in humans.
Arthropods with Two Body Regions
Spiders are important terrestrial predators.
Spiders produce protein threads from which they spin webs to snare prey and which they use to construct homes, safety lines for climbing, for mating structures, protection of developing young, and for dispersal.
Themes in the Evolution of Protostomes
Most protostome evolution took place in the oceans.
Some protostome lineages gradually evolved the ability to change their shape in complex ways and to move with greater speed as a result of subdivided body cavities that allowed better control of movement.
Many different lineages of animals evolved feeding structures designed to extract small prey from the water, most likely because this was the only type of food available during much of animal evolution.
Themes in the Evolution of Protostomes
Flowing water brings food with it. Thus, the sessile lifestyle evolved repeatedly during lophotrochozoan and ecdysozoan evolution.
The competition for space is a consequence of the sessile lifestyle, and many sessile animals have evolved mechanisms for overgrowing one another and engaging in toxic warfare when they come into contact.
Members of sessile colonies that are directly connected can share resources, an ability that enables some individuals to specialize for particular functions.
Themes in the Evolution of Protostomes
External body coverings probably evolved as a result of the selective pressures resulting from predation.
Body coverings evolved independently in many lophotrochozoan and ecdysozoan lineages.
The evolution of animals is, in one sense, a complex arms race among predators and prey, with each side developing structures and behaviors to deal with the selective pressures applied by the other.
Although greater complexity has evolved in many animal lineages, several lineages that have remained simple have been very successful.
Video 33.1 The nematode Caenorhabditis elegans
Video 33.2 The crustacean Daphnia pulex
Video 33.3 The molting of the fruit fly larva Drosophila melanogaster
Video 33.4 The metamorphosis from pupa to adult in the fruit fly Drosophila melanogaster
Video 33.5 The metamorphosis of a lepidopteran from larva to adult