# Lecture 13: Animal Diversity — Invertebrates II
## General Biology II — Organismal, Evolution & Ecology
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## Learning Objectives
By the end of this lecture, students will be able to:
1. Describe the characteristics of Ecdysozoa, including the significance of molting
2. Compare the biology and diversity of Nematoda and Arthropoda
3. Identify the major arthropod subphyla and their distinguishing features
4. Describe the characteristics of Echinodermata and their phylogenetic position as deuterostomes
5. Explain the ecological and medical importance of nematodes and arthropods
6. Describe the key evolutionary innovations of arthropods that contributed to their success
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## Lecture Content
### I. Ecdysozoa: The Molting Animals
The Ecdysozoa is a major clade of protostomes united by a shared characteristic: ecdysis, the periodic shedding of an external exoskeleton or cuticle to permit growth. This process is regulated by the hormone ecdysone. The clade encompasses two enormously successful phyla -- Nematoda (roundworms) and Arthropoda (insects, crustaceans, arachnids, and their relatives) -- that together account for the majority of animal species on Earth.
### II. Phylum Nematoda (Roundworms)
Nematodes are triploblastic pseudocoelomates with unsegmented, cylindrical bodies tapering at both ends. They are covered by a tough, flexible cuticle that must be molted as the worm grows. Unlike flatworms, nematodes possess a complete digestive tract with both a mouth and an anus, establishing the tube-within-a-tube body plan that characterizes most bilaterians. They lack dedicated circulatory and respiratory systems, relying on diffusion for gas and nutrient transport, and possess only longitudinal muscles, which produces their characteristic thrashing movement.
Nematodes are among the most abundant and diverse animals on the planet. They inhabit virtually every environment -- soil, freshwater, marine sediments, and the bodies of other organisms -- and a single square meter of soil can contain millions of individuals. Free-living nematodes play important roles as decomposers in soil ecosystems. The species Caenorhabditis elegans has become one of the most important model organisms in biology: its body consists of exactly 959 somatic cells (the complete cell lineage of which has been mapped), it has approximately 20,000 genes, it was the first multicellular organism to have its genome fully sequenced, and its transparent body and three-day generation time make it ideal for developmental and genetic studies.
Parasitic nematodes have an enormous medical and agricultural impact. Ascaris lumbricoides, the giant intestinal roundworm, infects approximately 800 million people worldwide. Enterobius vermicularis (pinworm) is the most common helminth infection in developed countries. Trichinella spiralis causes trichinosis, transmitted through undercooked pork, with larvae encysting in muscle tissue. Wuchereria bancrofti, a filarial worm transmitted by mosquitoes, causes elephantiasis by blocking lymphatic drainage. Necator americanus (hookworm) attaches to the intestinal wall and causes chronic anemia. In agriculture, root-knot nematodes of the genus Meloidogyne devastate crops worldwide.
### III. Phylum Arthropoda
Arthropods are the most species-rich phylum on Earth, with more than one million described species and estimates suggesting five to ten million total -- roughly 80% of all known animal species. Their extraordinary evolutionary success rests on a suite of key innovations. A hard **exoskeleton** made of chitin (reinforced with calcium carbonate in crustaceans) provides protection, structural support, and a barrier against desiccation, though it must be molted periodically to allow growth. **Jointed appendages** -- segmented legs, antennae, and mouthparts -- are remarkably versatile, having been modified by evolution for walking, swimming, flying, feeding, sensing, and reproduction. The arthropod body is divided into **segments** that are often fused into functional groups called tagmata, such as the head, thorax, and abdomen of insects. Well-developed **sensory organs** include compound eyes, antennae, and sensory hairs. An **open circulatory system** pumps hemolymph through sinuses, while gas exchange is accomplished through tracheae (in insects), book lungs (in arachnids), or gills (in aquatic forms). Insects are the only invertebrates capable of powered **flight**, an ability that evolved approximately 350 million years ago and opened vast new ecological opportunities.
A phylogenetic overview of major arthropod subphyla with representative organisms. A cladogram shows the branching relationships among four subphyla. Subphylum Chelicerata: illustrated with a spider showing chelicerae, pedipalps, 4 pairs of walking legs, and two body segments (cephalothorax and abdomen). Subphylum Myriapoda: illustrated with a centipede (one pair of legs per segment) and a millipede (two pairs per segment). Subphylum Crustacea: illustrated with a crayfish showing biramous appendages, antennae, cephalothorax with carapace, and abdomen. Subphylum Hexapoda (Insecta): illustrated with a grasshopper showing three body regions (head, thorax, abdomen), three pairs of legs, two pairs of wings, compound eyes, and antennae. Key synapomorphies are labeled at each branch point.
#### A. Subphylum Chelicerata
Chelicerates are distinguished by chelicerae -- pincer-like or fang-like mouthparts -- and the absence of antennae. Their body is divided into a cephalothorax (prosoma) and an abdomen (opisthosoma), and they bear four pairs of walking legs. The class **Arachnida** includes spiders (with venom glands, silk-producing spinnerets, and book lungs for gas exchange), scorpions, and the medically important ticks and mites, many of which are ectoparasites and vectors for diseases such as Lyme disease and Rocky Mountain spotted fever. **Merostomata** includes the horseshoe crabs -- living fossils whose blue, copper-based blood is used in pharmaceutical testing for bacterial contamination. **Pycnogonida** (sea spiders) are bizarre deep-sea arthropods.
#### B. Subphylum Myriapoda
Myriapods have elongated bodies with many segments and a single pair of antennae. **Centipedes** (Chilopoda) bear one pair of legs per segment, are predatory, and possess venomous forcipules -- modified first legs used to subdue prey. **Millipedes** (Diplopoda) bear two pairs of legs per segment, have cylindrical bodies, are detritivores that feed on decaying plant matter, and defend themselves with chemical secretions.
#### C. Subphylum Crustacea
Crustaceans are predominantly aquatic, inhabiting both marine and freshwater environments, though some (such as the terrestrial isopods, or pill bugs) have colonized land. They are the only arthropods with two pairs of antennae and characteristically possess biramous (two-branched) appendages. Many crustaceans bear a carapace, a hardened dorsal shield covering the cephalothorax, and their exoskeletons are calcified with calcium carbonate in addition to chitin. Gas exchange occurs through gills. Key groups include the **Decapoda** (crabs, lobsters, shrimp, and crayfish), **Isopoda** (pill bugs and marine isopods), **Copepoda** (tiny planktonic crustaceans that dominate the zooplankton and are critical links in marine food webs), **Cirripedia** (barnacles, which are sessile filter feeders cemented to hard surfaces), and **Branchiopoda** (brine shrimp and water fleas such as Daphnia).
#### D. Subphylum Hexapoda (Class Insecta)
Insects are the most species-rich group of organisms on Earth, with approximately one million described species. Their body plan consists of three distinct regions -- head, thorax, and abdomen -- with three pairs of legs attached to the thorax, one or two pairs of wings (the only wings among invertebrates), one pair of antennae, compound eyes, and diverse mouthparts adapted for chewing, piercing-sucking, siphoning, or sponging. Gas exchange occurs through a tracheal system -- a branching network of air-filled tubes that open to the exterior through spiracles and deliver oxygen directly to tissues. Excretion is handled by Malpighian tubules that empty waste into the gut.
Insect development follows one of two patterns. **Complete metamorphosis** (holometabolous development) involves four distinct stages -- egg, larva, pupa, and adult -- as seen in butterflies, beetles, flies, and bees. **Incomplete metamorphosis** (hemimetabolous development) proceeds through egg, nymph (which resembles a miniature adult), and adult, as seen in grasshoppers, dragonflies, and true bugs. Among the major insect orders, Coleoptera (beetles) is the most species-rich with approximately 400,000 species, followed by Lepidoptera (butterflies and moths), Hymenoptera (ants, bees, and wasps, many of which are eusocial), Diptera (flies and mosquitoes, which possess only one pair of wings with halteres as modified hindwings for balance), and Hemiptera (true bugs). Ecologically, insects serve as pollinators, decomposers, and foundational links in food webs, while some are agricultural pests and disease vectors of immense medical importance -- mosquitoes transmit malaria, dengue, Zika, and yellow fever; tsetse flies transmit sleeping sickness; fleas transmit plague; and lice transmit typhus.
### IV. Phylum Echinodermata (Echinoderms)
The echinoderms occupy a surprising position on the animal phylogenetic tree: as deuterostomes, they are more closely related to chordates (and thus to humans) than to any protostome group. They are exclusively marine and display a suite of features found nowhere else in the animal kingdom.
Adult echinoderms exhibit **pentaradial (five-part) symmetry**, though their larvae are bilaterally symmetrical, revealing their bilaterian ancestry. Their body is supported by an **endoskeleton** of calcareous ossicles (calcium carbonate plates) embedded beneath the skin, often bearing spines. Their most distinctive feature is the **water vascular system**, a unique hydraulic network. Seawater enters through the madreporite (a sieve plate on the body surface) and flows through a series of canals -- the stone canal, ring canal, and radial canals -- ultimately reaching the tube feet (podia). By contracting muscular ampullae, the animal can extend and retract these tube feet, using them for locomotion, feeding, gas exchange, and attachment. Echinoderms lack cephalization and a centralized brain, instead possessing a decentralized nerve ring with radial nerves. Most species can regenerate lost arms or even large portions of their body.
The major classes include the **Asteroidea** (sea stars, typically five-armed predators that can pry open bivalve shells by everting their stomach into the prey), **Ophiuroidea** (brittle stars, with thin, flexible arms sharply distinct from the central disk), **Echinoidea** (sea urchins and sand dollars, which lack arms and use Aristotle's lantern -- a complex jaw apparatus -- to scrape algae), **Holothuroidea** (sea cucumbers, with elongated soft bodies and the dramatic defense mechanism of evisceration), and **Crinoidea** (sea lilies and feather stars, the oldest surviving echinoderm lineage, which are suspension feeders).
A diagram of the water vascular system of a sea star. Panel A: A dorsal view of a sea star with one arm shown in cutaway to reveal internal anatomy. The madreporite on the aboral (top) surface is labeled, with arrows showing water flow into the stone canal, then to the ring canal encircling the mouth on the oral surface, then into five radial canals running down each arm. Along each radial canal, lateral canals connect to ampullae and tube feet (podia). Panel B: A magnified cross-section of one arm showing the arrangement of tube feet in two rows along a groove on the oral surface, each connected via a lateral canal to an ampulla (bulb). Arrows show how contraction of the ampulla pushes fluid into the tube foot, extending it; a sucker at the tip provides grip. Panel C: A sea star using its tube feet to open a bivalve shell, with the everted stomach labeled between the two shell valves.
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