Chapter Outline
INTRODUCTION
Development of Jointed Appendages
Success of the Arthropods
Economic Importance
GENERAL CHARACTERISTICS OF ARTHROPODS
Possess Characteristic Jointed Appendages
Other External Characteristics
Bodies are segmented
Have rigid external skeleton: exoskeleton
Members are generally small in size
MAJOR GROUPS OF ARTHROPODS
Primarily Categorized by Modifications of Anterior Appendages fig 41.2
Mandibulates have modified mandibles
Chelicerates lack mandibles and instead possess chelicerae
Secondarily Categorized Anatomy of All Appendages
Aquatic mandibulates
Terrestrial mandibulates
EXTERNAL FEATURES
Exoskeleton
All arthropods covered by hardened skeleton or cuticle
Functions of exoskeleton
Growth requires periodic ecdysis
Compound Eye fig 41.5
Composed of many ommatidia: independent visual units
Apposition eyes
Superposition eyes
Ocelli are simple eyes with single lenses fig 41.5b
INTERNAL FEATURES fig 41.6
Reduction of Coelom Through the Course of Evolution
Circulatory System fig 41.6b
Respiratory System
Excretory System
Nervous System fig 41.6b
SUBPHYLUM CHELICERATA: THE CHELICERATES
Class Arachnida: The Arachnids
General characteristics
Order Scorpiones: the scorpions fig 41.1;8
Order Araneae: the spiders fig 41.9
Order Acari: the mites fig 41.12
Order Opiliones: the daddy longlegs fig 41.14
Class Merostomata: The Horseshoe Crabs fig 41.15
Class Pycnogonida: The Sea Spiders fig 41.17
SUBPHYLUM CRUSTACEA: CRUSTACEANS
Arrangement and Nature of Appendages
General Biology
Diversity of Crustaceans
Decapods ("ten-footed") include lobsters, shrimp and crabs fig 41.18
Terrestrial crustaceans
Planktonic crustaceans
Barnacles, order Cirripedia fig 41.22,24.11
SUBPHYLUM UNIRAMIA
General Characteristics
Classes Diplopoda and Chilopoda: The Millipedes and Centipedes fig 41.23
Class Insecta: The Insects
Largest group of organisms on earth
External features fig 41.6
Orders classified by structure of mouthparts, feeding habits
Thorax consists of three fused segments (tagmata)
Structure of insect wings
Internal organization fig 41.6
Sense receptors
Insect communication
Life histories
DEUTEROSTOMES
PHYLUM ECHINODERMATA: THE ECHINODERMS
General Ecology of Echinoderms
Basic Features of Echinoderms
Possess a water vascular system
Feeding strategies of echinoderms
Sexual reproduction and fertilization is external
Diversity of Echinoderms
Class Crinoidea: the sea lilies and feather stars fig 41.33c,36
Class Asteroidea: the sea stars
Class Ophiuroidea: the brittle stars fig 41.33d
Class Echinoidea: the sea urchins and sand dollars fig 41.33e,f
Class Holothuroidea: the sea cucumbers fig 41.33b
Answers to Review Questions
1. The two groups are (1) chelicerates, with mouthparts formed from the most anterior pair of appendages, the second pair of appendages are set pincerlike or feelerlike, and the remaining pairs are legs (examples: spiders, mites, scorpions), and (2) mandibulates, with the most anterior appendages, sensory antennae, and jaws formed by the second or third set of appendages (examples: crustaceans, insects, millipedes, and centipedes).
2. The subgroup with fundamentally biramous appendages is the crustaceans, specifically aquatic mandibulates. The subgroup with uniramous appendages is the terrestrial mandibulates. Biramous appendages are double branched, while uniramous are single branched.
3. The exoskeleton provides a surface for the muscles to work against, protects against predators and injury, and reduces water loss. During ecdysis a new exoskeleton is grown under the old one, with fluid separating them; then the outer skeleton is shed and the body is expanded by blood circulation and air intake, and the soft new skeleton hardens as it is exposed to air or water. The control for this process involves hormones.
4. The arthropod circulatory system is an open one, with a dorsal, longitudinal heart. The blood flows from anterior end to the head, through internal body spaces toward the posterior end and back in the dorsal vessel. This one-way flow is maintained by valves in the posterior region of the heart.
5. The respiratory system transports oxygen throughout the arthropod's body. This is different from vertebrates in that vertebrates use the circulatory system. The resultant impact is a limitation of the size of the organism, with each cell necessarily within diffusion distance of a respiratory structure in the arthropod. The respiratory system of a typical arthropod is numerous, small, branched, cuticle-lined air ducts called tracheoles with an opening to the outside via spiracles that can be closed to reduce water loss. The special systems are as follows: spiders-book lungs; horseshoe crabs-book gills, crustaceans-gills.
6. Malpighian tubules are projections from the digestive tract, between the midgut and the hindgut. The wastes are processed as follows: fluid is absorbed from the blood through the walls of the Malpighian tubules, nitrogenous wastes are concentrated, then they are emptied into the hindgut and eliminated. Water loss is regulated because water and salts are reabsorbed by the hindgut and returned to the circulation.
7. The three classes of Chelicerata are Arachnids (spiders), Merostomata (horseshoe crabs), and Pycnogonida (sea spiders). The arrangement of appendages in the largest class, Arachnids, is a pair of chelicerae, a pair of pedipalps, and four pairs of walking legs. These appendages function as follows: chelicerae-fangs attached to poison glands; pedipalps-specialized for copulation in male spiders, but also serve sensory and feeding functions; walking legs-locomotion. The class feeds by pumping digestive enzymes into its prey and sucking up the digested material. What remains is a dried husk of the original prey.
8. The ancient arthropods that are ancestors to horseshoe crabs are trilobites. Horseshoe crabs exhibit external fertilization. Sea spiders regulate body wastes and gas exchange by direct diffusion across the body surface.
9. Crustaceans have two pairs of antennae, three pairs of chewing appendages, and various numbers of walking legs. The legs are on the abdomen as well as the thorax. The labyrinth functions to regulate the osmotic condition of the body primarily, and secondarily to excrete salts, amino acids, and water.
10. Millipedes and centipedes are both members of Uniramia, with bodies that consist of a head region followed by numerous segments with paired appendages. However, centipedes have one pair of legs per segment and are carnivores. Millipedes have two pairs of legs per segment (actually two segments fused to form one) and are herbivores.
11. Most insects possess a tubular digestive tract that is somewhat coiled. Insects that feed on juices low in protein have a greatly coiled tract to provide greater opportunity to absorb nutrients and to allow their digestive enzymes, which are weaker, more length over which to work. Respiratory adaptations in insects include (1) tracheae enlarged to form air sacs surrounded by muscles that help force air deep into the body, (2) permanently closed spiracles, and (3) gases that cross between the trachea and exoskeleton via diffusion.
12. An instar in this context refers to stages between molts. The two different kinds of metamorphosis are simple-if wings are present, they develop externally during juvenile stages with no resting stage before the last molt into adulthood (immature stages-nymphs); and complete-wings develop internally during the juvenile stages and appear only during the resting stage prior to the adult (immature stages-larvae; resting stage-pupa or chrysalis).
13. The symmetry is radial and there is a five-part body plan in adult echinoderms. The skeleton is calcium-rich plates called ossicles that make up the internal skeleton covered by epidermis beset with numerous spines. Their nervous system is not centralized, with no head or brain, but rather the system is composed of central nerve rings that branch. The water vascular system is a ring canal with five radial canals so that water enters through the madreporite, flows to the ring canal through the stone canal, and ultimately into tube feet. They extend and contract their tube feet through the contraction of muscular ampullae at the base of each tube foot, which forces fluid into the tube foot, causing it to extend; then when the muscles contract, fluid is forced back into the ampulla. In sea cucumbers, sea lilies, and brittle stars the tube feet are specialized for feeding.
14. Echinoderms respire via skin gills, which are fingerlike projections of epidermis that occur near spines. The digestive system is complete with mouth, gut, and anus. The ability to regenerate lost body parts means that they can asexually reproduce from parts, and sea cucumbers in particular can eject intestinal parts as a defensive mechanism. Reproduction is usually sexual, with the sexes separate and externally indistinguishable; there is external fertilization although some species brood eggs in cavities or underneath bodies. The larvae are bilaterally symmetrical.
15. Sea cucumbers have soft bodies, with leathery skin, and lie on their sides so that the radial symmetry is less evident. Their tube feet form tentacles that surround the mouth. To eat, the tube feet exude mucus that captures small organisms, then the tentacles are brought to the mouth and the food organisms are wiped off within the esophagus. Their skeleton is widely separated microscopic plates. The respiratory tree has internal branches from the cloaca that function in gas exchange as water is pulled in and out. Reproductively, some are hermaphroditic.