33.1 Mollusks were among the first coelomates.
Mollusk Body Plan
• The evolution of a coelom was a significant advance in animal body structure because it repositions fluid and allows complex tissues and organs to develop. (p. 652)
• Mollusks are extremely diverse; they exhibit a wide range of body forms and inhabit a large variety of habitats. (p. 652)
• The molluscan body plan is bilaterally symmetrical, with an efficient excretory system and a muscular foot for locomotion. (p. 654)
• Most mollusks have distinct sexes, and most aquatic mollusks engage in external fertilization. (p. 655)
Major Classes of Mollusks
• Major classes of mollusks include the class Polyplacophora (chitons), class Gastropoda (snails and slugs), class Bivalvia (bivalves), and class Cephalopoda (octopuses, squids, and nautiluses). (pp. 656-657)
33.2 Annelids were the first segmented animals.
Segmented Bodies
• Segmentation was a key transition in animal body plans, because similar segments can be individually controlled for different functions. (p. 658)
• Three principal features of annelids are repeated segments, specialized segments, and connections between segments. (p. 658)
• The annelid body plan is a tube within a tube. (p. 658)
• Locomotion is carried out by a squeezing motion as the hydrostatic skeleton forces fluid from one segment to the next. (p. 658)
Major Classes of Annelids
• The roughly 12,000 recognized annelid species are divided into three classes: Polychaeta (polychaetes), Oligochaeta (earthworms), and Hirudinea (leeches). (pp. 660-661)
33.3 Lophophorates appear to be a transitional group.
Lophophorates
• The marine phyla Phoronida (phoronids), Ectoprocta (bryozoans), and Brachiopoda (brachiopods) are characterized by a ridge around the mouth bearing ciliated, hollow tentacles (a lophophore) that functions in gas exchange and food collection. (pp. 662-663)
33.4 Arthropods are the most diverse of all animal groups.
Arthropod Body Plan
• Over two-thirds of all named species on the earth are arthropods. (p. 664)
• Jointed appendages used as legs, antennae, and mouthparts, and an exoskeleton of chitin and protein used for protection and water loss prevention, have allowed arthropods to become the most diverse phylum on the planet. (p. 664)
• Arthropods are segmented, with some segments becoming fused into tagmata (functional groups). (pp. 665-666)
• The exoskeleton is secreted by, and fused with, the epidermis, and provides a hard surface for muscle attachment. (p. 666)
• All arthropods have an open circulatory system; some have adaptations such as compound eyes, a respiratory system composed of tracheae, and an excretory system composed of Malpighian tubules. (pp. 666-667)
A Major Group of Arthropods: Crustaceans
• Most crustaceans (35,000 species) have two pairs of antennae, three types of chewing appendages, and various pairs of legs. (p. 668)
• Crustaceans are found in marine, freshwater, and terrestrial habitats. (pp. 668-669)
Major Classes of Arthropods: Arachnids
• Arachnids (57,000 species) have a pair of chelicerae, a pair of pedipalps, and four pairs of walking legs. (p. 670)
• Two major orders are Araneae (spiders) and Acari (mites and ticks). (p. 670)
Major Classes of Arthropods: Centipedes and Millipedes
• Centipedes (class Chilopoda) and millipedes (class Diplopoda) are made of a head region followed by numerous similar segments. Centipedes have one pair of legs per segment, and millipedes have two pairs per segment. (p. 671)
Major Classes of Arthropods: Insects
• Class Insecta is the largest group of organisms on the planet, living in nearly every possible habitat. (p. 672)
• Most are relatively small, and contain three body sections: head, thorax, and abdomen, with three pairs of legs attached to the thorax, and one pair of antennae. (p. 674)
• Most insects have compound eyes. (p. 674)
• Sensory hairs, tympanal organs, and chemoreceptors all act as sense receptors in insects. (p. 675)
• Many insects undergo either simple or complex metamorphosis. (p. 675)
33.5 Echinoderms are radially symmetrical as adults.
Deuterostome Development and an Endoskeleton
• Echinoderms are marine animals with hard calcium plates forming a true endoskeleton in young individuals. (p. 676)
Echinoderm Body Plan
• All echinoderms are bilaterally symmetrical during larval development, and become radially symmetrical as adults. (p. 678)
• Echinoderms have a five-part body plan with a central, branched nerve ring and an endoskeleton composed of calcium-rich plates (ossicles). (p. 678)
• Many echinoderms can regenerate lost parts, but in most of them, reproduction is sexual and external. (p. 679)
Major Classes of Echinoderms
• There are six living classes of Echinoderms: Crinoidea (sea lilies and feather stars), Asteroidea (sea stars or starfish), Ophiuroidea (brittle stars), Echinoidea (sea urchins and sand dollars), Holothuroidea (sea cucumbers), and Concentricycloidea (sea daisies). (p. 680)