ASIM'S WEBWORLD View: Main School Main AP Biology Chapter Guides RELATED LINKS Review Questions and Answers		AP BIOLOGY: Chapter Forty Outline INTRODUCTION "Higher" Invertebrates Possess a Coelom Mollusks and Annelids Are Two Major Coelomic Phyla fig 40.1 Lophophorate Phyla Are Intermediate Between Protostomes and Deuterostomes LOPHOPHORATES Include Three Phyla of Marine Animals Ectoprocts (formerly Bryozoa) fig 40.1a Brachiopoda Phoronida Biology of the Lophophorates Lophophore is a circular or U-shaped ridge around the mouth fig 40.2 Coelomic cavity lies within lophophore and its tentacles Functions in gas exchange and food collection Share features of protostomes and deuterostomes Cleavage is radial as in deuterostomes Formation of coelom varies, resembles deutero- or protostomes Ribosomes of all are protostomic Phylum Phoronida: The Phoronids Superficially resemble tube worms Individuals secrete a tube made of chitin Possess U-shaped gut within a sac Development Develop as protostomes Anus develops secondarily Some exhibit spiral cleavage Phylum Ectoprocta: The Ectoprocts fig 40.2 Formerly called Bryozoa Commonly called moss animals Anus (proct) is external to lophophore (ecto) Form colonies, freshwater and marine forms Secrete a zoecium chamber and live within it Development Develop as deuterostomes Mouth develops secondarily Exhibit radial cleavage Phylum Brachiopoda: The Brachiopods fig 40.4 Superficially resemble clams, but shells are on dorsal and ventral surfaces Some attach to substrate with a stalk Lophophore located within shell Few living species, many extinct species Example: Lingula, most ancient surviving genus of all animals Development Develop as protostomes Exhibit radial cleavage Coelomic development follows pattern of deuterostomes PHYLUM MOLLUSCA: THE MOLLUSKS Introduction to the Mollusks Include snails, clams, scallops, oysters, cuttlefish, octopuses and slugs May possess durable shells Widespread and abundant in marine, freshwater and terrestrial habitats Terrestrial forms occur in seasonally moist places Surpassed by only arthropods in terms of success on land Economic importance Sources of human food Production of pearls and shell material Destructive to submerged timbers Zebra mussels have negatively impacted American aquatic ecosystems Extensive crop and flower damage caused by snails and slugs Serve as intermediate hosts for many serious parasitic diseases Includes largest invertebrates Giant squid exist in great numbers, but are rarely caught Giant clam may reach 1.5 meters and 270 kilograms Body Plan of the Mollusks fig 40.4 Have distinct bilateral symmetry Possess a visceral mass and a muscular foot May have a well-defined head at the anterior end of the body Digestive, reproductive and excretory organs located within visceral mass Folds from dorsal body wall form mantle Gills or lungs located within mantle cavity Gills are specialized portion of mantle Comprised of filamentous projections rich in blood vessels Highly efficient, extract 50% of oxygen from water Outer surface of mantle may secrete protective shell Horny protein outer layer Calcium carbonate middle layer Pearly inner layer Bivalves may produce pearls of shell material around foreign objects Some forms can withdraw into mantle cavity Continuous stream of water flows through mantle Mantle may be modified for propulsion as in squid and octopuses Muscular foot adapted for locomotion, attachment or food capture Cephalopod foot divided into ares or tentacles Foot of free-swimming, pelagic forms modified into fins All mollusks except bivalves possess a rasping, tongue-like radula fig 40.5 Circulatory system consists of a heart and open flowing system Three-chambered heart: two collect from gills, third pumps to body Cephalopods have a closed system of vessels and auxiliary hearts Coelom is represented by small area around the heart Excretory system is more efficient than that of lower invertebrates Nitrogenous wastes removed by tubular nephridia Funnel-shaped, cilia-lined nephrostome collects waste from coelom Coiled tube from nephrostome connects to bladder Bladder connected to excretory pore Waste discharged into mantle cavity Nutrients and salts reabsorbed to maintain osmotic balance Reproduction in Mollusks Most have separate sexes, few hermaphroditic forms Cross-fertilization is the rule, even in hermaphrodites Some may change sex within one season Mollusks dispersed through larval forms Many form free-swimming trochophore larvae fig 40.6 A second free-swimming veliger stage may precede adult form fig 40.7 The Classes of Mollusks Seven classes provide information on evolutionary relationships Probable ancestor was dorsoventrally flattened and unsegmented Most closely related to present chitons: class Polyplacophora fig 40.8 Class Gastropoda: the snails and slugs fig 40.9 Possess single shell or are derived from shelled forms Body divided into head, foot and visceral mass Shell of marine forms closed by a door-like operculum Head possess paired tentacles that may have terminal eyes Mouth may be simple or modified into proboscis Visceral mass asymmetrical because of torsion during development One side of larva grows faster than other Associated with other anatomical changes Display varied feeding habits Terrestrial herbivores are serious garden pests Whelks bore into other mollusk shells, suck out insides Cone shells, predator with harpoon-like radula fig 40.9a Nudibranchs, possess nematocysts from Cnidarian polyps fig 40.9d Terrestrial forms evolved a rudimentary lung under mantle Class Bivalvia: the bivalves fig 40.3;10 Have two shells hinged together dorsally (left and right sides) Held together by a ligament Close with contraction of adductor muscles Mantle forms incurrent and excurrent siphons Pair of gills located under folds of the mantle Lack distinct heads and radulas fig 40.6 Foot adapted for locomotion and anchoring Most forms are filter feeders with palps located on sides of mouth Disperse in larval stage Scallops are unique, very mobile forms fig 40.11 Abundant in marine and freshwater habitats fig 40.10 Class Cephalopoda: the octopuses, squids and nautilus fig 40.14 Active predators that compete successfully with fish Foot has evolved into a series of tentacles fig 40.14c Have highly developed nervous systems Rapid responses result from giant nerve fibers attached to mantle Eyes are elaborate with retina similar to that in vertebrates Are most intelligent invertebrates Lack external shells except for the few nautilus species Take water into mantle and expel it through siphon for propulsion Sexes are separate Specialized tentacle transmits spermatophore to female Eggs fertilized as they leave the oviduct PHYLUM ANNELIDA: THE ANNELIDS Segmented Worms Abundant in all habitats Segments characterized by ringlike structures along body Internal segments divided by septa Digestive and excretory organs repeated in each segment Body Plan of Annelids fig 40.13 Tube (digestive tract) within a tube (coelom) runs from mouth to anus Anterior segments may be modified with well-developed brain Possess diverse sensory organs Separate ganglia located in each segment, connected by nerve cords Muscles provide locomotion fig 40.14 Coelomic fluid serves as hydrostatic skeleton Each segment is one hydrostatic unit that contracts independently Each segment has setae: external bristles of chitin Help provide anchorage during movement Lacking in most leeches Polychaetes develop from trochophore larva similar to mollusks fig 40.6 Indicate common unsegmented ancestor Segmentation evolved early in the development of annelids Common ancestor of mollusks and annelids probably unsegmented Segmentation suggests arthropods share a common ancestor with annelids Segmentation in vertebrates likely evolved independently Possess a more efficient closed circulatory system Lack respiratory systems and exchange gases across body surfaces Earthworms have five pulsating blood vessels that serve as hearts fig 40.13 Blood may have hemoglobin respiratory pigments dissolved in it Excretory units similar to those of mollusks Repeated in each segment Transport waste out of coelom Classes of Annelids Diverse forms that appear in many different habitats Free-living, marine polychaetes most primitive Terrestrial, free-living Oligochaetes evolved from polychaetes Predatory or bloodsucking leeches, Hirudinea Possess clitellum as do oligochaetes Evolved from oligochaetes by specialization in habits Class Polychaeta: the polychaetes Great variety of marine worms fig 40.1b;16 Many commensal with sponges, mollusks, echinoderms, crustaceans Well-developed heads with specialized sense organs Possess distinct paddle-like parapodia Function in locomotion Provide increased surface area for gas exchange Sexes separate, fertilization generally external Males lack permanent gonads Sperm produced directly from cells lining coelom or on septa Produce mobile trochophore larvae Class Oligochaeta: the earthworms fig 40.13 Literally eat their way through the soil Contraction of pharynx sucks in organic debris Muscular gizzard grinds food with aid of soil particles Castings (undigested materials) are deposited outside burrows Lack eyes, but have light- touch- and moisture-sensitive organs Have fewer setae than polychaetes, no parapodia Are hermaphroditic, individuals trade gametes during mating fig 40.17 Mucus from clitellum holds worms together, forms cocoon Passes along body after separation, picks up deposited sperm Contains fertilized eggs which ultimately hatch into young worms Class Hirudinea: the leeches fig 40.18 Most are freshwater, few marine and terrestrial forms Usually dorsoventrally flattened Are hermaphroditic, seasonally develop a clitellum, cross-fertilize Coelom is reduced, continuous through the body, and unsegmented Have a sucker at one or both ends, for attachment and locomotion Lack setae, except for one species Most are predators or scavengers, some suck blood Example: Hirudo medicinalis, medicinal leech Mouth has chitinous teeth, secretes an anticoagulant Used to remove blood after special surgery Collected for anticoagulant THE FIRST TWO LARGE PROTOSTOME PHYLA Mollusks and Annelids Arthropods Examined in the Next Chapter