Chapter 30 - Protostomes

  • Protostomes: develop from mouth to anus

  • Protostomes are divided into two major clades based on DNA sequencing:

    • Lophotrochozoans

    • Ecdysozoans

Lophotrochozoans: bilateral protostomes and have spiral cleavage

  • The defining characteristic that separates lophotrochozoans from ecdysozoans:

    a cuticle that is thin and not molted in lophotrochozoans, but is thick and constantly molted in ecdysozoans

  • Some animals in this group have a lophophore (a feeding structure), a circular or U-shape ring of hollow ciliated tentacles around the mouth used for feeding and gas exchange

    • Examples: bryozoans, brachiopods, and phoronids

  • Some have a trochophore larva, a form that moves by beating a band of cilia, which also helps bring plankton closer to where they can be captured

    • Examples: mollusks, annelids, ribbon worms, entoprocts

  • Flatworms/Platyhelminthes (lophotrochozoans)

    • Acoelomates

    • Dorsoventrally flattened; no gas transport system

    • Most species are endoparasites- tapeworms and flukes

      • the mouth opens into a blind gut- incomplete digestive tract (one opening which is both the mouth and anus) → this is because their hosts provide all the nutrition they need, so they do not require elaborate feeding or digestive organs and can devote most of their body to reproduction

    • Free-living flatworms have cephalization and glide over surfaces by broad bands of cilia

    • Some flatworms cause serious diseases such as schistosomiasis

    • Perform both sexual and asexual reproduction

    • Flatworms have an excretory system with a network of tubules attached to flame cells


Annelids (lophotrochozoans)

  • earthworms, leeches, etc

  • Coelomates

  • Clearly segmented

  • A separate nerve ganglion controls each segment + have cephalization

  • Most have a thin, permeable body wall that serves for gas exchange; restricted to aquatic or moist habitats

  • Have an excretory system that consists of organs called nephridia

  • Live in aquatic environments and damp soils

  • Polychaetes (class of Annelida)

    • bristle worms/fan worms/feather duster worms

    • Mostly marine; burrow in soft sediments

    • Have one or more pairs of tentacles for prey capture or filter feeding

    • Parapodia (paddle-like appendages) on each segment are extensions of the body wall; they function in gas exchange, locomotion, and sensory roles

  • Clitellates (class of Annelida)

    • oligochaetes and leeches

    • Oligochaetes (earthworms) lack parapodia, eyes, and tentacles

      • Have few setae

      • Earthworms burrow in and ingest soil

      • Oligochaetes are hermaphroditic; each individual gives and receives sperm during copulation (sexual intercourse)

      • They sense light and movement using photoreceptors (since they have no eyes)

    • Leeches also lack parapodia and tentacles

      • Segments at either end are modified to form suckers – temporary anchors that aid in movement

      • Leeches are freshwater and terrestrial

      • Most leeches are ectoparasites that feed on blood

      • Medicinal leeches are used to treat diseases caused by excess blood coagulation (Hirudino medicinalis)

Mollusks (lophotrochozoans)

  • Bilateral symmetry

  • Coelomates

  • Foot: large, muscular structure, for locomotion and support; has been modified in different groups

  • Visceral mass: heart, digestive, excretory, and reproductive organs

  • Mantle: fold of tissue that covers organs in the visceral mass; secretes the calcareous shell

  • Have gills for gas exchange

  • Mollusks, except cephalopods, have an open circulatory system

  • Have hemocoel – fluid inside the body cavity analogous to blood

  • Chitons (class Polyplacophora of phylum Mollusca)

    • Eight overlapping calcareous plates, surrounded by a girdle, protect the organs and muscular foot

    • Most are marine omnivores that scrape rocks with a radula

    • They cling tightly to rock surfaces with the large, muscular foot

  • Gastropods (class of Mollusca)

    • Snails, slugs, nudibranchs

    • Nudibranchs (sea slugs) and slugs have lost their shells

      • many nudibranchs are toxic and have brilliant colors for warning others- have camouflaged coloration

    • Land snails and slugs are the only mollusks that live in terrestrial habitats

    • The mantle tissue is modified into a highly vascularized lung

  • Bivalves (class of Mollusca)

    • Clams, oysters, scallops, mussels

    • Have hinged, two-part shells

    • Many use their foot to burrow into mud or sand

    • Lack a radula → are suspension (filter) feeders

      • Feed by bringing water in the incurrent siphon and filtering food particles with large gills; water exits through the excurrent siphon

  • Cephalopods (class of Mollusca)

    • squids, octopi, nautiluses

    • The only mollusks with a closed circulatory system

    • Complex brain and nervous system/sensory organs (more cephalization)

    • High degree of mobility: water can be forcibly ejected from the siphon (muscular tube), creating “jet propulsion”

    • Eyes are comparable to vertebrates (advanced)

    • Cephalopods are active predators; they use their arms and tentacles to capture prey























Ecdysozoans: bilateral protostomes and have radial cleavage

  • Have an external cuticle (made of chitin- a fibrous substance made of polysaccharides), secreted by the epidermis

    • to grow, the cuticle must be shed, or molted, and replaced with a larger one (ecdysis)

  • Nematoda (roundworms) - Ecdysozoans

    • Unsegmented, thick, multilayered cuticle

    • Gas and nutrient exchange occurs through the cuticle and the gut wall

    • The pharynx, a muscular anterior organ, moves materials through the gut

    • Move by contracting longitudinal muscles

    • Many are scavengers in soil, freshwater, and marine sediments, predators feeding on protists and other animals, and parasites of plants and animals

    • Have a complete digestive system (2 openings: mouth and anus)

  • Caenorhabitis elegans (C. elegans) is used as a model organism in genetics and developmental biology research

    • It is easy to cultivate, matures in 3 days, and has a fixed number of body cells

    • Its genome has been completely sequenced

Arthropods - Ecdysozoans

  • Have jointed, paired appendages (but evolved from ancestors with unjointed appendages)

  • The largest group and most diverse

  • Have segmented bodies

  • Exoskeleton made of chitin

  • Hox genes have allowed great diversity in form and function

    • hox genes: group of genes that regulate the development of body parts in animals

  • Body cavity is called hemocoel with a fluid called hemolymph

  • Chelicerates (subphylum of Arthropoda)

    • sea spiders, horseshoe crabs, archnids

    • Two-part body: cephalothorax and abdomen

    • Two pairs of chelicerae (appendages modified into mouthparts)

    • Four pairs of walking legs

    • Horseshoe crabs have book gills

    • Spiders have book lungs

  • Mandibulates (subphylum of Arthropoda)

    • myriapods, crustaceans, hexapods

    • have mandibles instead of chelicerae

      • mandibles can be used for chewing, biting, and holding food

    • Have sensory antennae on the head

    • Myriapods: centipedes and millipedes

      • Head and a long, flexible trunk

      • Centipedes – 1 pair of legs per segment; prey on insects and other small animals (larger jaw)

      • Millipedes – 2 pairs of legs per segment (2 segments are fused); scavenge and eat plants (smaller jaw)

  • Crustaceans (subphylum of Arthropoda)

    • Dominant marine arthropods today; also terrestrial and freshwater

    • Most crustaceans have 3 body regions: head, thorax, and abdomen

    • Appendages are specialized for different functions: gas exchange, chewing, food capture, sensing, walking, swimming

    • Many species have a carapace, a fold of exoskeleton that extends over the head and thorax region (a shell)

    • Excretory waste of crustaceans is through the antennal gland (aka green gland)

  • Insects (subphylum Hexapoda of phyla Arthropoda)

    • Head, thorax, abdomen

      • Head with one pair of antennae

      • Thorax with three pairs of legs and two sets of wings in most groups

      • Abdomen with no appendages

    • Insects have external mouthparts

    • Unique gas exchange system: air sacs and tubular channels called tracheae extend from the external openings (spiracles) into tissues throughout the body

    • Malpighian tubules function as the primary excretory organ in insects

    • Metamorphosis: substantial physical changes occur between life stages

      • Incomplete metamorphosis: changes are from egg, nymph, and adult (ex: grasshopper) → nymph looks like adult

      • Complete metamorphosis: changes are dramatic (ex: butterflies) → 85% of all insects do complete

        • Allows for specialized tasks at different life stages

        • In many species, larvae are specialized for feeding and growing; adults are specialized for reproduction and dispersal