Protostome Notes

Protostomes

33.1 The Clades of Protostomes

33.2 Flatworms (Platyhelminthes)

33.3 Rotifers (Rotifera)

33.4 Mollusks (Mollusca)

33.5 Annelids (Annelida)

33.6 Ribbon Worms (Nemertea)

33.7 Bryozoans (Bryozoa) and Brachiopods (Brachiopoda)

33.8 Roundworms (Nematoda)

33.9 Arthropods (Arthropoda)

What does it mean to be a protostome?

• Protostomes:
  1. Develop the mouth from the blastopore. The anus, if present, develops from the blastopore or another region of the embryo.
  2. Use spiral cleavage.
  3. Have determinant cell fate (cell fate is determined early).
  4. Cells move apart to form the coelom.

The Clades of Protostomes

• All protostomes belong to either the Lophotrochozoa or the Ecdysozoa.

Lophotrochozoa

• Embryos develop using spiral cleavage.
• Most live in water.
• Move using cilia or contractions of the body musculature.
• Two characteristics define members of this group:
  1. Trochophore: a free-living larva.
  2. Lophophore: a horseshoe-shaped crown of ciliated tentacles surrounds the mouth used in filter-feeding.

Ecdysozoa

• Contains animals that molt.
• Exoskeleton.
• Two large groups:
  1. Arthropods.
  2. Nematodes.

Protostomes: Lophotrochozoa

Phylum Platyhelminthes

• Flatworms
• Simple bodies with no circulatory or respiratory systems but a complex reproductive system
• Includes marine and freshwater planarians and parasitic flukes and tapeworms.
• Flatworms are ciliated, soft-bodied animals. Bodies are solid aside from an incomplete digestive cavity.

Additional Flatworm Features

• Only one opening to the digestive cavity.
• Muscular contractions in the pharynx allow food to be ingested and torn into small bits.
• Lack circulatory system.
• Diffusion for gas transport.
• The gut functions in digestion and food distribution.
• Some particles digested extracellularly.
• Cells engulf particles by phagocytosis.
• Tapeworms (parasitic flatworms) lack digestive systems – absorb food directly through body walls.

Excretion and Osmoregulation

• Excretory and Osmoregulatory System:
  • Network of fine tubules runs through the body.
  • Flame cells are located on the side branches.
  • Flagella move water and excretory substances into the tubules and then to pores located between the epidermal cells through which the liquid is expelled.
  • Metabolic wastes are excreted into the gut and eliminated through the mouth.

Nervous System and Reproduction

• Simple Nervous System
  • Anterior cerebral ganglion and nerve cords.
  • The eyespot can distinguish light from dark.
• Reproduction
  • Most are hermaphroditic.
  • Undergo sexual reproduction.
  • Also have the capacity for asexual regeneration.

Major Groups of Flatworms

• Subphylum Neodermata
  • Includes the parasitic flatworms
  • Two subgroups
    1. Trematoda (Flukes)
    2. Cercomeromorpha (Tapeworms)

Trematoda (Flukes)

• Attach within the host body by suckers, anchors, or hooks.
• The life cycle may have 2 or more hosts.
• Includes the miracidium, sporocyst, redia, and cercaria stages.
• Clonorchis sinensis (liver fluke).
• Schistosoma.

Schistosoma

• One of the most important trematodes to human health are blood flukes Schistosoma.
• Afflict 200 million people in tropical Asia, Africa, Latin America, and the Middle East.
• About 200,000 people die each year from schistosomiasis or bilharzia.
• The fertilized egg must break through the wall of the blood vessels in the intestine or the urinary bladder to get out.

Cercomeromorpha (Tapeworms)

• The adult hangs onto the inner wall of the host intestine using the scolex.
• Most of the tapeworm body is proglottids.
  • A complete hermaphroditic unit, containing both male and female reproductive organs.
• Beef tapeworm, Taenia saginata
  • A frequent human parasite from eating uninspected rare beef.

Phylum Rotifera

• Bilaterally symmetrical, unsegmented pseudocoelomates
• Highly developed internal organs
• Corona – “wheel animals”
• Conspicuous ring of cilia at the anterior end.
  • Used for locomotion and sweeping food into the mouth.

Phylum Mollusca

• Second in diversity only to arthropods
• Include snails, slugs, clams, octopuses, and others
• Some have a shell; some do not

Features of the Phylum Mollusca

• Range in size from microscopic to huge
• Giant clams may weigh 270 kg.
• Evolved in the oceans, and most groups have remained there.
• An important source of human food
• Economically significant in other ways
  • Pearls are produced in oysters.
  • Mother-of-pearl is produced in the shells of abalone.
• Pests – Zebra mussel (Dreissena polymorpha).

Mollusk Body Plan: Mantle and Foot

• Mantle
  • Thick epidermal sheet.
  • Bounds mantle cavity.
  • Secrete shell (if there is one).
• Foot
  • The primary means of locomotion for many.
  • Divided into arms or tentacles in cephalopods.

Mollusk Body Plan: Internal Organs

• The coelom is highly reduced.
• Limited to small spaces around the excretory organs, heart, and part of the intestine.
• Digestive, excretory, and reproductive organs are concentrated in a visceral mass.
• Ctenidia – gills in aquatic mollusks.
• Also filter food in most bivalves.

Mollusk Body Plan: Shell

• Protects against predators and adverse environments.
• Secreted by the outer surface of the mantle.
• Clearly not essential – repeated loss or reduction.
• A typical shell has 2 layers of calcium carbonate.
• The internal layer may be mother-of-pearl or nacre.
• Pearls are formed by coating a foreign object with nacre to reduce irritation.

Mollusk Body Plan: Radula

• Characteristic of most mollusks.
• Rasping, tongue-like structure used in feeding.
• Used to scrape up algae.
• In predatory gastropods, modified to drill through clam shells.
• In Conus snails, modifies into a harpoon with a venom gland.
• Bivalves do not have a radula.
• Gills used in filter-feeding.

Waste Removal

• Nitrogenous waste removal: Nephridia
  • Consist of cilia-lined openings called nephrostomes.
  • Tube to excretory pore to mantle cavity.

Circulatory System

• Open circulatory system
  • Hemolymph sloshes around hemocoel.
  • 3-chambered heart.
• Cephalopods have a closed circulatory system.

Mollusk Reproduction

• Most mollusks are gonochoric (individuals are either male or female).
• A few are hermaphroditic.
• Some oysters change sex.
• Most engage in external fertilization
• Gastropods have internal fertilization.
• The mollusk zygote undergoes spiral cleavage.

Mollusk Life Stages

• Trochophore
  • Free-swimming larval stage.
• Veliger
  • Second free-swimming larval stage.
  • Only in bivalves and most marine snails.
• Both forms drift widely in the ocean.

Classes of Mollusks

• There are 7 or 8 recognized classes. Here are 3 of them:
  1. Gastropoda – limpets, snails, slugs
  2. Bivalvia – clams, oysters, scallops
  3. Cephalopoda – squids, octopuses, cuttlefishes, and chambered nautilus

Class Gastropoda

• Limpets, snails, slugs
• A primarily marine group – some freshwater and only terrestrial mollusks
• Most have a single shell – some lost it
• Heads typically have pairs of tentacles with eyes
• Torsion
  • Unique among animals.
  • The mantle cavity and anus are moved from the posterior to the front.
• Coiling is the spiral winding of the shell

Class Bivalvia (Bivalves)

• No radula or distinct head
• Have 2 shells (valves) hinged together
• Adductor muscles counter hinge ligament.
• Water enters through the inhalant siphon and exits through the exhalant siphon
• Most marine, some freshwater
• Includes clams, scallops, mussels, oysters, and others

Class Cephalopoda

• More than 700 strictly marine species
• Active marine predators
• Only mollusk with a closed circulatory system
• The foot has evolved into a series of arms equipped with suction cups
• Beak-like jaws, toxic saliva.
• Largest relative brain sizes among invertebrates
• Highly developed nervous system

The Siphon and the Skin

• Living cephalopods lack an external shell
  • Except chambered nautilus.
• Squid and cuttlefish have internal shells.
• Jet propulsion using siphon
• Ink can be ejected from the siphon
• Chromatophores allow for changing skin color for camouflage or communication

Phylum Annelida

• Segmented worms
• The body is built of repeated units
  • Allows for specialization
• Segmentation may have evolved multiple times

Annelid Body Plan

• The head has a well-developed cerebral ganglion.
• Ventral nerve cord
• Sensory organs in ring-like segments.
  • Many species have eyes.
• Segments divided internally by septa.
• Each segment has a pair of excretory organs, a ganglion, and a locomotory structure.
• Closed circulatory system.

Other Annelid Features

• Each part of the digestive tract is specialized for a different function
• Locomotion:
  • Coelomic fluid creates a hydrostatic skeleton.
  • Alternating muscle contractions allow complex movements.
  • Chaetae – bristles of chitin found in most groups.
• Closed circulatory system: Gas exchange by diffusion across body surfaces.
• Excretory system: Nephridia similar to mollusks

Annelid Clades

• Roughly 32,000 described species of annelids occur in many habitats
• Formerly classified as Polychaeta, Oligochaeta, and Hirudinea
• Now grouped into 2 clades:
  1. Errantia: Includes clamworms, scaleworms, lugworms, sea mice, tubeworms, and others
  2. Sedentaria

Sedentaria

• Little movement
• May burrow or live in tubes
• May feed using tentacles or may filter-feed

Riftia

• Adults are gutless
• Sulfur-oxidizing bacteria housed on Riftia synthesize organic compounds used by the worm
• Found near hydrothermal vents

Clade Clitellata

• Clitellum found in all members

Earthworms

• Head not well differentiated.
• No parapodia.
• Few chaetae project from the body wall.
• Hermaphroditic but cross-fertilize.
• The clitellum secretes a mucus cocoon.
• 100–175 similar segments
• Chaetae project from the body wall
• Eat their way through the soil
• No eyes
• Can sense light, touch, and have chemoreceptors

Leeches

• Occur mostly in freshwater
• Usually flattened dorsoventrally
• Hermaphroditic and cross-fertilization
• The clitellum develops only during the breeding season.
• Coelom reduced, not divided into segments
• Suckers at both ends of the body
• No chaetae (except for one species)
• Some eat detritus, others suck blood

Protostomes: Ecdysozoa

Phylum Nematoda

• Vinegar eels, eelworms, and other roundworms
• Members of this phylum are found everywhere – abundant and diverse
• Marine, freshwater, parasites, free-living

Additional Nematode Characteristics

• Bilaterally symmetrical and unsegmented
• Covered by a flexible, thick cuticle that is molted as they grow
• The digestive system is well developed
  • Stylets – piercing organs near the mouth.
  • Pharynx – creates sucking action.
  • Anus

Reproduction and Composition

• Sexual reproduction
  • Most gonochoric
  • Sexual dimorphism – male smaller with hooked end
  • Internal fertilization.
  • Indirect development – egg, larva, adult
• Cellular composition
  • Adults consist of a fixed number of cells.
  • Caenorhabditis elegans has only 959 cells.
  • Important in genetic and developmental studies.

Nematodes and Human Disease

• About 50 species cause human diseases

Hookworms

• Common in the southern U.S.
• Produce anemia.

Trichinella

• Causes trichinosis
• Forms cysts in muscles.
• Infection from eating undercooked meat.

Additional Nematodes Associated with Human Disease

Pinworms, Enterobius vermicularis

• Infects 11% of the U.S. population.
• Causes itching of the anus.

Ascaris lumbricoides

• Intestinal roundworm
• Infects 1 in 6 worldwide.
• The adult female can be 30 cm long.
• Rare in areas with modern plumbing

Serious tropical nematode diseases

• Filariasis
• Elephantiasis

Phylum Arthropoda

• By far the most successful animals
• Well over 1,200,000 species (2/3 of all named species).
• Arthropods affect all aspects of human life
• Divided into four extant classes
  1. Chelicerata.
  2. Crustacea.
  3. Hexapoda.
  4. Myriapoda.

TABLE 33.1 Major Groups of the Phylum Arthropoda

Arthropod Success

1. Segmentation
   • In some classes specialized into tagmata.
     • Head, thorax, abdomen.
     • The head and thorax may be fused into the cephalothorax or prosoma.
2. Exoskeleton
   • Made of chitin and protein.
   • Protects against water loss.
   • Must undergo ecdysis – molting
3. Jointed appendages
   • May be modified into antennae, mouthparts, or wings.

Circulatory and Nervous Systems

• Open circulatory system
• Nervous system
  • Double chain of segmented ganglia.
  • Ventral ganglia control most activities.
  • Can eat, move, or copulate with the brain removed

Arthropod Eyes

• Compound eyes are found in many arthropods
  • Composed of independent visual units called ommatidia.
• Other arthropods have simple eyes, or ocelli
  • May be in addition to compound eyes.
  • Have single lenses
  • Distinguish light from darkness

Respiratory system

• Some tiny arthropods lack any structure for gas exchange
• Many marine arthropods have gills
• Terrestrial arthropods use tracheae
  • Branch into tracheoles in direct contact with cells.
  • Connected to the exterior by spiracles.
  • Valves control water loss.
• Many spiders use book lungs
  • Leaflike plates

Excretory system

• In aquatic arthropods, much of the waste diffuses out of gills
• Terrestrial insects and some others use Malpighian tubules
  • Eliminates nitrogenous wastes as concentrated uric acid or guanine.
  • Efficient conservation of water.

Class Chelicerata

• Spiders, ticks, mites, scorpions, daddy long-legs, horseshoe crabs, sea spiders
• Most anterior appendages are called chelicerae
  • May function as fangs or pincers.
• The body is divided into 2 tagmata
  • Anterior prosoma bearing all appendages.
    • Pedipalps and 4 pairs of walking legs.
  • Posterior opisthosoma contains reproductive organs.

Order Araneae (Spiders)

• About 35,000 species
• Particularly important as predators
• Many spiders catch their prey in silk webs
  • Silk protein is forced out of spinnerets found on the posterior of the abdomen.
• Other spiders actively hunt their prey
• All spiders have poison glands with channels through their chelicerae

Order Acari (Mites and Ticks)

• The most diverse of the chelicerates
• Predators and parasites
• Most mites are small
  • The cephalothorax and abdomen are fused into an unsegmented ovoid body.
• Ticks are larger
  • Blood-suckers
  • Can carry many diseases
    • Rocky Mountain spotted fever, Lyme disease

Class Crustacea

• Crabs, shrimps, lobsters, barnacles, crayfish, copepods, pill bugs, sand fleas
• Largely marine, some freshwater
• Have three tagmata
  • Cephalon and thorax fused to form a cephalothorax.
  • 2 pairs of antennae, 3 pairs of appendages for chewing, and various pairs of legs
  • Most appendages are biramous.
• Gas exchange through gills or across the cuticle

Crustacean Reproduction

• All crustaceans but barnacles are gonochoric (separate sexes)
• Many kinds of copulation
• Nauplius stage
  • Several stages before maturity.
  • Evidence of a common ancestor for a diverse group.

Decapod crustaceans

• Have 10 feet – 5 pairs of thoracic appendages
• The exoskeleton is usually enforced with $CaCO_3$
• Cephalothorax covered by carapace
• Lobsters and crayfish
• Swimmerets used in reproduction and swimming.
• Uropods paddle on either side of the telson.
• The crab has a small abdomen held under the carapace
• Shrimps, lobsters, crabs, and crayfish

Class Hexapoda

• Insects are by far the largest group of animals
  • Number of species and number of individuals.
• More than half of all named animal species
• Approximately one billion billions $(10^{18})$ insects are alive at any one time

TABLE 33.2 Major Orders of Insects

External features

• Three body regions:
  1. Head
  2. Thorax has three segments, each with a pair of legs
     • May have one or two pairs of wings – outgrowths of the body wall.
  3. Abdomen
• Most insects have compound eyes

Insect Mouthparts

• Insect mouthparts all have the same basic structure
• Modifications reflect feeding habits
• Mosquito, butterfly, housefly

Internal organization

• The digestive tract is a tube
• Digestion takes place in the stomach (midgut)
• Excretion takes place through Malpighian tubules
• Winged insects have dilated tracheae forming air sacs
  • Form bellows to force air deep into the body.
  • Spiracles through which air enters the tracheal system.
• Some parasitic or aquatic forms have permanently closed spiracles – use diffusion

Sensory receptors

• Sensory setae are hair-like structures
  • Detect chemical and mechanical signals.
• Tympanum – a thin membrane associated with tracheal air sacs
  • Detect sound.
  • May also use sensory hairs to detect sound.
• Pheromones are also used for communication
  • Mating signals, trail markers.

Insect Life Histories

• Many insects undergo metamorphosis

Simple metamorphosis

• Immature stages like adults

Complete metamorphosis

• Immature larva are wormlike.
• A resting stage, pupa or chrysalis, precedes the final molt into adult form.

Class Myriapoda

Centipedes (subclass Chilopoda)

• One pair of appendages per segment
• Carnivorous – poison fangs

Millipedes (subclass Diplopoda)

• Two pairs of appendages per segment.
• Each segment is a tagma of 2 segments
• Herbivores
• Complex glands produce bad-smelling fluid in defense.

Characteristics

• Head regions followed by numerous segments
• Gonochoric, internal fertilization, lay eggs
• Young add segments as they grow.