BilateriaBilateria

Having completed the radially Having completed the radially symmetrical Cnidarians and symmetrical Cnidarians and Ctenophores we now move on to the Ctenophores we now move on to the remaining animals, all of which are remaining animals, all of which are bilaterally symmetrical (or bilaterally symmetrical (or secondarily radially symmetrical [the secondarily radially symmetrical [the Echinoderms]). Echinoderms]).

These are the Bilateria.These are the Bilateria.

BilateriaBilateria

All Bilateria are triploblastic and the All Bilateria are triploblastic and the members are assigned to two major members are assigned to two major groups, which we have already met, the groups, which we have already met, the Protostomia and the Deuterostomia.Protostomia and the Deuterostomia.

Deuterostomes include the Echindoerms, Deuterostomes include the Echindoerms, Hemichordates and Chordates.Hemichordates and Chordates.

Protostomes include all the other bilateral Protostomes include all the other bilateral invertebrates including Platyhelminthes, invertebrates including Platyhelminthes, Annelida, Mollusca, and Arthropoda.Annelida, Mollusca, and Arthropoda.

Protostomia divisionsProtostomia divisions Classification of invertebrates is in a state of flux for Classification of invertebrates is in a state of flux for

several reasons:several reasons: molecular phylogenetic studies have cast doubt on traditional molecular phylogenetic studies have cast doubt on traditional

invertebrate classification.invertebrate classification. many invertebrates are soft-bodied so fossils of many groups many invertebrates are soft-bodied so fossils of many groups

are rare or unknown, which makes relationships between are rare or unknown, which makes relationships between groups hard to decipher.groups hard to decipher.

Traditional major groups of protostomes (acoelomate, Traditional major groups of protostomes (acoelomate, pseudocoelomate and coelomate) appear not to be pseudocoelomate and coelomate) appear not to be monophyletic.monophyletic.

For this class, we will use the For this class, we will use the Deuterostome/Protostome arrangement and Deuterostome/Protostome arrangement and recognize two majors groups of Protostomes: the recognize two majors groups of Protostomes: the Lophotrochozoa and the Ecdysozoa.Lophotrochozoa and the Ecdysozoa.

Protostome Divisions: the Protostome Divisions: the Lophotrochozoa and Lophotrochozoa and

Ecdysozoa.Ecdysozoa.

Protostomes are divided into two Protostomes are divided into two large groups the Lophotrochozoa and large groups the Lophotrochozoa and the Ecdysozoa.the Ecdysozoa.

The relatedness of phyla The relatedness of phyla withinwithin these these two groups is not entirely clear and two groups is not entirely clear and will likely change in the future.will likely change in the future.

Lophotrochozoa and Lophotrochozoa and EcdysozoaEcdysozoa

Lophotrochozoa: members generally Lophotrochozoa: members generally possess a trochophore larva [free-possess a trochophore larva [free-swimming oval or pyramidal ciliated larva swimming oval or pyramidal ciliated larva with a band of cilia around the body] or a with a band of cilia around the body] or a lophophore [tentacle bearing arm which lophophore [tentacle bearing arm which contains within it an extension of the contains within it an extension of the coelomic cavity].coelomic cavity].

Ecdysozoa: members shed their cuticle as Ecdysozoa: members shed their cuticle as they grow they grow

Trochophore larvahttp://www.microscopy-uk.org.uk/mag/imgjan09/image006.jpg

Lophophore: characteristic feeding structure of members of theBrachiopoda, Bryozoa and Phoronida

Bryozoan lophophore http://www.bryozoans.nl/pictures/figuren/anatomy.jpg

Lophotrochozoa and Lophotrochozoa and EcdysozoaEcdysozoa

Lophotrochozoa members:Lophotrochozoa members: Platyhelminthes, Annelida, Mollusca, and Platyhelminthes, Annelida, Mollusca, and

a diverse array of “lesser phyla” a diverse array of “lesser phyla” including Nemertea, Gnathostomulida, including Nemertea, Gnathostomulida, Rotifera, and Sipuncula.Rotifera, and Sipuncula.

Ecdysozoa members:Ecdysozoa members: Nematoda, Arthropoda and “lesser Nematoda, Arthropoda and “lesser

phyla” including Onychophora, phyla” including Onychophora, Tardigrada, and Priapulida.Tardigrada, and Priapulida.

““Worms”Worms”

The term worm is loosely employed in biology The term worm is loosely employed in biology and is applied to very different animals and is applied to very different animals including the segmented worms (Annelids), including the segmented worms (Annelids), roundworms (pseudocoelomates) and a roundworms (pseudocoelomates) and a variety of acoelomate bilateral animals.variety of acoelomate bilateral animals.

““Worm” describes any bilaterally Worm” describes any bilaterally symmetrical, legless, soft-bodied animal at symmetrical, legless, soft-bodied animal at least 2-3 times as long as it is wide.least 2-3 times as long as it is wide.

One group of worms traditionally recognized One group of worms traditionally recognized was the acoelomate worms.was the acoelomate worms.

Acoelomate wormsAcoelomate worms

Three phyla of acoelomate worms Three phyla of acoelomate worms were traditionally grouped together were traditionally grouped together because they lacked a coelom and because they lacked a coelom and had a solid body filled with had a solid body filled with parenchyma cells:parenchyma cells: Platyhelminthes: flatwormsPlatyhelminthes: flatworms Nemertea: ribbon wormsNemertea: ribbon worms Gnathostomulida: jawed wormsGnathostomulida: jawed worms

Baseodiscus delineatus (Nemertea) http://tolweb.org/images/Nemertea/2489

http://tolweb.org/images/Gnathostomulida/2481

Acoelomate wormsAcoelomate worms

However, molecular evidence based on However, molecular evidence based on ribosomal RNA sequences suggests the ribosomal RNA sequences suggests the Nemertea are more closely related to Nemertea are more closely related to Annelids than Platyhelminths. Annelids than Platyhelminths.

Similarly, Gnathostomulids do not have free Similarly, Gnathostomulids do not have free swimming larvae and their sperm morphology swimming larvae and their sperm morphology is very different from the Platyhelminthes, is very different from the Platyhelminthes, which suggests they aren’t close relatives.which suggests they aren’t close relatives.

Acoelomate wormsAcoelomate worms

Because of the dubious relatedness of the Because of the dubious relatedness of the phylum Platyhelminthes to these other phylum Platyhelminthes to these other groups we will not deal with them until groups we will not deal with them until later in the semester.later in the semester.

However, the phylum Platyhelminthes, is a However, the phylum Platyhelminthes, is a very important group which includes a very important group which includes a wide variety of parasitic forms such as the wide variety of parasitic forms such as the flukes and tapeworms.flukes and tapeworms.

Phylum PlatyhelminthesPhylum Platyhelminthes

Unlike other animals encountered so Unlike other animals encountered so far, Platyhelminthes:far, Platyhelminthes: have evolved have evolved cephalizationcephalization with their with their

sense organs concentrated at the head sense organs concentrated at the head end. end.

Possess the beginning of a ladder-type Possess the beginning of a ladder-type nervous system.nervous system.

are bilaterally symmetrical.are bilaterally symmetrical. Are dorsoventrally flattenedAre dorsoventrally flattened

Phylum PlatyhelminthesPhylum Platyhelminthes

In addition they are In addition they are triploblastic, but lack a coelom. Instead, triploblastic, but lack a coelom. Instead,

they have a solid body filled with they have a solid body filled with parenchyma cells.parenchyma cells.

have evolved organs and in some cases have evolved organs and in some cases organ systems. organ systems.

The first (and simplest) excretory or The first (and simplest) excretory or osmoregulatory systems and circulatory osmoregulatory systems and circulatory systems are found in members of these systems are found in members of these groups.groups.

Phylum PlatyhelminthesPhylum Platyhelminthes

Platyhelminthes typically have dorsoventrally Platyhelminthes typically have dorsoventrally flattened bodies, usually slender and leaflike or flattened bodies, usually slender and leaflike or ribbonlike.ribbonlike.

The advantage of a flat body is that it increases The advantage of a flat body is that it increases surface area and allows the animal to exchange surface area and allows the animal to exchange gas and lose wastes by diffusion.gas and lose wastes by diffusion.

Four classes in the Platyhelminthes. The Four classes in the Platyhelminthes. The Turbellaria are free living whereas as members of Turbellaria are free living whereas as members of the Monogenea, Trematoda and Cestoda are the Monogenea, Trematoda and Cestoda are parasitic.parasitic.

NutritionNutrition

The digestive system includes a mouth, The digestive system includes a mouth, pharynx, and blind intestine (the gut is pharynx, and blind intestine (the gut is incomplete)incomplete)

In the free-living Turbellarians the pharynx In the free-living Turbellarians the pharynx can be everted from the mouth. can be everted from the mouth.

Food is sucked into the intestine where a Food is sucked into the intestine where a combination of extracellular and combination of extracellular and intracellular digestion takes place. intracellular digestion takes place.

http://www.thaigoodview.com/library/contest2551/science04/119/kingdon_animalia/images/turbellaria4.jpg

“Organization of the blind digestive cavity of polyclads [a group of Turbellarians] with highly branched diverticles (ventral view)”

http://www.rzuser.uni-heidelberg.de/~bu6/Introduction04.html

NutritionNutrition

Undigested food exits via the Undigested food exits via the pharynx. pharynx.

In the Cestoda the digestive tract is In the Cestoda the digestive tract is absent and all nutrients are absorbed absent and all nutrients are absorbed across the tegument (the syncytial across the tegument (the syncytial membrane/body covering found in all membrane/body covering found in all parasitic Platyhelminthes).parasitic Platyhelminthes).

Excretion/OsmoregulationExcretion/Osmoregulation

The osmoregulatory system consists of a series of The osmoregulatory system consists of a series of canals that end in flame cells or protonephridia.canals that end in flame cells or protonephridia.

The flame cell consists of a fine-meshed cup that The flame cell consists of a fine-meshed cup that contains cilia. The beating of the cilia draws fluid contains cilia. The beating of the cilia draws fluid which is filtered as it passes into the cup. which is filtered as it passes into the cup.

This system appears mainly intended to remove This system appears mainly intended to remove

excess fluid, but retain essential ions. It is most excess fluid, but retain essential ions. It is most developed in freshwater Turbellarians, but developed in freshwater Turbellarians, but reduced or absent in marine species, which do reduced or absent in marine species, which do not have to remove excess water.not have to remove excess water.

http://www.cartage.org.lb/en/themes/Sciences/Lifescience/GeneralBiology/Physiology/ExcretorySystem/Invertebrate/flatwormexcret.gif

Nervous system and sense Nervous system and sense organsorgans

Flatworms possess a simple brain and one Flatworms possess a simple brain and one to five pairs of longitudinal nerve cords that to five pairs of longitudinal nerve cords that are cross connected to form a ladder-like are cross connected to form a ladder-like arrangement.arrangement.

There has been a tendency towards There has been a tendency towards reduction of the number of pairs of nerve reduction of the number of pairs of nerve cords and increased development of the cords and increased development of the ventral pair. A similar evolutionary ventral pair. A similar evolutionary pathway may have led to the development pathway may have led to the development of the ventral nerve cord found in annelids of the ventral nerve cord found in annelids and arthropods.and arthropods.

Nervous system of Dugesiahttp://biodidac.bio.uottawa.ca/ftp/BIODIDAC/ZOO/PLATYHEL/DIAGCL/TURB007C.GIF

Nervous system and sense Nervous system and sense organsorgans

Neurons are specialized for different Neurons are specialized for different tasks e.g. sensory and motor functions, tasks e.g. sensory and motor functions, which is an important advance in the which is an important advance in the evolution of nervous systems.evolution of nervous systems.

There are a number of different sensory There are a number of different sensory cells found in flatworms and tactile and cells found in flatworms and tactile and chemoreceptive cells are abundant.chemoreceptive cells are abundant.

Nervous system and sense Nervous system and sense organsorgans

In freshwater Planarians In freshwater Planarians concentrations of sensory cells form concentrations of sensory cells form two ear-like structures (the auricles) two ear-like structures (the auricles) found on the side of the head.found on the side of the head.

Light sensitive eyespots or ocelli are Light sensitive eyespots or ocelli are common in all classes but Cestoda. common in all classes but Cestoda.

Freshwater Planarians:http://www.aecos.com/CPIE/flatworm.jpg

ReproductionReproduction

Reproduction in the Platyhelminthes Reproduction in the Platyhelminthes can be asexual or sexual. However, can be asexual or sexual. However, most are hermaphroditic and cross most are hermaphroditic and cross fertilize.fertilize.

In parasitic forms sexual and asexual In parasitic forms sexual and asexual reproduction may alternate in reproduction may alternate in different stages of the life historydifferent stages of the life history

Classification of Classification of PlatyhelminthesPlatyhelminthes

There are four classes in the There are four classes in the Platyhelminthes:Platyhelminthes: Class Turbellaria: free-living flatworms.Class Turbellaria: free-living flatworms. Class Trematoda: endoparasitic flukesClass Trematoda: endoparasitic flukes Class Monogenea: parasitic flukes that Class Monogenea: parasitic flukes that

are mainly ectoparasitesare mainly ectoparasites Class Cestoda: tapewormsClass Cestoda: tapeworms

Class TurbellariaClass Turbellaria

Class Turbellaria contains about 3000 Class Turbellaria contains about 3000 species. There is considerable debate species. There is considerable debate about the classification of the class and it about the classification of the class and it is likely that the class is not monophyletic. is likely that the class is not monophyletic.

Most species are marine and benthic Most species are marine and benthic (move around on the bottom in aquatic (move around on the bottom in aquatic environments). environments).

Some also found in fresh water or in moist Some also found in fresh water or in moist temperate and tropical terrestrial habitats. temperate and tropical terrestrial habitats.

8.2

Figure 14.10

Marine turbellarian

Dugesia tigrina, a freshwater turbellarian

© Mauricio A. Muñoz

Class TurbellariaClass Turbellaria

Most Turbellarians are predators of invertebrates Most Turbellarians are predators of invertebrates smaller than themselves. Other species are smaller than themselves. Other species are herbivores or scavengers. herbivores or scavengers.

In many species the pharynx is protrusible and In many species the pharynx is protrusible and can be inserted into the prey to begin digesting it.can be inserted into the prey to begin digesting it.

Turbellarians move by swimming, creeping or Turbellarians move by swimming, creeping or crawling. They combine muscular contractions crawling. They combine muscular contractions with ciliary movement to move. with ciliary movement to move.

Class TurbellariaClass Turbellaria

Turbellarians’ outer surface is ciliated and the Turbellarians’ outer surface is ciliated and the animal secretes mucus. Beating of cilia within this animal secretes mucus. Beating of cilia within this viscous mucus allows movement to occur.viscous mucus allows movement to occur.

Turbellarians may also use waves of muscle Turbellarians may also use waves of muscle movement to move. The muscular contractions movement to move. The muscular contractions push and pull the animal forward. push and pull the animal forward.

To help grip the substrate when generating muscle To help grip the substrate when generating muscle contractions turbellarians use paired secretory contractions turbellarians use paired secretory glands on their underside to secrete first a viscous glands on their underside to secrete first a viscous glue and later a second chemical which breaks the glue and later a second chemical which breaks the attachment to the surface.attachment to the surface.

“Polycladida moseleyi is distributed throughout the Mediterranean Sea and the temperate eastern Atlantic. Its favored food are tunicates (Clavelina sp.). “http://www.rzuser.uni-heidelberg.de/~bu6/flat0431.html