Download Marine turbellarian Dugesia tigrina

Bilateria


Having completed the radially symmetrical
Cnidarians and Ctenophores we now move
on to the remaining animals, all of which
are bilaterally symmetrical (or secondarily
radially symmetrical [the Echinoderms]).
These are the Bilateria.
Bilateria



All Bilateria are triploblastic and the members
are assigned to two major groups, which we
have already met, the Protostomia and the
Deuterostomia.
Deuterostomes include the Echindoerms,
Hemichordates and Chordates.
Protostomes include all the other bilateral
invertebrates including Platyhelminthes,
Annelida, Mollusca, and Arthropoda.
Protostomia divisions

Classification of invertebrates is in a state of flux for several
reasons:




molecular phylogenetic studies have cast doubt on traditional
invertebrate classification.
many invertebrates are soft-bodied so fossils of many groups are rare
or unknown, which makes relationships between groups hard to
decipher.
Traditional major groups of protostomes (acoelomate,
pseudocoelomate and coelomate) appear not to be monophyletic.
For this class, we will use the Deuterostome/Protostome
arrangement and recognize two majors groups of
Protostomes: the Lophotrochozoa and the Ecdysozoa.
Protostome Divisions: the
Lophotrochozoa and Ecdysozoa.


Protostomes are divided into two large
groups the Lophotrochozoa and the
Ecdysozoa.
The relatedness of phyla within these two
groups is not entirely clear and will likely
change in the future.
Lophotrochozoa and Ecdysozoa


Lophotrochozoa: members generally possess a
trochophore larva [free-swimming oval or
pyramidal ciliated larva with a band of cilia
around the body] or a lophophore [tentacle
bearing arm which contains within it an
extension of the coelomic cavity].
Ecdysozoa: members shed their cuticle as they
grow
Trochophore larva
http://www.microscopy-uk.org.uk/mag/imgjan09/image006.jpg
Bryozoan lophophore
http://www.bryozoans.nl/pictures/
figuren/anatomy.jpg
Lophophore: characteristic feeding structure of members of the
Brachiopoda, Bryozoa and Phoronida
Lophotrochozoa and Ecdysozoa

Lophotrochozoa members:


Platyhelminthes, Annelida, Mollusca, and a
diverse array of “lesser phyla” including
Nemertea, Gnathostomulida, Rotifera, and
Sipuncula.
Ecdysozoa members:

Nematoda, Arthropoda and “lesser phyla”
including Onychophora, Tardigrada, and
Priapulida.
“Worms”



The term worm is loosely employed in biology
and is applied to very different animals including
the segmented worms (Annelids), roundworms
(pseudocoelomates) and a variety of acoelomate
bilateral animals.
“Worm” describes any bilaterally symmetrical,
legless, soft-bodied animal at least 2-3 times as
long as it is wide.
One group of worms traditionally recognized was
the acoelomate worms.
Acoelomate worms

Three phyla of acoelomate worms were
traditionally grouped together because
they lacked a coelom and had a solid body
filled with parenchyma cells:



Platyhelminthes: flatworms
Nemertea: ribbon worms
Gnathostomulida: jawed worms
Baseodiscus delineatus (Nemertea) http://tolweb.org/images/Nemertea/2489
http://tolweb.org/images/Gnathostomulida/2481
Acoelomate worms


However, molecular evidence based on
ribosomal RNA sequences suggests the
Nemertea are more closely related to Annelids
than Platyhelminths.
Similarly, Gnathostomulids do not have free
swimming larvae and their sperm morphology is
very different from the Platyhelminthes, which
suggests they aren’t close relatives.
Acoelomate worms


Because of the dubious relatedness of the
phylum Platyhelminthes to these other groups
we will not deal with them until later in the
semester.
However, the phylum Platyhelminthes, is a very
important group which includes a wide variety of
parasitic forms such as the flukes and
tapeworms.
Phylum Platyhelminthes

Unlike other animals encountered so far,
Platyhelminthes:




have evolved cephalization with their sense
organs concentrated at the head end.
Possess the beginning of a ladder-type
nervous system.
are bilaterally symmetrical.
Are dorsoventrally flattened
Phylum Platyhelminthes

In addition they are



triploblastic, but lack a coelom. Instead, they
have a solid body filled with parenchyma cells.
have evolved organs and in some cases organ
systems.
The first (and simplest) excretory or
osmoregulatory systems and circulatory
systems are found in members of these
groups.
Phylum Platyhelminthes



Platyhelminthes typically have dorsoventrally flattened
bodies, usually slender and leaflike or ribbonlike.
The advantage of a flat body is that it increases surface
area and allows the animal to exchange gas and lose
wastes by diffusion.
Four classes in the Platyhelminthes. The Turbellaria are
free living whereas as members of the Monogenea,
Trematoda and Cestoda are parasitic.
Nutrition



The digestive system includes a mouth, pharynx,
and blind intestine (the gut is incomplete)
In the free-living Turbellarians the pharynx can
be everted from the mouth.
Food is sucked into the intestine where a
combination of extracellular and 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
Nutrition


Undigested food exits via the pharynx.
In the Cestoda the digestive tract is
absent and all nutrients are absorbed
across the tegument (the syncytial
membrane/body covering found in all
parasitic Platyhelminthes).
Excretion/Osmoregulation



The osmoregulatory system consists of a series of canals
that end in flame cells or protonephridia.
The flame cell consists of a fine-meshed cup that
contains cilia. The beating of the cilia draws fluid which
is filtered as it passes into the cup.
This system appears mainly intended to remove excess
fluid, but retain essential ions. It is most developed in
freshwater Turbellarians, but reduced or absent in
marine species, which do 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 organs


Flatworms possess a simple brain and one to
five pairs of longitudinal nerve cords that are
cross connected to form a ladder-like
arrangement.
There has been a tendency towards reduction of
the number of pairs of nerve cords and
increased development of the ventral pair. A
similar evolutionary pathway may have led to
the development of the ventral nerve cord found
in annelids and arthropods.
Nervous system of Dugesia
http://biodidac.bio.uottawa.ca/ftp/BIODIDAC/ZOO/PLATYHEL/DIAGCL/TURB007C.GIF
Nervous system and sense organs


Neurons are specialized for different tasks
e.g. sensory and motor functions, which is
an important advance in the evolution of
nervous systems.
There are a number of different sensory
cells found in flatworms and tactile and
chemoreceptive cells are abundant.
Nervous system and sense organs


In freshwater Planarians concentrations of
sensory cells form two ear-like structures
(the auricles) found on the side of the
head.
Light sensitive eyespots or ocelli are
common in all classes but Cestoda.
Freshwater Planarians:
http://www.aecos.com/CPIE/flatworm.jpg
Reproduction


Reproduction in the Platyhelminthes can
be asexual or sexual. However, most are
hermaphroditic and cross fertilize.
In parasitic forms sexual and asexual
reproduction may alternate in different
stages of the life history
Classification of Platyhelminthes

There are four classes in the
Platyhelminthes:




Class Turbellaria: free-living flatworms.
Class Trematoda: endoparasitic flukes
Class Monogenea: parasitic flukes that are
mainly ectoparasites
Class Cestoda: tapeworms
Class Turbellaria



Class Turbellaria contains about 3000 species.
There is considerable debate about the
classification of the class and it is likely that the
class is not monophyletic.
Most species are marine and benthic (move
around on the bottom in aquatic environments).
Some also found in fresh water or in moist
temperate and tropical terrestrial habitats.
Figure 14.10
8.2
Marine turbellarian
Dugesia tigrina, a freshwater turbellarian
© Mauricio A. Muñoz
Class Turbellaria



Most Turbellarians are predators of invertebrates smaller
than themselves. Other species are herbivores or
scavengers.
In many species the pharynx is protrusible and can be
inserted into the prey to begin digesting it.
Turbellarians move by swimming, creeping or crawling.
They combine muscular contractions with ciliary
movement to move.
Class Turbellaria



Turbellarians’ outer surface is ciliated and the animal
secretes mucus. Beating of cilia within this viscous mucus
allows movement to occur.
Turbellarians may also use waves of muscle movement to
move. The muscular contractions push and pull the
animal forward.
To help grip the substrate when generating muscle
contractions turbellarians use paired secretory glands on
their underside to secrete first a viscous glue and later a
second chemical which breaks the 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