Lophotrochozoa Flashcards
Flatworms - What are they?
Flatworms, flukes and tapeworms i.e. free living / parasitic lifestyles
~ 20,000 species
Reproduce asexually & sexually
Triploblastic development
Mesodermal layer
“Acoelomates” – no body cavity
Parasitic group (Neodermata- with neodermis)
Triploblastic organisms have 3 tissue layers
Triploblastic development
(acoelomates)
No organs for gas exchange, gases have short distance to diffuse so no need
Turbellaria
Phylum: Platyhelminthes, Class: Turbellaria, Order: Polycladida
Many colourful marine species – other live in freshwater / brackish water
Turbellaria - Nutrition and digestion
Gastrovascular cavity –simple or branched
Proteolytic enzymes for extracellular digestion (external digestion)
Cells phagocytise small food particles in the intestine (intracellular digestion)
Sac like gut: undigested items ejected via mouth
No circulatory system. Food distributed by branched Gastrovascular system
The pharynx connects the mouth and intestines- and can be protuded to capture prey
Many are carnivores, some feed on algae / detritus
Some catch their prey by entangling them with slime
Turbellaria - Locomotion
The body is covered by cilia
Cilia provide force for propulsion
Turbellaria - Excretion and osmoregulation
Nitrogenous waste is eliminated across the general body surface
Osmoregulation is also achieved with protonephridia
The protonephridium are of the flame-cell type
Bundle of flame cells = protonephridium
PROTONEPHRIDIA (network of tubules)
a simple apparatus for excretion and osmoregulation
Turbellaria - Asexual reproductio and regeneration
Asexual reproduction occurs via fission
20% of adult cells are pluripotent stem cells- highly regenerative
Turbellaria - Nervous system
Nerve cords with a simple brain
Bilaterally symmetrical ie have head
Eye spots & photosensitive behaviour
Can learn to overcome light avoidance- but ‘memory’ retained even if have to grow new head
Rotifera, Bryozoa and Brachiopoda
Clade named after Lophophore
Crown of Ciliated tentacles
Present in some members of clade
And after trochophore larva
Shaped like a spinning top
Very small and generally translucent
Prominent circlet of cilia
Present in some members of clade
But some members of the clade lack both these features
Bryozoa and Brachiopoda
Lophophorates
Crown of ciliated tentacles
U-shaped alimentary canal
No head
Sessile
“Coelomates”
Bryozoa
Bryozoa meaning “moss animals”
~ 4000 species
Small (~0.5mm)
Colonial
Widespread and numerous
Hard exoskeleton
Most species live in the sea, where they are widespread and numerous sessile animals.
Brachiopods
Sessile bottom dwellers
Resemble bivalve Molluscs e.g. mussels, cockles
Shells on dorsal & ventral sides (unlike bivalves)
All marine
~ 350 extant species
~ 30,000 fossil species in
Paleozoic & Mesozoic seas
Anatomy
The pedicle valve (dorsal) is larger than the brachial valve (ventral)
The pedicle attaches the animal to the seabed
Organs in coelom with contractile heart
Lingula
Often cited as oldest extant genus on earth
Apparently virtually unchanged for at least 400 million years
Some species now considered threatened
Rotifera
Name means wheel bearer
- referring to crown of cilia at anterior end
They have a mouth, stomach and anus
Anterior end forms the head region
The corona→ water currents to mouth
Used for feeding & locomotion
Modified muscular pharnyx (Mastax)
Mastax has intricate jaws (trophi)
Trunk contains visceral organs
Not a coelomate (hemocoel/pseudocoel)
Rotifera typical body plan
Have specialized organs but limited number of cells (around 1000)
Cloacal bladder collects excretory / digestive wastes
Protonephridia with flame bulbs (same function as in Platyhelminthes)
Foot segmented, telescopic, 1-4 toes
Rotifera - reproduction
Gonochristic reproduction in seisonidea
Parthenogenesis alternating with sexual reproduction in Monogononta
Obligate parthenogenesis in Bdelloidea
Bdelloidea:exclusivley asexual reproduction
No sex for 40-100 million years
Platyhelminthes (class tubellaria) - sexual reproduction
Simultaneous hermaphrodites
Most have one or two pairs of testes and ovaries
Eggs are fertilized internally by copulation
The forms of impregnation are bizarre
Self fertilization is rare
Trematode ‘flukes’
Snails as intermediate host – infected by miracidium =ciliated larvae
Definitive host infected by cercaria (motile larvae)
Larvae disperse in water
Cestoda
Tapeworms
~4000 gut endoparasites parasitic
Definitive hosts vertebrates
Intermediate hosts arthropods/vertebrates
Gastrovascular cavity lost
Scolex with hooks & suckers
Proglottids - reproductive units
Dangers of eating meat with cysticeri
Eating meat with cysticerci produces intestinal infection. Infection with eggs / gravid proglottids (from faeces) means that cysticerci can form in connective tissue & brain.
Cysticerci in brain can cause epilepsy & death
The generalised mollusc
Phylum both conspicuous and familiar
Includes forms such as clams, oysters, squids, octopods and snails
Strong morphological divergence from hypothetical ancestral body plan
Mollusc - mantle morphology
Dorsal surface covered by shell that protects internal organs
Epidermis called mantle secretes the shell
Epidermal secretions are continual so shell increases
Mollusc - gills
A pair of gills project from mantle cavity
Water enters the lower mantle cavity, up, through the gills and posteriorly back
Water pushed through cavity by beating of cilia
Two blood vessels run over gills and are oxygenated
Mollusc - Bivalvia
Mussels, clams, scallops, oysters, etc
Size range ~2mm→ 1.2m Tridacna
Most marine, few brackish and fresh water forms
Sedentary suspension feeders
No head or radula
No brain: nerve network with a few ganglia
Left and right valves that attach and articulate with each other
Mollusc - Shell
Dorsally held together by hinge ligament
When the valves are closed (with Adductor muscle) the outer hinge ligament is stretched and the inner compressed
The valves are locked together by teeth or ridges that are located on the hinge
Mollusc shell morphology
- The valves of the shell are pulled together by adductor muscles
- Shell is made of calcite, aragonite and protein (conchiolin)
- The adductors are attached anteriorly and posteriorly
- There are scars on the inner surface of the valve where the muscles attach
- There are several benefits to have shell + adductors; They help defend, prevent desiccation, support internal organs
In contrast to the ‘generalised mollusc’ body plan, in Bivalves…
- The edges of the mantle folds form incurrent and excurrent siphons
- Cilia direct water flow over gills for gas exchange and filter feeding
- The radula is absent
Mollusc - feeding
Most r filter feeders
The gills have been modified to increase food capture
The cilia on the gills capture food particles
Food particles are transported in a steady stream of mucus to the mouth
Mollusc - digestion
Cilia in the stomach cause the crystalline style to rotate and churn the contents
This directs the suitable particles to the digestive gland for intracellular digestion
Waste material is pushed to the rectum and is removed on the exhalent water stream via the anal pore
Generalised bivalve body plan
- The foot is attached ventrally
- Gills are covered by mantle folds
- Edges of mantle folds form
incurrent and excurrent siphons - Cilia of gills mantle direct water
flow over gills for gas exchange & filter feeding - Mantle takes up some of function in gas exchange
Bivalve - feeding and digestion
Most are suspension feeders, but…
Giant clams receive most of their nutrients from zooxanthellae
Some have unusual diets- such as driftwood. Teredo ‘worm’ is actually a bivalve
A big problem in the days of wooden ships- addressed by using copper plated bottoms.
Cellulytic digestion aided by symbiotic bacteria
