L5 The growing complexity of triopoblastic animals
Bilateral symmetry leads to cephalization
Cephalization development of a head cells and sensory structures (eyes, nose, ears) are connected at the anterior end
Heads can be simple, like flatworms with eyespots capable of detecting the intensity of light
Eg, sharks posses well-developed brains and multiple sensors that make them intelligent and fearsome predators
Capable of detecting prey hidden within sandy seabed through electromagnetic sensors
Body cavities are lined by mesodermal cells
Body cavities formed from coelom are internal spaces between the gut cavity and the body outer surface
Alloes organs more space to grow and move independently
The fluids in body cavity acts as a hydroskeleton against which the muscles can act and allow movement
The fluids also transport nutrients, gases and wastes to and from different body regions
Animals can be classified by the presence of a body cavity
Acoelomate animals do not possess a body cavity
Coelomate animals possess a body cavity that is completely lined with tissue that develops from the mesoderm
Pseudocoleomate animals possess a small or partially-lined coelom
Platyhelminth and Nemertea: Acoelomate triploblasts
Phyla Platyhelminthes and Nemertea
Flatworms and ribbon worms are exceptions - bilateral, triploblastic but have solid body
Phylum Platyhelminthes ia commonly known as flatworms while Nemertea is known as ribbon worms
Named for their dorsal ventrally flatten bodies
They are found in fresh and marine waters and in damp soil
Regenerative power of Planarians (free-living flatworms)
The presence of stem cells throughout the bodies allow injured wirms to regenerate into complete organisms
Stem cells are undifferentiated cells that have potential to turn into any cell type in the animal, eg: brain cells or a new head
Model organisms for the study if stem cells at different stages (they are the only ones capable of growing a head)
Flukes and tapeworms are parasitic flatworms
Most flatworms live within the bodies of their hosts from which they obtain their nutrients
Specially adapted for the parasitic lifestyle which result in their different appearance from typical flatworms
They have suckers and hooks for holding onto their hosts
Can resist the digestive chemicals secreted by their hosts
Ribbon worms are also called proboscis worms
The proboscis is a long, hollow, muscular tube that can be rapidly expelled from the anterior end of the body
This is used to capture prey or as defensive behaviout when threathened
Many are marine predators that can swallow prey that are more than double its body width, eg: crusteceans, snails and clams
Key differences between Platyhelminthes and Nemertea
Flatworms (platyhelminthes) lack a digestive tract (or it may be greatly simplified)
Cells obtain oxygen via diffusion through the body wall
Proboscis worms (ribbon worms, Nemertea) have a complete gut (separate mouth and anus) and a closed circulatory system
Protostomes - 2 major clades
Bilateral is slit into the protostomes and deuterostomes
2 major clades within protostomes are the Lophotrochozoa and Ecdysozoa
Most have bilateral symmetry (at least in their larval stages)
Lophotrochozoa
The clade is named after 2 features
The lophophore a set of ciliated tentacles surrounding a mouth for feeding
The trochophore larva that is characteristic of marine annelids and most mollucs
Phylum Annelida: Animals with repeating segments
Annelids possess body cavity
They are coelomates
The digestive and othe organ systems like excretory and circulatory systems are contained within a body cavity
They have separat emouth and anus
Phylum Annelida are known as segmented worms
The term Annelida is from the latin word meaning ‘little rings’
Its body and coelom is organized as a series of ring-like segments
Each segment has muscles that are capable of independent elongation and shortening (ie, one part of the body lengthens whilst another parts contracts
Internal systems are well developed
Closed, circulatory system
Digestive system is complete with separate mouth and anus
Every segment possess repeating locomotion structures
Terrestrial segmented worms possess chitin bristles called setae on most segments
Marine segmented worms possess many setae and parapodia
Paarapodia act like swimming paddled in the water
Setae anchor on solid surfaces to allow body to move
Annelid diversity
Polychaetes ir bristle worms: mainbly marine with trochophore larvae
Clitellates: No parapodia ir trocophore larvae, less developed heads
Blood sucking leeches
Parasites that feed on blood of vertebrates including humans, and mollusks
No setae nor parapodia
A sucker enables attachement to the host
Subclass Hirudinea
Named after Hirudin - an anesthetic, aunty clotitng polypeptide, that is released into the host bloodstream
Medical leech therapy (MLT) is used for pain relief, increasing blood circulation and to break up blood clots in damaged or reconstructed tissues
Molluscs: efficient predators
Phylum Mollusca - protected by shells
Mollusca means soft-bodied
Usually protected by a dorsal shell composed mainly of calcium carbonate
They inhabit freshwater and marine and terrestrial habitats
They have a true coelom that occurs around certain organs
The main body cavity is typically a hemocoel (a space containing blood) but it is not to be confused for a coelom
Broad, muscular foot which is used for locomotion (moving)
Body organs (viscera) are concentrated as a visceral mass located above the foot.
The mantle is a flaplike extension from the body that secretes the shell.
The mantle overhangs the visceral mass to form a mantle cavity containing gills or a lung
Predatory molluscs possess radula for feeding. It is a tongue-like structure equipped with rows of chitinous teeth
Mollusc diversity
Bivalves: clams, oysters, and mussels
Gastropods: snails, slugs, and nudibranchs
Cephalopods: squid and octopuses
The phylum Mollusca consists of 8 clades
Gatropods - the most diverse
Twisting my insides
Gastropoda means ‘stomach foot’
Torsion – the 180° twisting of the visceral mass is a unique feature of the group
Protected by single spiral shell in which they withdraw when threatened.
Torsion is unrelated to the coiling of the shell.
Some gastropods like slugs do not possess shells
Bivalves
Bivalves with two-part shells
The soft bodies are laterally compressed (squeezed from the sides).
Protected and enclosed by a two-part shell hinged at the upper (dorsal) side
Their names are derived from these two-part shells.
When threatened, the bivalve contracts muscles to close the shells and hold them tightly
The foot in some species help them to burrow and hide within the seabed
Radula and a distinct head are absent in these filter feeders
Pearl formation is a defense mechanism
Pearls are the result of a biological process that occurs in response to foreign material being lodged between the oyster’s shell and mantle.
The mantle secretes and covers the irritant with layers of nacre (mother-of-pearl) to protect itself
Most bivalves filter feed
Bivalves actively take in water through a tube called incurrent siphon.
Food particles are trapped in the gills and directed to the mouth and digestive system.
Non-food particles like sediment form bigger particles and are removed via excurrent siphon
Bivalves are important indicators of water pollution they help clarify water and improve water quality by retaining excess nutrients and toxic pollutants
Invasive molluscs
The assassin snail was introduced to Singapore by the aquarium pet trade to control snail pests
It has been found in local reservoirs.
The American brackish water mussel was introduced to Singapore’s northern coast by ships
It displaces native Asian green mussel grown for sale by fish farmers and competes with vulnerable horseshoe crabs for habitat
Cephalopods
Cephalopods have their foot on their head
Cephalopod means head-foot
The foot is modified into tentacles to catch prey.
Suckers along tentacles seize and hold onto prey.
Two strong beaks kill and tear prey into small pieces
Cephalopods lack an external shell
Cephalopods do not possess an external shell, except nautilus.
Nautilus have been around for 550 million years and are called living fossils
The nautilus shell is divided into chambers. Most of the chambers are filled with gas to allow the animal to float.
Some cephalopods possess internal shell
Cuttlefish and squid possess an internal shell, that is commonly called bone (cuttlefish) or pen (squid)
The cuttlefish internal shell is porous It can be filled with gases to keep the animal buoyant
Cephalopods have 3 hearts
An active lifestyle requires cells to have access to high amounts of energy and oxygen.
Cephalopods use copper-based blood to transport oxygen. Copper-based blood transports lesser oxygen than iron-based blood in humans.
The presence of three hearts in cephalopods allow the animals to overcome the limitation of copper-based blood with an efficient circulation in the body
Anti predator behaviors
Ink is released from an ink-sac to confuse and temporary blind the predator and allow for escape
Expelling water from their bodies through a siphon creates a jet propulsion effect and speedy escape.
masters of camouflage
Pigment cells called chromatophores allow cephalopods to change colours rapidly - as a camouflage, warning signal or means to escape from predators
Cephalopods have good eyesight and brain
Eyes are as complex as a human eye. They can focus and form images.
Octopuses have 9 brains: 1 main brain and 1 mini brain in each tentacle to allow independent and more effective movement of tentacles.
Exhibit intelligence - short-term memory, solve mazes and tasks for food and can escape from aquarium