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