BIOSCI 108: Lecture 27 - Molluscs & Echinoderms

Protestome vs. Deuterostome Development

How they develop in the zygote phase differs.

• Cleavage: Protostomes experience spiral & determinate. Deuterostomes experience radial & intermediate.

• Coelom Formation: Protostomes develop them near the blastopore at the bottom. Deuterostomes develop them opposite to the blastopore

• Mouth: Protostomes devleop the mouth first. Deuterostomes develop the mouth second (anus first)

Hox Genes

Groups of genes that specify how structures develop different segments of the body.

All animals have variation in the primary group of Hox genes which determines their morphology. Some groups may have gained new similar ones, or lost some groups.

Morphogenesis

The changes that occur as the body and organs develop. This involved specific changes in cell shape, position, and survival.

How the cell is shaped, positioned, and so on will essentially decide the shape of the body, organs and so on.

• This includes things such as adhesion, fusion, cell migration, proliferation, and so on

Molluscs

A diverse group of protestome organisms that have a soft body, some having a shell, and are bilateral.

They’re non-segmented coelomates, have gills, and live in a variety (marine, freshwater, terrestrial) of habitats.

• They make up ¼ of all marine life

They have two main distinctive morphological characters

• Lophophore

• Trochophore

Lophophore

A feeding structure with ciliates crown around the mouth, used for filter feeding.

Trochophore

The specific diamond-shaped larval stage that molluscs have.

Molluscs Basic Body Plan

There’s three main body regions:

• Head

• Mantle

• Radula

They’re also covered in a mucus that serves as protection and can also help with feeding.

They also have a ventral foot.

Mollusc Head

The visceral body region with midgut, intenstine, gonoperidcardial system, and lateral visceral nerve cords.

Like many bilaterians, the brain and sensory organs are focused there.

Mollusc Mantle

An extension of the dorsal body wall, making a visceral flappy mass. It has various uses including movement, protection (with a shell), camouflage, communication, and so on.

• Gastropods mantles are hidden inside their shell and excretes the shell

• Bivalves have a coloured mantle which protects it from bright sunlight

• Cephalopods mantles are the outer body walls and fins on the head that aid in movement (and can sometimes camouflage)

Mollusc Radula

A spikey ribbon-like structure (like a coveyor belt of spikes) found in the mouths of all molluscs besides bivalves.

Mollusc Foot

A visceral mass on their ventrical that is used for movement, adhesion, digging, and so on.

• Gastropods use them for crawling and adhesion

• Bivalves use them for digging, burrowing, and crawling

• Cephalopods modified them into arms and tentacles for swimming and feeding

Variation in Mollusc Body Plans

The diversity seen in molluscs is caused by variation in the main body plan parts mentioned prior as well as variations in the shape and structure of their embryonic tissue.

Hox genes control how specific parts will develop and where.

• It is thought that the ventral and dorsal parts have been decoupled, allowing for more specialisation and differents between the two.

Echinoderms

Are deuterostomes that develop into bilateral larva, but later develop into pentamerous radial symmetrical adults.

• This is where there is symmetry within a circle, with each part of it being repeated. So, for example, each arm of a starfish has the same internal organs and parts

They only live in marine habitats. To move around, they have a water vascular system where coelomic tubes are fluid filled and have tube feet at the end.

They also secrete a shell to make an endoskeleton that is embedded in their dermis. They also have amazing regenerative capabilities.