Question

1)Compare / contrast the following and list an example organism for each: Acoelom, Pseudocoelom, True Coelom.

2) List the major component organisms that makeup lichens. Provide the name of each of the 3 main growth forms of lichens.

3) Explain why Echinoderms are classified as Bilaterians when their adult form most frequently shows radial symmetry.

Answer 1

1) Coelom is the body cavity of almost all organism. Based on the presence or absence of coelom or the type of coelom, animals can be divided into 3. Acoelomate, Pseudocoelomate and True coelomate.

Acoelom	Pseudocoelom	True coelom
Acoelome is the absence of body cavity or coelom. Mesoderm completely covers the space between Gut and bodywall	Pseudocoelome or false coelom is a second bodycavity (other than the gut). It is present between the mesoderm and endoderm.	True coelom is the presence of body cavity (fluid filled) between the gut and bodywall.
No peritonium present	Peritonium lines the body cavity partially.	Peritonium lines the body cavity completely.
No transport medium	Nutrients are circulated through osmosis and diffusion	Nutrients are circulated through the blood stream.
Organs are embeded in the mesoderm derived parenchymatic cells	Not a well organised organs.	Well organised organs.
Organisms without a body cavity is called acoelomate organisms.	Organisms with pseudocoelom are called hemocoelomates or blastocoelomates	Organisms with true coelome are called coelomates.
Seen in invertibrates	Seen in invertibrates	Seen in both vertibrates and invertibrates.
Eg : Platyhelminthes (flatworms)	Eg: Nematodes, Rotifers, Entoprocta etc	Eg: Annelida, Arthropoda, Mollusca etc

2) Lichen is the association of algae (cyanobacteria) and fungi. Here the fungus provides shelter to the algae and the algae provides food to the fungus.

Three main growth forms of lichen are as follows : Foliose, Fruticose and Crustose

• Foliose lichen has a leaf like shape which is lobed.
• Fruticose lichen have upright branches which is pendulous.
• Crustose lichen forms a crust like structure.

3) Even though echinoderm adults are radially symmetrical, they are classified as bilaterally symmetric organisms. This is because they are developed from bilateria group. Their larvae is bilaterally symmetrical and they show radial symmetry only in certain stages of their life as an adaptation for their sessile character.