Question

Which category of organism exhibits cell aggregate body plan?

Answer 1

Hint: Cell aggregate body plan is a type of body plan where cells are scattered randomly but perform common functions together. Cells are the basic unit in the level of organisation and perform vital functions in an organism. They do not form tissue in lower animals.

Complete answer:
The members of phylum Porifera possess a cell aggregate body plan. These are commonly known as Sponges. They are also known as 'pore- bearers' as their body wall possess numerous tiny pores. All sponges are aquatic; most of them are marine but a few species are fresh-water like Spongilla.
Sponges are sessile organisms. They obtain the food and other substances through the water entering their body.Sponges are the primitive multicellular animals. Most of them are asymmetrical. The cells in their body do not form tissues.
All cells act more or less independently and show little coordination. The numerous pores in the body wall of sponges are meant for entry and exit of water current. Minute pores present in the body through which the water enters are called ostia. The water exits outside through a large opening known as the osculum. The canals and the spongocoel are internally lined by characteristic flagellated cells called choanocytes.
Functions of the canal system include food gathering, respiratory exchange, removal of waste and transfer of sperms. Digestion of sponges include food vacuoles as they are the filter feeders. Almost all sponges possess an internal skeleton called an endoskeleton. This internal skeleton provides the support to the body and comprises spicules or spongin fibres that may be calcareous or siliceous. All sponges are hermaphrodite i.e. sexes are not separate. They are also called bisexual. Both asexual and sexual reproduction occurs. They undergo an internal fertilization that results in the formation of diploid zygote.

Note:
Some sponges host photosynthesizing micro organisms and green algae as endosymbionts. These alliances often produce more food and oxygen than they consume. This alliance is significant along Australia's Great Barrier Reef. The sponges here have flexible whips, fingers and fans adapted for coping with high currents.