Porifera (Sponges) Overview

Study Notes

Porifera (Sponges)

Phylum Porifera is part of the Kingdom Animalia.
Sponges are placed in the Parazoa group due to their evolutionary difference from protozoa and metazoa.
They are multicellular but lack true tissues.
Sponges are mostly marine, but some live in freshwater.
They come in various shapes (spherical, flat, vase-shaped, irregular).
Color variations include red, yellow, orange, purple, green, and black.
Symmetry is typically radial in primitive species, but most are asymmetrical.
The body wall is pierced by tiny pores (ostia) that lead to channels.
A prominent feature is the oscula (exhalant openings).
Sponges have specialized cells, but they lack coordination among these cells, not true tissues.
Sponges have skeletons of lime (calcium carbonate), silica, or organic fibers (spongin).
They don't have sensory or nerve cells, thus their responses to stimuli are limited.
Nutrition is holozoic, meaning they take in food through phagocytosis.
They lack a digestive tract.
The interior space is called a spongocoel (paragastric cavity).
Choanocytes (collar cells) with flagella are present in the spongocoel for feeding and water circulation.
Respiration and excretion occur by diffusion.
Sponges reproduce asexually by budding or forming gemmules.
They also reproduce sexually by forming gametes.
They have no circulatory or muscular systems; they rely on water flow generated by choanocytes to obtain food and oxygen and remove waste.
About 5,000 living species are classified into three classes: Calcarea, Desmospongia, Hexactinellida, based on skeleton composition.

Cellular Structure

Sponges have three cell layers:
- Pinacoderm (dermal layer): Outer layer of flattened cells.
- Choanoderm (gastral layer): Inner layer of flagellated choanocytes.
- Mesohyl (mesenchyme): Gelatinous matrix between the two layers, containing amoebocytes and skeletal elements (spicules or spongin).
Pinacocytes: Flat, leathery cells lining exterior.
Archaeocytes (amoebocytes): Amoeboid cells, capable of differentiating.
Sclerocytes: Produce spicules.
Porocytes: Surround canal openings (pores).
Spongocytes: Secrete fibrous collagen called spongin.
Myocytes: Contractile cells.

Sponge Types

Asconoid: Simplest type, small, tubular form.
Syconoid: More complex, with folded body wall creating channels.
Leuconoid: Most complex, with numerous chambers.

Nutrition

Sponges are filter feeders, obtaining food from the surrounding water.
Microorganisms and organic particles are filtered through the sponge's structure.
Food is phagocytosed by choanocytes and archaeocytes.
Some species can also capture prey with hooks, spicules, or tentacles.

Respiration and Excretion

Respiration occurs via simple diffusion of dissolved oxygen in the water.
Excretion mainly happens through ammonia diffusion.
Fresh water sponges use contractile vacuoles to remove excess water.

Reproduction

Sponges reproduce asexually through budding or creating internal buds called gemmules.
Gemmules are resistant to harsh conditions, facilitating survival during unfavorable periods.
Reproduction can also be sexual, with sponges being hermaphroditic.
External fertilization happens in surrounding water.
Internal fertilization is also possible, where sperm enters through the aqueous system.
Development involves blastula and amphiblastula.