Types of Cells in Sponges

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Study Notes

Sponges are simple multicellular organisms, lacking tissues and organs
Fundamental processes like respiration and excretion occur through diffusion on a cellular level.
Pinacocytes are thin, flat, epithelial-type cells covering the sponge's exterior and some interior surfaces.
Mesohyl is a gelatinous matrix containing skeletal elements and amoeboid cells.
Sclerocytes secrete spicules (skeletal elements).
Spongocytes secrete spongin (skeletal elements).
Collenocytes secrete collagen (skeletal elements).
Choanocytes are flagellated collar cells that generate water currents through the sponge and ingest suspended food.
Archaeocytes (amoeboid cells) receive food particles from choanocytes and perform digestion.
Digestion occurs entirely within cells, without a digestive tract.

Sponges are suspension feeders, capturing food particles suspended in water that flows through their bodies.
Water enters through incurrent pores called dermal ostia.
Choanocytes collect food particles from the water current.
Choanocytes phagocytize small particles and pinocytize protein molecules.
Sponges can also absorb dissolved nutrients from the water.

Most sponges are hermaphrodites, functioning as both male and female.
Gametes are derived from archaeocytes.
Most sponges are viviparous, retaining the fertilized zygote and nourishing it, releasing ciliated larvae later.
Some sponges are oviparous, releasing gametes into the water.

Sponges have impressive regeneration capabilities.
Regeneration from fragmentation is a form of asexual reproduction.
External buds can break off to form new sponges.
Internal buds (gemmules) in freshwater sponges can remain dormant during droughts.

Cnidaria and Ctenophora are radially symmetrical, diploblastic animals (two embryonic tissue layers).

All animals except sponges belong to the Eumetazoa clade (animals with true tissues).
Cnidaria is one of the oldest groups within Eumetazoa.
Cnidarians are mostly marine, with some freshwater species.

Cnidarians exhibit radial symmetry.
Two body plans are common in their life cycles: polyp (cylindrical body) and medusa (umbrella shape).

Polyps are sessile (stationary).
They have a cylindrical body with an opening at one end, the mouth, surrounded by tentacles.
Examples include hydra, sea anemones, and corals.

Cnidarian body wall consists of three layers:
- Outer epidermis (ectoderm-derived).
- Inner gastrodermis (endoderm-derived) lining the gastrovascular cavity.
- A gelatinous material called mesoglea between the two layers.
Cnidarians exhibit tissue-level organization rather than organ-level.

Cnidarians have stinging cells called nematocysts.
Nematocysts are specialized cells containing a barbed or venomous coiled thread that can be projected for defense or prey capture.

Hydrozoans, jellyfish, sea anemones, and corals represent diverse cnidarian groups.

Hydrozoan colonies usually contain two types of polyps:
- Feeding polyp (gastrozooid).
- Reproductive polyp (gonozooid).

Most hydrozoans exhibit an alternation of generations between polyp and medusa stages.

Hydroid medusae are usually smaller than those in the class Scyphozoa.
A muscular ring called the velum controls the diameter of the opening, aiding in swimming.
Specialized organs:
Statocysts for balance, detecting gravity and orientation.