Study Notes on Cnidarians and Sponges
Physiological Processes in Sponges
Existence and Longevity
- Sponges have existed on Earth for over 500 million years despite their simple structure.
Digestive System
- Sponges lack a true digestive system.
- Energy intake occurs through the intracellular digestion performed by specialized cells known as choanocytes.
- Limitation: Food particles must be smaller than individual sponge cells for digestion to occur.
Other Physiological Processes
- Gas Exchange, Circulation, and Excretion:
- These processes occur via diffusion between water and sponge cells.
Reproduction
- Sponges can reproduce both sexually and asexually.
- Asexual reproduction methods:
- Fragmentation: A portion of the sponge breaks off and develops into a new individual.
- Budding: An outgrowth arises from the parent sponge and eventually detaches.
- Gemmule Formation: Unique to freshwater sponges, gemmules are clusters of cells encased in a tough outer layer, which allows them to survive harsh environmental conditions and later develop into new sponges when conditions improve.
Sexual Reproduction
- Sponges are monoecious (hermaphroditic), capable of producing both eggs and sperm.
- Sequential hermaphroditism occurs, where sponges produce eggs first and then sperm:
- Eggs develop from amoebocytes and are kept within the spongocoel.
- Sperm develops from choanocytes and is released through the osculum, where it can fertilize eggs from other sponges carried by water currents.
- The early stages of larval development happen inside the sponge before free-swimming larvae are released through the osculum, marking the only mobile phase of their life cycle.
Adult Form
- Adult sponges are sessile, meaning they remain fixed to a substrate.
Cnidarians
General Overview
- The phylum Cnidaria exhibits radial or biradial symmetry and is diploblastic.
- Approximately 99% of cnidarian species are marine organisms.
Cnidocytes and Nematocysts
- Cnidarians possess specialized cells called cnidocytes, or