Case Study
Passage with linked questions
Case Set 1
Case AnalysisPassage
During a marine biology field trip, students collected water samples near a coral reef and observed small, irregularly shaped organisms attached to rocks. Under a microscope, they noticed the organisms had numerous tiny pores on their surface and a larger opening at the top. The teacher explained that water enters through the smaller pores and exits through the larger opening after passing through an internal cavity. Inside the organism, specialised collar cells were visible lining the internal canals. The skeleton appeared to be made of needle-like structures. The teacher further noted that these organisms are among the most primitive multicellular animals and that digestion occurs entirely within individual cells rather than in a central cavity. Students were asked to identify the phylum and analyse the functional significance of the structures observed.
Question 1: Identify the phylum to which the observed organisms belong and name the two types of openings described in the passage.
- The organisms belong to Phylum Porifera (sponges), which are primitive multicellular marine animals with cellular level of organisation.
- The smaller pores through which water enters are called ostia, and the larger opening through which water exits is called the osculum.
- The internal cavity into which water from ostia collects before exiting through the osculum is called the spongocoel.
Question 2: What are the collar cells (choanocytes) mentioned in the passage, and what is their role in the life of sponges?
- Choanocytes or collar cells are specialised flagellated cells that line the spongocoel and the internal canals of sponges.
- They create water currents by beating their flagella, ensuring a continuous flow of water through the canal system for filter feeding.
- They trap and engulf food particles (bacteria, organic matter) suspended in the water passing through the canals, carrying out intracellular digestion.
Question 3: The needle-like skeletal structures observed are called spicules. Explain the types of skeletal materials found in sponges, and analyse how the water canal system serves multiple physiological functions simultaneously.
- Sponge skeletons are made of either spicules (composed of calcium carbonate or silica, providing rigidity) or spongin fibres (a flexible protein material, as in Euspongia used commercially as bath sponges), or a combination of both.
- The water canal system simultaneously performs food gathering (choanocytes filter food particles from incoming water), respiratory exchange (oxygen from water diffuses into cells, CO2 diffuses out), and waste removal (metabolic wastes are carried out with exiting water).
- This multi-functionality of a single system is a hallmark of the cellular level of organisation in Porifera, where there are no specialised organs — the canal system integrates nutrition, respiration, and excretion into one continuous water flow.