Experiment 01Invertebrate BiologyMicroscopic Whole Mounts
Microscopic Examination of Invertebrate Whole Mounts: Protozoan Micromorphology & Hydra Cnidocyte Eversion
Lab Duration: 3 Hours • Paramecium caudatum & Hydra vulgaris • High-Power 400x
🎯 Aim: To observe living cultures and permanent stained whole mounts of *Paramecium caudatum* under brightfield microscopy, identify nuclear dimorphism and contractile vacuole cycles, stimulate trichocyst extrusion, and analyze the discharging mechanics of *Hydra vulgaris* penetrant (stenotele) cnidocytes.
🔬 Principle & Functional Invertebrate Morphology
Unicellular and basal diploblastic invertebrates possess specialized subcellular organelles and cellular effector complexes that perform functions analogous to organ systems in higher metazoans:
- Paramecium caudatum (Phylum Ciliophora): An asymmetric slipper-shaped ciliate enclosed within a rigid elastic pellicle covered by $sim 2,500$ coordinated cilia. Key adaptations include:
– Nuclear Dimorphism: Possesses a large polyploid Macronucleus (directs somatic metabolism and protein synthesis) and a small diploid Micronucleus (maintains germline DNA for sexual conjugation).
– Osmoregulation: Two pulsatile Contractile Vacuoles (anterior and posterior) surrounded by radiating feeder canals cycle between diastole (filling) and systole (discharging water to the exterior through a pellicular pore).
– Defense Effector (Trichocysts): Subpellicular spindle-shaped organelles that discharge fine barbed filaments in response to mechanical or chemical irritants (dilute acetic acid). - Hydra vulgaris (Phylum Cnidaria): A diploblastic freshwater polyp exhibiting radial symmetry. The ectoderm contains specialized stinging cells termed Cnidocytes:
– The cnidocyte houses a fluid-filled collagenous capsule (nematocyst) containing a coiled invaginated hollow thread bathed in toxic Hypnotoxin (a mixture of neurotoxic proteins and phenols).
– Mechanical contact of prey with the modified sensory cilium (cnidocil), combined with chemical cues (glutathione), triggers a massive increase in capsular hydrostatic pressure ($> 150,text{atm}$) due to rapid water influx.
– The operculum pops open, and the shaft with 3 large recurved barbs and helical spine rows everts inside-out in $< 3,mutext{s}$ (accelerations reaching $5 times 10^6,g$), penetrating prey cuticle to inject paralyzing venom.
Figure 1.1: Cellular morphology of Paramecium caudatum alongside the high-speed discharge mechanics of a Hydra stenotele cnidocyte.
📋 Step-by-Step Observation Protocol
- Live Paramecium Culture Mount: Harvest a drop from the scum layer of a rich hay infusion culture of *Paramecium*. Place on a clean slide. Because *Paramecium* glides rapidly via ciliary beating ($> 1,text{mm/s}$), add a drop of $10%$ methylcellulose (or a strand of absorbent cotton wool) to create a physical mesh that slows swimming. Apply a coverslip.
- Organelle Observation & Vacuolar Cycling: Examine under $100times$ and $400times$. Locate the anterior oral groove, cytostome, and food vacuoles circulating via cyclosis. Focus on the posterior contractile vacuole: time the interval (in seconds) between successive systolic discharge contractions. Note that the posterior vacuole contracts faster than the anterior due to higher water influx from the cytopharynx.
- Induced Trichocyst Discharge: Introduce a drop of $1%$ dilute acetic acid (or methylene blue) at the edge of the coverslip, drawing it across with a strip of filter paper. The chemical shock instantly triggers explosive extrusion of needle-like trichocyst shafts surrounding the pellicle.
- Permanent Stained Whole Mounts: Examine permanent slides of *Paramecium* stained with Heidenhain’s iron hematoxylin to clearly resolve the kidney-shaped macronucleus and closely nestled micronucleus.
- Hydra Cnidocyte Eversion: Mount a living tentacle of *Hydra vulgaris* in a drop of pond water. Add a drop of $1%$ acetic acid or dilute methylene blue while observing under $400times$. Watch the instantaneous explosive discharge of penetrant (stenotele), volvent (desmoneme), and glutinant (isorhiza) nematocysts. Measure the length of the discharged barbed shaft and hollow thread.
