Q.67 The following statements are related to sensory tissues of animals.

1. Proprioceptors are commonly called ‘stretch receptors’ that respond to mechanically-induced changes.
2. Certain flagellated protozoa like Euglena that contains chlorophyll possess a mass of bright red photoreceptor granules called stigma.
3. ‘Neuromasts’ are the primary photoreceptors in sharks, some fishes and aquatic amphibians.
4. Meissner’s corpuscles and Pacinian corpuscles primarily help to detect chemical stimulus in animals.
5. Most reptiles possess Jacobson’s (Vomeronasal) organs for olfactory reception.

Choose the set of correct statements from the options given below:

1. A, B and E only
2. B, C and D only
3. C, D and E only
4. A, D and E only

   The correct answer is A, B and E only.

   These statements accurately describe muscle spindle proprioceptors as stretch detectors, Euglena’s red-pigmented eyespot (stigma) for phototaxis, and reptiles’ vomeronasal organs for pheromonal detection.

   Option Analysis

   A, B and E only
   A: Correct—proprioceptors (muscle spindles, Golgi tendon organs) detect mechanical stretch/tension changes for position sense. B: Correct—Euglena stigma (red carotenoid granules) shades paraflagellar swelling for light direction phototaxis. E: Correct—Jacobson’s organ (vomeronasal organ) detects non-volatile pheromones via vomeronasal nerve to accessory olfactory bulb. Correct set.

   B, C and D only
   C incorrect: Neuromasts detect mechanosensory lateral line water movement (cupula deflection), not photoreception. D incorrect: Meissner (light touch), Pacinian (vibration/pressure) corpuscles are mechanoreceptors. Incorrect.

   C, D and E only
   C, D both false (neuromasts mechanoreceptors; cutaneous corpuscles mechanical stimuli). E true. Incorrect.

   A, D and E only
   A, E correct, but D false—encapsulated nerve endings detect deformation, not chemicals (chemoreceptors = olfactory, gustatory). Incorrect.

   Sensory tissues of animals transduce environmental/mechanical/chemical stimuli via specialized receptor proteins, enabling survival-critical perception across phyla.

   Proprioceptive Stretch Detection

   Proprioceptors (A)—primarily muscle spindles (intrafusal fibers + Ia/II afferents)—monitor sarcomere length changes, velocity via dynamic/static nuclei. Golgi tendon organs sense active tension. Stretch reflexes maintain posture, prevent injury via spinal monosynaptic arcs.

   Photoreception in Protozoa

   Euglena stigma (B): paraflagellar swelling + red carotenoid granules screen light, creating photoresponse gradient across single cell. Enables positive phototaxis toward light-optimal photosynthetic depths. Not true eye, but functional photoreceptor analog.

   Vomeronasal Pheromone Detection

   Jacobson’s organ (E): paired vomeronasal pits in reptile nasal floor house TRPC2+ vomeronasal sensory neurons detecting lipophilic pheromones (vomeronasal nerve → AOB). Mediates reproductive behavior, conspecific recognition vs. main olfactory epithelium volatile odorants.

   Common Misconceptions

   Neuromasts (C): lateral line canal gel cupula + hair cells detect water acceleration (prey, predators). Meissner/Pacinian (D): Merkel/Meissner light/discriminative touch; Pacini deep pressure/vibration—pure mechanoreception.