The balancing organ equal to our ear in aquatic arthropods as in prawn is
A) Otolith
B) Lateral line organ
C) Otocyst
D) Statocyst

Hint: In aquatic arthropods, the end-product of biochemical reactions that metabolise nitrogen is ammonia, which is so toxic that it needs to be diluted as much as possible with water.

Complete Answer:
- Arthropods are characterized by their jointed limbs and cuticle made of chitin, often mineralised with calcium carbonate. The arthropod body plan consists of segments, each with a pair of appendages.
- The rigid cuticle inhibits growth, so arthropods replace it periodically by moulting. Arthropods are bilaterally symmetrical and their body possesses an external skeleton. Some species have wings.
- The statocyst is a balanced sensory receptor present in some aquatic invertebrates in prawns. Tine statocysts or the balancing organs of prawn communicate with the exterior through minute pores. There are two statocysts situated one on the base of each first antenna.
- A statocyst lies inside the basal segment or precoxa of each antennule, attached to its dorsal wall. It opens dorsally on the concave surface of the pre coxa through a minute statocysts aperture. A small statocystic branch of antennules nerve supplies the statocyst.

Therefore from the above explanation, the correct option is D).

Note: Statocyst perceives the direction of force of gravity & functions as the organ of orientation and equilibrium. The sand particles serve as statoliths. Any change in the position of the swimming prawn causes a corresponding displacement of sand particles. This displacement presses against some of the sensory setae & stimulates them.