What is endostyle?

The correct answer is Longitudinal ciliated groove on the ventral wall of the pharynx of Amphioxus.

Explanation:

• The endostyle is a key anatomical feature found in certain chordates, particularly in primitive species such as Amphioxus (a cephalochordate) and larval forms of tunicates.
• It is a longitudinal ciliated groove located on the ventral wall of the pharynx. Its main function is to produce mucus and aid in trapping food particles during filter feeding.
• In evolutionary terms, the endostyle is considered homologous to the thyroid gland in vertebrates. This evolutionary link is supported by the fact that both structures share a role in iodine metabolism.
• The endostyle secretes mucus, which captures small food particles suspended in water as the organism filters it through its pharyngeal slits.
• The cilia lining the endostyle help in moving the mucus-bound food particles toward the digestive tract for further processing.

Fig: Amphioxus (Internal structure)

Other Options:

• Stylus in the endoderm of the intestine – This is incorrect because the endostyle is not a stylus (rod-like structure) and is not located in the intestine. It is specifically a ciliated groove in the pharynx of chordates.
• Crystalline style in mollusca – This is incorrect because the crystalline style is a structure in mollusks, particularly in bivalves, that aids in digestion by releasing enzymes to break down food. 
• An endodermal derivative in the coelom – Endostyle is of endodermal origin, it is not located in the coelom (body cavity).