Question

An aquatic living fossil, with ancient origin and many primitive characteristics which respire through book lungs, is ______.
A. Limulus
B. Cancer
C. Lucifer
D. Daphnia

Answer 1

Book lung is a respiratory organ found in arachnids, a class belonging to the arthropods. They have a folded book appearance hence the name. Book gills and book lungs have quite a few structural resemblances. It is said that the book’s lungs are an advanced or evolved form of book gills. Book gills are external organs, unlike the book lungs.

Complete answer:

Limulus (horseshoe crab), a marine arthropod, has many fossilized ancestors. Atlantic horseshoe crab is the only known living member at present. These organisms have book gills to help in the respiration process. Their gills have lamellae with capillaries, separated by a thin membrane from the external environment, which also helps in osmoregulation. It is the movement of the gills that enables respiration through the gaseous exchange.
Cancer (crab) belongs to Crustacea which is a part of phylum arthropods. They use gills as their organ of respiration situated inside a body cavity. In land crabs, this respiratory organ grows in size and acts as modified lungs.
Lucifer (prawns) belongs to the Luciferidae family. There is little known about this organism. The organism has a long body and bears no gills. They are known for their bioluminescence.
Daphnia belongs to the family Daphniidae, they can survive in freshwater or even acidic swamps. Diffusion is the process by which they respire. The hemoglobin present in their blood carries oxygen molecules to various parts of the body.

The correct answer is Limulus, option (A).

Note: A living fossil is a living organism that is very much similar to an extinct fossilized one. They have hardly evolved over a long period. Their morphological features match that of the fossil even after millions of years have passed between the organisms. It is very difficult to examine a fossil at times, in those times the scientists can study a living fossil (if any) that has hardly evolved in ages.