Question

In Limulus (Horseshoe crab), belonging to subclass Xiphosura, the respiratory organs are
A. Book gills
B. Gills
C. Tracheae
D. Absent

Answer 1

Hint: The Limulus species of crab, have flap-like external appendages that are present at the base of the abdomen which serves as gills for gas exchange underwater. They are book-shaped structures where there are several thin membrane-like lamellae, which are rich in capillaries. They are external appendages as they evolved from legs.

Step by step answer:The subclass of Xiphosura is an order of arthropods which are commonly called horseshoe crabs. This organism has a peculiar respiratory system, where its gills are known as book gills. Book gills are appendages that have several hundred thin membranes of lamellae. Here the membranes are arranged in a way that resembles the leaflet of a book, thereby acquiring the name book gills.
Book gills are currently seen only in horseshoe crabs. In Limulus (horseshoe crab), there are five sets of book gills present. These are present externally and the functioning book gill is present at the base of the abdomen. They are external appendages as it is believed to have evolved from limb buds as these structures also aid in swimming in young crabs. The book gills present at the base of the abdomen has a protective membrane which prevents the capillaries of the lamellae from being directly exposed to seawater.
Therefore, the correct option is A.

Note: Limulus is a genus of horseshoe crab which belongs to the order of Xiphosura. The marine king crab is a species of large segmented carb of the order Decapoda. Marine king crab and the horseshoe crab are different. Book gills evolved from limb buds, where the tissues evolved into thin membranes. This is why book gills are also called modified legs.
Book gills are similar to book lungs, except for the fact that book lungs are internal structures and not appendages and book gills are external appendages. Book gills and book lungs should not be used interchangeably.