Which of the following has haemocoel?

The correct answer is Cockroach.

To understand why this is the correct choice, let us break down what a haemocoel is and examine the body cavity type of each option provided.

1. Understanding Haemocoel:
A haemocoel is a body cavity (coelom) that is filled with blood or hemolymph. Unlike a true coelom where the blood is confined within blood vessels (closed circulatory system), a haemocoel acts as the primary cavity in an open circulatory system. Organs within the animal are bathed directly in the circulating fluid (hemolymph) to facilitate the exchange of nutrients, gases, and wastes.

2. Analyzing the Options:

• Cockroach: Cockroaches belong to the phylum Arthropoda. A characteristic feature of arthropods is an open circulatory system where the coelom is reduced, and the primary body cavity is a blood-filled space called the haemocoel. Therefore, the cockroach has a haemocoel.
• Earthworm and Nereis: Both of these organisms belong to the phylum Annelida. Annelids possess a true coelom (schizocoelom) and a closed circulatory system, meaning their blood flows through a closed network of vessels rather than filling the body cavity. Thus, they do not have a haemocoel.
• Hydra: Hydra belongs to the phylum Cnidaria. Cnidarians are diploblastic animals and do not possess any coelom (they are acoelomate). They only have a simple central gastrovascular cavity (coelenteron) with a single opening.

Therefore, among the given options, the Cockroach is the organism that possesses a haemocoel.