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Difficulty: MediumHigher Invertebrates: Annelida, Mollusca, Arthropoda, and Echinodermata

Arrange the following physiological events in the correct sequence to describe the path of hemolymph through the open circulatory system of an insect (phylum Arthropoda), starting from the initiation of heart contraction.

  1. 1Contraction of the dorsal vessel pumps hemolymph anteriorly through the aorta.
  2. 2Hemolymph exits the aorta and enters the head cavity and primary body sinuses (hemocoel).
  3. 3Hemolymph directly bathes internal organs and tissues, facilitating nutrient and waste exchange.
  4. 4Hemolymph re-enters the relaxed dorsal vessel through the valve-like lateral ostia.

Answer

The correct sequence begins with the contraction of the dorsal vessel propelling hemolymph forward through the aorta, followed by its release into the head cavity and hemocoel, direct bathing of the tissues and organs, and finally its return to the relaxed dorsal vessel via the ostia.
In arthropods, the open circulatory system operates in a defined directional loop: contraction of the dorsal heart drives hemolymph forward through the aorta into the head cavity and body sinuses (hemocoel). There, hemolymph directly bathes the internal organs before being drawn back into the relaxed heart through lateral ostia.

Step-by-Step Solution

1
Identify the primary pumping action that initiates blood flow.
The muscular dorsal vessel contracts, driving hemolymph anteriorly into the aorta.
Circulation in insects is powered by peristaltic waves of contraction in the dorsal heart.
2
Determine where hemolymph travels after exiting the vessel.
Hemolymph discharges from the open anterior aorta into the head space and body sinuses (hemocoel).
Arthropods possess open circulatory systems lacking a continuous capillary network.
3
Identify the functional interaction between hemolymph and organs in the hemocoel.
Hemolymph flows backwards through the body, directly bathing visceral organs and tissues.
Direct contact between interstitial fluid and organ membranes allows efficient nutrient and metabolic exchange.
4
Trace the return path of hemolymph back to the heart.
As the dorsal heart relaxes, hemolymph enters the cardiac chambers through lateral openings called ostia.
Ostia contain one-way valves that open during relaxation to refill the vessel.

Key Concept

Open circulatory system and pathway of hemolymph in Arthropoda
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Topics

Higher Invertebrates: Annelida, Mollusca, Arthropoda, and Echinodermata

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