Anatomy of finfish and shellfish

Unit 12 - Arthropoda

12.3.3. Circulatory System
In a crab, circulation of blood takes place much as in a shrimp or lobster. Arteries carry blood dorsally from the heart forward into the head and viscera and backward into the abdomen. A sternal artery carries blood ventrally, where main branches direct it both forward and back. Further branching of main arterial vessels leads the blood into thin- walled capillaries, where exchange of gases and foodstuffs between blood and tissues can occur. The blood collects in venous sinuses, goes to the gills, and then enters the pericar¬dial sinus surrounding the heart. Here, when the heart re¬laxes and its three pairs of ostia open, the blood enters the heart. In keeping with the breadth of a crab's body, the heart is broad, filling much of the pericardial sinus, or pericardium. In the small tail of a crab, the ventral abdominal artery is of relatively small size compared with the same artery in the muscular tail of a lobster.