Anatomy of finfish and shellfish

Unit 12 - Arthropoda

12.3.4. Nervous System
The nervous system of all crabs, except: the most primitive, has undergone a high degree of fusion. All ventral ganglia are fused into a single thoracic ganglionic mass, which lies near the floor of the cephalothorax and through which the sternal artery descends. From the periphery of the thoracic gan¬glionic mass, nerves radiate out to the appendages all the way from the mandibles to the last thoracic legs. An abdominal nerve emerges posteriorly at the midline and supplies the muscles and appendages of the tail.
Connecting the thoracic ganglionic mass with the brain, or supraesophageal ganglion, are the two long, large nerves that pass on either side of the esophagus and are known as the circumesophageal connectives. Slight swellings on the con¬nectives mark the position of the stomatogastric, or connec-tive, ganglia that supply nerves to the foregut. The trito¬cerebral commissure links the two connectives in crabs, as in shrimps and lobsters.
The kidneys of crabs lie on the interior ventral surface of the body, just posterior to a position between each antenna and the corresponding eyestalk on the same side. Due to their color, which is pale green, yellow, or green-brown, the kid¬neys are also called green glands; due to their position on the second, or antennal, segment, they are also often called antennal glands.

In structure, the antennal glands of crabs are similar to those of lobsters. There is a glandular portion, which secretes urine and regulates salts, and a large, many-lobed, thin- walled bladder, in which urine is temporarily stored. The main lobe of the bladder lies above the glandular portion of each kidney, but the remaining lobes extend out in several directions. Because of the delicacy of the lobes, it is almost impossible to see them unless they are fixed in alcohol or injected with India ink or a powdered dye, such as carmine. Urine passes from the glandular portion of each antennal gland upward into the bladder and then to the exterior via a duct. The opening of the duct, which lies at the base of the antenna, is covered by a calcified, movable cover, called an operculum.