Nervous System in Chaetognatha

Abstract

The nervous system of chaetognatha consists of 6 ganglia in the head region, one ventral ganglion in the body, nerves connecting these ganglia and peripheral nerves passing out of these ganglia. The ganglia in the head region are the cerebral ganglion, a pair of vestibular ganglia, a pair of oesophageal ganglia, and a suboesophageal ganglion. Putative sensory organs are a pair of eyes, the corona (a ciliary loop) behind the eyes, ciliary tufts on the body surface and papillae around the mouth. Decapitated worms lose photoresponsiveness but can swim normally. When the ventral ganglion is ablated, the worms can no longer swim, suggesting that a motor center is located in this ganglion. The nerves from the eyes and the corona run directly to the cerebral ganglion where the sensory information must be integrated. Except for the swimming movements, we can see movements of hooks which participate in grasping prey. A pair of vestibular ganglia have been considered to control these movements. Neither large neurons nor thick fibers have been detected. However, axonal pathways of relatively thick fibers are detected by vital staining using methylene blue. Immunohistochemical studies also gave information on axonal pathways, including neurotransmitter candidates in the nervous system. Serotonin-, methionin-enkephalin-, and neurotensin-like immunoreactivities exist in the nervous system, suggesting that these substances are neurotransmitter candidates in chaetognatha.