They are similar to the epidermal receptors except that they have broader outer ends. They are tactile in function and also respond to chemical stimuli and changes in the temperature.īuccal receptors: These receptors are restricted only to the epithelium of the buccal chamber. They are surrounded on all sides by ordinary supporting epidermal cells. Each receptor has an elevated cuticle covering a group of tall, slender and columnar receptor cells, bearing small hair-like processes at their outer ends and connected with nerve fibers at their inner ends. They are more abundant on the lateral sides and ventral surface of the body. Pheretima has three types of sense organs:Įpidermal receptors:These receptors are well distributed all over the epidermis. These sense organs are simple in structure consisting of one single cell or a group of specialized ectodermal cells. In earthworm, impulse is also travel from one ganglion to another ganglion through ventral nerve cord.Įarthworms have well-developed sense organs. Motor fibers (efferent fibers) carry impulse back to effective organs or different parts of body from central nervous system. Sensory fibers (afferent fibers) carry impulse to central nervous system. Working of nervous system:The sensory cells from different parts of the body receive impulse. Sympathetic nervous system: It consists of nerve plexuses extensively branched and distributed beneath epidermis, alimentary canal that is connected to circumpharyngeal connectives. From each segmental ganglion, three pairs of nerves arise and supply to respective segment.From subpharyangeal ganglia, three pairs of nerve arise and supply to 2nd, 3rd and 4th segment.From circumpharyngeal connectives, two pairs of nerves arise and supply to 1st and 2nd segment.From cerebral ganglia, 8-10 nerves arise and supply to prostomium, buccal chamber, and pharynx.Peripheral nervous system: It includes nerve fibers or nerve, which arises from central nervous system. The outer covering of ventral nerve cord is called peritoneum.
There are 4 giant fibers on mid dorsal side of nerve cord which conducts impulses rapidly. Ventral nerve cord is composed of nerve cells and nerve fibers.
Actually, there are two cords fuse together to form single ventral nerve cord. In each segment, ventral nerve cord swells which is called segmental ganglia. Ventral nerve cord-It is white rod like structure, which starts running from sub- pharyngeal ganglia towards posterior end. The fused portion is called sub pharyngeal ganglia. They encircle pharynx and fuse at 4th segment. The nervous system of earthworm consists of three parts namely,Ĭentral Nervous system: It includes following structures:Ĭerebral ganglia-There are two pear shaped cerebral ganglia fused together at 3rd segment called brain.Ĭircumpharyngeal connective- Two circumpharyngeal connectives arise from each cerebral ganglion laterally.
0 kommentar(er)
