The nervous system of Palaemon resembles that of annelidas, it shows
2. Peripheral nervous system, and
3. Autonomic nervous system.
1. Central nervous system: It includes a pair of cerebral ganglia, a pair of circum esophageal connectives and a double ventral nerve cord.
i) Cerebral ganglia:
- They lie in front of the junction of the oesophagus with the cardiac stomach. It is a white bilobed mass.
- It is also called brain.
ii) Circum oesophageal connectives:
- These are nerve cords which originate from the postero-lateral parts of the brain, run downwards round the oesophagus to unite with ventral nerve cord.
- Each connective shows a small commissural ganglion which gives a mandibular nerve to mandible.
iii) Double Ventral nerve cord:
- It lies in the mid-ventral line of the body, it bears seventeen pairs ganglia.
- Tare anterior eleven pairs of ganglia belong to the cephalothorax and are fused to form a single and large mass called the Cephalo thoracic ganglionic mass.
- The posterior six pairs of ganglia of the nerve cord belong to the abdomen and they remain separate.
- The sixth and abdominal ganglion is larger than others.
2. Peripheral nervous system: It contains paired nerves that arise from the central nervous system to innervate body parts.
a) Nerves from brain: They are 5 pairs.
- Antennary and
- Tegumental nerves.
- The optic nerves extend outwards and forwards through the eye-stalks and innervate the eyes.
- The opthalmic nerves arise close behind the optic nerves. They supply the oscular muscles in the eye stalks.
- The Antennulary nerves arise from the brain antero-ventrally. Each nerve enters the antennule of its side. It also gives the statocystic nerve which goes to the statocyst.
- The antennary nerves arise from .the ventral side of the brain. Each nerve enters the antenna of its side.
- The tegumental nerves arise behind the antennary nerves. They innervate the labrum.
b) Nerves from Cephalothoradc ganglionic mass: Eleven pairs of nerves originate from the cephalo thoracic ganglionic mass.
- A pair of Mandibular nerves to mandible.
- A pair of Maxilhilary nerves to maxillulae.
- A pair of Maxillary nerves to maxillae.
- Three pairs of Maxillipede nerves to maxillipede.
- Five pairs of walking leg nerves to walking legs.
c) Nerves from Abdominal ganglia:
- Three pairs of nerves arise from each abdominal ganglion.
- The fifth abdominal ganglion gives off only two pairs of nerves.
- The sixth abdominal ganglion gives six pairs of nerves.
3. Autonomic nervous system:
- A slender nerve arises from the hind end of the brain and has two small visceral ganglia.
- The anterior visceral ganglion is connected with the commissural ganglia of the circum-oesophageal connectives by short transverse connectives.
- The posterior visceral ganglion gives two pairs of nerves to the muscles of the wall of the oesophagus and stomach.
SENSE ORGANS OF PALAEMON
Palaemon shows different sense organs. They are
- TACTILE SETAE,
- OLFACTORY SETAE,
- COMPOUND EYES.
- In palaemon a pair of statocysts can be seen.
- The precoxa of each antennule will contain a statocyst. A statocyst is a round sac.
- It is 1 to 1.5 mm wide. Its outer opening is covered by a small integument. Inside it an oval ring of setae, and heap of sand grains will be present.
- A receptor setae has two parts 1) a swollen base and 2) a slender shaft. The base is articulated with the wall of the sac by a membrane.
- It is supplied with nerve from the statocystic nerve.
- The shaft is bent and fine bristles are present above the bent part.
- The statocysts are the organs which maintain equilibrium in the normal position. The sand grains lie on the floor of the statocyst.
- The sand particleswork like statoliths.
- When the position of the animal in water is changed the sand grains press against some of the receptor setae.
- Nerve fibres are stimulated. It is conveyed to the brain. Position is corrected.
2) TACTILE SETAE: The tactile setae are seen on the antennae and on other parts of the body. A tactile setae has two parts.
- Stout Proximal segment: Articulating with the integument by a flexible arthrodial membrane.
- Distal segment: It is touch sensitive. It is also called blade. It shows two rows of barbs.
3) OLFACTORY SETAE:
- The olfactory setae, are on the middle small feeler of each antennule.
- Each setae has a stalk and a blade.
- Each setae gets a small nerve from antennary nerve.
- There are a pair of compound eyes.
- They are stalked.
- The eye stalks are in orbital notches near the base of the rostrum.
- Each eye-stalk has two segments.
- 1. Proximal segment attached to the integument.
- 2. Distal segment with eye at its free end.
Structure: the compound eye consists of a large number of Ommatidia. They are radially arranged.
1) Dioptric part:
- The eye is covered by a transparent comea.
- The corneal lens is biconvex.
- It is secreted by a pair of comeageal cells. Below these cells crystallirje cells are pressure.
- The crystalline cone is secreted by four vitrellae cell.
- All the above structures collectively form dioptric part of the ommatidium.
2) Retinal part:
- Below the dioptric part a long, refractile rod, the rhabdome is present.
- It is secreted and surrounded by seven long cells, the retinulae.
- The end of each retinal cells is prolonged into a nerve fiber that joins the optic nerve.
- The rhabdome and the retinulae from the retinal part of the ommatidium.
- Each ommatidium is covered by pigmented cells which will separate it from the nearby ommatidium.
- During bright light, ommatidia are completely covered by the pigment cells.
- The ommatidia are separate from one another.
- Thus many images are developed. Such-an image is called the opposition or mosaic image and the eye is said to have a mosaic vision.
- During dim light pigment cells separate apart and expose the ommatidia. With the result, the rays of light entering several adjacent corneae will overlap The image is formed by overlapping points of light.
- Such an image is known superposition image.