Respiratory system and Nervous system of Asterias.

The respiratory system of the Asterias.

The respiration takes place by exchange of gaseous through the thin-walled tube feet, gills or papulae. In Asterias, papulae play a chief important role in the respiratory organ. The papulae are simple, contractile, transparent, hollow outgrowths of body wall on the aboral surface heaving ciliated epithelium at outer and inner surfaces. These cavities are continued with coelom.Mayer (1935), in Asterias Rubens, with covered ambulacral grooves oxygen consumption decreases by nearly 60 percent.

In sea water exchange of gases take place between oxygen and carbon dioxide. and body fluid of gills. The cilia of epithelial of coelomic fluid and in creating respiratory water currents in sea water. The other thin-walled, richly vascularized and moist body parts also act as respiratory organs.

Nervous system of theAsterias.l

The nervous system present in Asterias is of primitive and simple type. It is composed of nerve fibre and nerve nets which are closely related with the epidermis. The nervous system of Asterias comprises into following four units placed at different levels in the disc and arm.

(i) Oral or Ectoneural or epidermal Nervous system:

Oral nervous system lies just beneath the epidermis. They are sensory in nature and also the main part of the sensory nervous system. It consists the following parts.

1. Nerve ring.
2. Radial nerve.
3. Sub-epidermal nerve.

1. Nerve ring.

It is pentagonal in shape and is circum- orallies around the mouth in the peristomial membrane. It supplies nerve fibre to the peristomal and oesophagus and at each radius gives off a radial canal.

2. Radial nerve:

It consists of fibrils arranged in a layer. The radial nerve interspersed with bipolar and multipolar ganglion cells. From each angle of the pentagonal circumoral nerve, ring arises a radial nerve cord which runs through the length of the arm in the bottom of the ambulacral groove.In cross section, the radial nerve cord appears thick V-shaped mass continued on its outer side with the epidermal and separated on its inner side from the hypo neural sinuses only by a thin dermis and the coelomic epithelium.Each radial nerve terminates as a sensory cushion on the aboral side of the terminal tentacles. It provides fibre to the tube feet and ampulla.

3. Super-epidermal nerve complex:

The subepidermal nerve complex is in the form of an elaborate nerve net through the body wall. It is an extensive network of nerve cell and nerve fibres beneath the epidermal all over the body surface including the gills and pedicellariae. It is connected with the radial nerve cords by means of the nerve fibres.The subepidermal nerve complex is thickened into a cord which forms the

1. A nerve rings in the sucker of each tube foot.
2. It forms a pair of marginal or a radial nerve, each of which extends through the length of an arm on each side and gives off a longitudinal series of lateral motor nerve which supplies the ossicles, muscles, coelomic epithelium of that area .

(ii) Deep or hypo neural nervous system:

This type of nervous system consists occurs in the form of a nervous layer in the lateral part of the oral wall of the hypo neural sinus, beneath the coelomic epithelium lining the sinuses. This type of nervous layer is called Lange's nerve and is separated by the radial nerve only by a thin layer of dermal connective tissue.Lange's nerves are named after their discovery. Lange's nerve continued to the peristomal region , where it forms five interradial thickening in the floor of the ring sinus that lies aboral to the main nerve ring.Lange's nerve also give off a serious of nerve along the adjacent lower transverse muscle extending between the ambulacral ossicles in the roof of hypo neural sinus.The deep nervous system is a primary motor in function and its nervous system arises from the mesoderm .

(III) Aboral or coelomic nervous system:

This type of nervous system is represented by a thin nerve plexus situated in the aboral body wall, just above the parietal layer of coelomic epithelium. Aboral nervous consists of a nerve ring in a central disc and a nerve in each arm. This system also has a connection with marginal nerve by nerve fibres. It innervates the body muscle of aboral side . This system is the motor in function and mesodermal in origin.

(IV) Visceral nervous system:r

This type of nerve system occurs in the wall of the gut just outside the enteric epithelium. It innervates the muscle of the gut wall or alimentary canal and is connected with the visceral receptors.

Reference:

Bhamrah, H.S., and Kavita, Juneja. A Text Book of Invertebrates, New Delhi: Anmol Publications Pvt Ltd, 2011.

Jordan E.L. and P. S., Verma. Invertebrate Zoology, New Delhi,: S. Chand and Company Pvt. Ltd., 2011.

Kotpal, R. L., Modern Text Book of Zoology: Invertebrates, New Delhi, India: Rastogi Publications,2011.