Ophiura: Example of Phylum Echinodermata

In this article we will discuss about Ophiura:- 1. Habit and Habitat of Ophiura 2. External Structures of Ophiura 3. Body Wall 4. Coelom 5. Digestive System 6. Respiratory System 7. Water Vascular System 8. Haemal System or Blood Lacunar System 9. Locomotion 10. Excretory System 11. Nervous System 12. Reproductive System.

Contents:

1. Habit and Habitat of Ophiura
2. External Structures of Ophiura
3. Body Wall of Ophiura
4. Coelom of Ophiura
5. Digestive System of Ophiura
6. Respiratory System of Ophiura
7. Water Vascular System of Ophiura
8. Haemal System or Blood Lacunar System of Ophiura
9. Locomotion of Ophiura
10. Excretory System of Ophiura
11. Nervous System of Ophiura
12. Reproductive System of Ophiura

1. Habit and Habitat of Ophiura:

Ophiura usually lives in shallow to deep water of the ocean. They usually hide them- selves at daytime beneath the sea-weeds or stones. They can also burrow in the sand or mud. They are very active at night. They can move quite rapidly by their arms. They are carnivorous animals and devour small crus­taceans, worms and other small animals.

2. External Structures of Ophiura:

The body of Ophiura consists of a star- shaped disc and five highly flexible arms (Fig. 21.19). The arms are elongated, slender and fragile. The arms are sharply marked off from the disc. The arms are un-branched in Ophiura, but in Gorgonocephalus the arms are extensively branched. Like that of Asterias, mouth is situated at the centre of the oral surface and the oral-aboral axis is very short.

The mouth is pentagonal, and at each inter- radius, there is a more or less triangular projection, called oral papilla or jaw. The madreporite is situated in an inter-radius on the oral surface. The ambulacral grooves and the anus are absent. The pedicellariae are wanting and the spines are located on the lateral plates of the arms.

3. Body Wall of Ophiura:

The integument is non-ciliated and is covered by cuticle. The epidermis is vestigial and the dermis is devoid of muscles.

Skeleton:

The skeleton is well-developed in Ophiura. Each arm consists of similar segments and the interior of each segment is almost filled with solid skeletal blocks, called vertebral or ambulacral ossicles (Fig. 21.20). The outer side of the arms contains four rows of plates— two lateral, one aboral and one oral. Two radial plates or shields, one on each side of the arms, are present.

The centre of the aboral side of the disc is occupied by a large roundish central plate which is surrounded by many concentric plates. In the oral side, the mouth is surrounded by five large oral or buccal plates and five pairs of aboral plates along the inter-radial planes of the disc. One of the oral plates is perforated, and acts as madreporite. The oral papillae con­tain a few small plates.

4. Coelom of Ophiura:

Like all other echinoderms, the coelom in Ophiura is enterocoelic in origin. The peri­visceral coelom is quite spacious and is traversed by mesenteries which run between the body wall and the viscera. In the arms the coelom is restricted to a small and crescentic space on the aboral side of the vertebral ossicles.

The coelomic fluid con­tains coelomocytes. The coelomocytes are granular and contain fine pseudopodia which anastomose to form networks. The spheroid coelomocytes recorded in the axial gland suggest the fact that the coelomocytes are formed in this gland.

5. Digestive System of Ophiura:

The alimentary canal is present inside the disc. As described earlier, the mouth is placed at the centre of the oral side and is sur­rounded by five groups of movable plates serving as jaws. Many spines are extended over the mouth which acts as strainer.

The mouth leads into a very short oesophagus which opens into the stomach. The stomach is sac-like, non-protrusible and is attached with the body wall by mesenteries. Lack of anus, caeca and intestine are quite striking in the digestive system of Ophiura.

6. Respiratory System of Ophiura:

Along the sides of the arm bases, there are five pairs of sac-like invaginations on the oral surface, called bursae. Each bursa is supported by genital shields and opens by a slit-like aperture. The bursae occupy the spaces between the stomach pouches.

The bursae inside the disc coelom fuse at places. The wall of the bursa is histologically same as the body wall, but the dermis is very thin. Through the bursae a constant flow of water current is maintained which possibly helps in the process of respiration.

7. Water Vascular System of Ophiura:

The water vascular system is almost simi­lar to that of Asterias, but exhibits certain minor peculiarities. The madreporite is usu­ally one in this genus, but in Ophiactis virens, five madreporites are present. The axial com­plex is highly developed. The axial sinus encloses the axial organ and the stone canal (Fig. 21.21).

The wall of the stone canal lacks calcareous matter and opens to the exterior through the pore canal. These structures are shifted towards the oral side. The Tiedermann’s bodies appear to be wanting, but in Ophiacten sericeum and Gorgonocephalus, such structures or their equivalents are re­ported to be present.

The radial canals run through the substance of the vertebral ossicles as the cavity of the terminal tentacles. The lateral branches to the tube-feet from the radial canals are very short. They lack suck­ers and ampullae. The tube-feet are sensory in function and have nothing to do with locomotion.

8. Haemal System or Blood Lacunar System of Ophiura:

The haemal system is quite well-developed in Ophiura. The oral and aboral haemal channels are disposed in the similar fashion as seen in Asterias. The haemal channels are very shallow.

9. Locomotion of Ophiura:

The tube-feet, as described earlier, have nothing to do with locomotion. The move­ment is affected by the highly flexible arms. During locomotion the disc of the body is lifted from the substratum with the help of the arms. By the action of the arms, Ophiura in this state can move at considerable speed. The arms also help the animal to swim in water.

10. Excretory System of Ophiura:

There is no special excretory organ in Ophiura. The excretory products pass out of the body through the bursal wall. The excre­tory products in the form of black or yellow spherules are seen to be accumulated in the bursal wall.

11. Nervous System of Ophiura:

The nervous system like that of Asterias comprises of circumoral nerve ring and the radial nerves to the arms. The radial nerves bear ganglionic swellings, one at each verte­bral ossicle of the arms and terminate at the base of the terminal tentacles.

There is no special sense organ in Ophiura. The tube-feet are sensory in function.

12. Reproductive System of Ophiura:

The sexes are separate and sexual dimor­phism is absent. The gonads are sac-like which remain attached with the bursae and open into them. The bursae are ten in number and open to the exterior through slit-like apertures, situated one on each side of the base of the arms.

Tine matured gametes are discharged into the cavity of the bursae and from there the gametes are expelled to the exterior through the bursal apertures. For this particular function, the bursae are gen­erally called genital bursae.