Holothuria: An Example of Phylum Echinodermata

In this article we will discuss about Holothuria:- 1. Habit and Habitat of Holothuria 2. External Structures of Holothuria 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 and Development.

Contents:

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

1. Habit and Habitat of Holothuria:

Holothuria usually lives in sandy bottom of the sea and may remain partially buried in the sand. They are also found at all depths of the sea, ranging from the shallow to deep water. They are very sluggish by nature. They feed on organic particles present in sand or other minute living organisms.

2. External Structures of Holothuria:

Holothuria has an elongated body with greatly elongated oral-aboral axis (Fig. 21.16). The oral end is provided with mouth. The anus is situated at the opposite end. The mouth is surrounded by a thin peristomial membrane which is bordered by a row of branched tentacles.

The tentacles are pro­vided with circled of short terminal branches which again branch and become shield- shaped in appearance. The tentacles are re­tractile into the ampullae. The shield-shaped tentacular branches are characteristics of the genus.

The radial symmetry of the body has become replaced by bilateral symmetry. The ‘dorsal’ and ‘ventral’ sides can be distin­guished. The ‘ventral’ side is more or less flattened and possesses three radii opposite to the madreporic inter-radius with its two radii.

In other forms the tube-feet are arranged along the radii from the oral to the anal end of the body, but in Holothuria, the radial orientation of the tube-feet is obliterated and they spread over the inter-radii of the body. But the internal structures retain their radial disposition.

3. Body Wall of Holothuria:

The histological picture of the body wall reveals the presence of a thin cuticle over a non-ciliated epidermis. There are numerous small ossicles in the dermis. There are lon­gitudinal muscle-bands under the radii and transverse muscle-bands between the radii. There is a thin layer of ciliated coelomic epithelium lining the internal side of the body wall.

4. Coelom of Holothuria:

The perivisceral coelom between the ali­mentary canal and the body wall is quite well-developed. The coelom is filled with coelomic fluid containing various types of coelomocytes. One of the types of the coelomocytes, called haemocytes, contains red pigment, the haemoglobin.

The other types of coelomocytes include:

(i) Lymphocytes,

(ii) Phagocytes,

(iii) Coloured and colourless morula cells,

(iv) Fusiform cells,

(v) Crystal cells,

(vi) Vibratile cells, etc.

5. Digestive System of Holothuria:

The mouth leads into a short oesophagus which in turn opens into a muscular stomach (Fig. 21.17). The oesophagus is enclosed by a calcareous ring which is composed of five radial and five inter-radial ossicles. The stom­ach is joined with the thin-walled coiled intestine.

The intestine is much longer than the body length and becomes coiled. The posterior part of the intestine is connected to a muscular cloaca by short rectum. The cloaca is supported by remnants of mesenteries which are called cloacal suspensors. The rectum opens to the exterior through the anus.

6. Respiratory System of Holothuria:

There are two long and elaborately branched respiratory trees which open into the cloacal chamber. The terminal branches end in thin-walled ampullae. Through the wall of the ampullae, water from the cloaca, can pass into the coelom and thus help in conveying oxygen to the coelomic fluid and to other organs.

In Holothuria, the lower branches of the respiratory trees are con­verted into coloured blind tubes called Cuvierian organs. These tubules can be everted through the anus and they produce a sticky secretion so as to entangle the enemy or to retard the entry of enemies by liberation of toxin.

7. Water Vascular System of Holothuria:

The water vascular system comprises of an oral ring vessel which in turn gives off five radial vessels. Each radial vessel supplies branches to the tentacles anteriorly and passes posteriorly along the ambulacral areas to give branches to the tube-feet (Fig. 21.18). Each tube-foot has an ampulla.

The oral ring vessel is provided with a single large polian vesicle and one or more stone canals which communicate with the anterior of the coelom by perforated extremity of the madreporite canals. The madreporite is internally placed. The axial sinus is very small.

8. Haemal System or Blood Lacunar System of Holothuria:

The haemal system is quite well-formed in Holothuria. This system consists of a circumoesophageal haemal sinus which gives five radial haemal vessels at the ante­rior side. Posteriorly, the circumoesophageal haemal sinus gives two haemal sinuses to the alimentary canal which send numerous smaller branches to the intestine.

9. Locomotion of Holothuria:

Like other echinoderms, Holothuria move by the tube-feet or podia. The mechanism of movement is same as that in Echinus and Asterias.

10. Excretory System of Holothuria:

There is no specialised organ in Holothuria to act as excretory organ. The nitrogenous waste products are extracted by the cloacal wall and are eliminated to the exterior through the anus. The respiratory trees are also reported to play some role in excretion.

11. Nervous System of Holothuria:

The nervous system is represented by a circumoral nerve ring beneath the peristominal membrane. Five radial nerves emerge from the oral nerve ring along the ambulacral areas.

No special sense organ is present in Holothuria, excepting the tentacles which act as tactile organs.

12. Reproductive System and Development of Holothuria:

The sexes are separate. There is a single gonad in each sex consisting of a tubular follicle situated in the ‘dorsal’ inter-radius. The gonad has a gonoduct which opens to the dorsal surface of the oral end. The testis and ovary look alike. The cord-like genital stolon represents the axial organ of the ge­nus.

Fertilization is external and the prelarval developmental stages are similar to that of Asterias and Echinus. The larval form is known as Auricularia. The larval mouth persists in the adult. The detailed account of the larval form is described in the general notes on the phylum.