An Example of Phylum Mollusca: Unio

In this article we will discuss about Unio:- 1. Habit and Habitat of Unio 2. External Structures of Unio 3. Coelom 4. Digestive System 5. Locomotion 6. Respiratory System 7. Circulatory System 8. Excretory System 9. Nervous System 10. Reproductive System 11. Development.

Contents:

1. Habit and Habitat of Unio
2. External Structures of Unio
3. Coelom of Unio
4. Digestive System of Unio
5. Locomotion of Unio
6. Respiratory System of Unio
7. Circulatory System of Unio
8. Excretory System of Unio
9. Nervous System of Unio
10. Reproductive System of Unio
11. Development of Unio

1. Habit and Habitat of Unio:

Unio is an aquatic form and inhabits fresh­water lakes, ponds and rivers. They usually burrow in the mud at the bottom of the pond by their large ventral muscular foot.

They do not go deep in the burrow, because the pos­terior extremities of the valves is to be kept exposed for the ingress and egress of respira­tory water current. They usually stay in shal­low water during night, but migrate to deeper water during daytime. The food of Unio com­prises of microscopical plants and animals.

2. External Structures of Unio:

Unio has a bilaterally symmetrical body and the size varies from about 5 to 10 cm in length. The body is laterally flattened. The anterior side of the body is roughly oval in outline and the posterior end is slightly narrower. The body is enclosed by a hard calcareous shell (Fig. 16.28A) which consists of two equal valves hinged at one edge. Beneath the shell, a delicate layer called mantle envelopes the whole visceral mass.

The mantle has two epithelial layers with an intermediate connective tissue layer (Fig. 16.29). The epithelium just beneath the shell is composed of secretory cells and the inner epithelium is ciliated. The mantle consists of two lateral halves called the mantle lobes.

The mantle lobes at the aboral side produce two short tubes—the inhalant and exhalant siphons. The edge of the exhalant siphon is smooth and that of the inhalant siphon is produced into delicate processes. Sometimes the triangular tongue-shaped foot protrudes between the two valves towards the oral end.

Macroscopic Structure of Shell:

The two valves are united dorsally along a hinge-line by an elastic band, called hinge- ligament which helps the opening and clos­ing of the valves. The hinge-teeth are present in Unio. The teeth are so arranged that the teeth of one fit into the sockets of the other valve. Series of concentric lines of growth are present on the external surface of the shell.

The lines of growth start from an elevation— the umbo which is the thickest and the oldest part of the shell. Some characteristic markings are also observed on the inner side of the shell. Two large oval impressions of the ante­rior and posterior adductor muscles are present near the anterior and posterior ends of the shell respectively (Fig. 16.28B).

The scar mark of the anterior adductor muscle is slightly smaller than that of the posterior one. Impressions of the anterior retractor and protractor muscles of the foot are present near that of the anterior adductor muscle. Another small impression of the posterior retractor muscle lies near that of the posterior adductor muscle.

The protractor muscle that serves to extend the foot, and retractor mus­cle withdraws the extended foot of the bivalves. A streak known as the pallial line, produced by the insertion of muscular fibres of the mantle into the shell is present.

Microscopic Structure of Shell:

The shell has three distinct layers (Fig. 16.29). The outermost layer, known as periostracum, is composed of a substance, related to chitin, called conchiolin.

Beneath the periostracum is the prismatic layer formed of alternate layers of conchiolin and prisms of calcium carbonate. The innermost layer, called the nacreous layer, is formed of alternate linings of conchiolin and calcium carbonate. Such linings are arranged parallel to the surface.

3. Coelom of Unio:

The original coelomic cavity, in an adult, is replaced by connective tissue and is rep­resented by three small cavities—the pericar­dium, the cavities in the gonads and the cavities in the excretory organs. The general body cavity is a haemocoel.

4. Digestive System of Unio:

The digestive system includes alimentary canal and digestive glands (Fig. 16.30). The mouth is a transverse slit and situated below the anterior adductor muscles as a slit. It is bounded by two pairs of conical flaps, called the labial palps. One pair of labial palps are external and the other are internal.

The ex­ternal labial palps in front of the mouth unite to form the upper lip and the internal labial palps similarly unite behind the mouth to form the lower lip. The radula is absent in Unio. The mouth leads into a spacious sac­like stomach by a short gullet. A pair of irregular digestive glands (liver) surround the stomach and the secretion of the gland is poured into the stomach by small ducts.

From the posterior end of the stomach starts the intestine which enters into the visceral mass and forms coiled loop. The intestine then goes up and takes the level of the stomach. It then proceeds towards the pos­terior end through the pericardium and ven­tricle of heart as the rectum. It finally opens into exhalant siphon as anus.

The rectal por­tion of the intestine is provided with a lon­gitudinal ridge, known as the typhlosole, which forms two longitudinal grooves, one on each side. The typhlosole increases the absorptive area of the intestine.

From one of such grooves of the intestine, a gelatinous rod-like structure, known as crystalline style, projects into the stomach which probably helps in the digestion of cellulose and starch. It has been seen that in starving Unio the crystalline style disappears.

5. Locomotion of Unio:

The muscular foot is the primary locomo­tor organ. The foot protrudes between the antero-ventral sides of shell valves and bur­rows like ploughshare through the mud.

During progression, due to the flow of blood into the foot, the latter swells up and be­comes turgid. When the animal intends to move, the foot is extended forward as far as possible by influx of blood and its contrac­tion draws the body forward. So by such alternate extension and contraction of the foot, the animal moves very slowly.

6. Respiratory System of Unio:

Unio is exclusively an aquatic animal. The respiratory organ consists of two gills or ctenidia situated one on each side of the body (Figs. 16.31 and 16.32A). Each ctenidium has an external and an internal gill plates or gill laminae (Fig. 16.32B). Each gill lamina is a double structure formed of two similar plates, the outer and inner gill lamellae.

The internal and the external gill lamellae are united with one another on all sides except­ing the dorsal end. The spaces between the internal and external gill lamellae are subdi­vided by vertical bars, called interlammellar junctions, into a number of compartments, called the water tubes (Fig. 16.32C). Each lamina exhibits double striations.

The verti­cal striations are due to closely-set gill fila­ments that compose the lamellae. The longi­tudinal striations are due to the fact that these filaments are connected by horizontal interfilamentar junctions. Between the gill filaments, bounded by interfilamentar junc­tions, there are minute apertures, called ostia, opening into the water-tubes.

A transverse section of the gill filament reveals that it is covered by ciliated epithe­lium (Fig. 16.32E) and is supported by chitinous rods. The cavity of the gill filament is filled with blood.

The long cilia present in the gill filament produce a current which drives the water from the mantle to the water tubes (Fig. 16.32D). The arrangement of gills shows that the water tubes open into the supra-branchial chamber which contin­ues posteriorly and opens to the exterior by exhalant siphon.

Mechanism of Respiration:

By the action of cilia, a constant flow of water-containing dissolved oxygen passes through the ostia into the water tubes (Fig. 16.32E).

From the water tubes water enters into the supra-branchial chamber and finally goes out to the exterior from the supra­-branchial chamber by the exhalant siphon. Gill filaments are highly vascular structures and during the transit of water through ostia to water tubes gaseous exchange takes place.

In addition to gills, the highly vascular mantle acts as an accessory respiratory struc­ture.

7. Circulatory System of Unio:

The blood is colourless and consists of plasma and few leucocytes. The heart con­sists of a muscular ventricle and two thin- walled auricles enclosed by pericardium. The auricles communicate with the ventricle. This opening is guarded by valves. Two aortae originate from the two ends of the ventricle (Fig. 16.33), one is the anterior aorta situated above the rectum and the other is the poste­rior aorta located below the rectum.

One of the peculiar features in Unio is that the rectum passes through the pericardium and the ventricle (Fig. 16.34). The aortae give rise to arteries such as pallial, gastric, pedal and intestinal arteries which supply the whole body. The blood from the different parts of the body is collected into a large vena cava located in between the kidneys. From the vena cava blood returns to the auricles by two ways.

In one circuit, blood flows into the gills by afferent branchial veins and then returns to the auricle by efferent branchial veins. Before coming to the gills blood from the vena cava passes through the kidneys. The renal veins ultimately unite to form the afferent branchial veins.

In the other circuit, blood from the vena cava comes to the au­ricle directly without going through the kid­neys and gills. Mantle also acts as an acces­sory respiratory organ from where blood returns to the auricle through pallial venis.

The circulatory circuit is schematically represented below:

8. Excretory System of Unio:

The excretory organs of Unio consist of pair of kidneys, often called organ of Bojanus, after the name of discoverer. These are situated postero-dorsally one on either side of the body beneath the floor of the pericardial cavity.

Structure:

Each kidney consist of two parts and ‘U’-shaped in appearance with two parallel arms. The lower arm is yellow­ish, glandular and thick-walled, forming the kidney proper and opens into the pericar­dial cavity by renopericardial aperture (nephrostome).

The dorsal arm is non-glan­dular, thin-walled and ciliated, known as ureter or urinary bladder. It opens into the mantle cavity by a minute opening, called renal aperture, (nephridiopore) situated between the visceral mass and the inner gill lamina (Fig. 16.34).

Physiology of excretion:

The functions of kidney (organ of Bojanus) in bivalves is to remove the nitrogenous waste products from the body, and also in freshwater species such as in Unio excrete large amount of water through the nephridia.

The kidneys of fresh­water bivalves also maintain the solute con­tent of the body fluid. The pericardial glands contain some cells which are like vertebrate podocytes and are believed the sites for ultrafiltration.

The pericardial fluid and blood contain the nitrogenous waste products pro­duced in the body during metabolic activi­ties. These products after filtration are col­lected by the glandular part of the kidney and are excreted through the nephridiopore.

The products pass through the upper arm by the constant outward movements of the cilia lining the inner surface of the ureter. The salt reabsorption probably takes place in the glan­dular part of the kidney.

In addition to kidneys, a large reddish- brown glandular mass known as Keber’s organ or pericardial gland is regarded to be excretory in function. It is situated in front of the pericardium and discharges the excre­tory products into the pericardial cavity.

9. Nervous System of Unio:

The nervous system of Unio (Fig. 16.35) comprises ganglia (aggregation of nerve cells), commissures (nerves connecting two similar ganglia), connectives (nerves connecting two dissimilar ganglia) and nerves. There are two cerebral ganglia. These are present one each at the base of labial palps just outside the corners of the gullet and are connected by a cerebral commissure.

A bilobed pedal ganglion, formed by the fusion of two gan­glia, is located at the junction of the visceral mass with the foot. This ganglion is con­nected with the two cerebral ganglia by two nerves situated one on each side of the body, called the cerebropedal connectives. Another bilobed ganglion, called the visceral ganglion, is present on the ventral side of the posterior adductor muscles.

The visceral ganglion is very large in size and is con­nected with the two cerebral ganglia by two cerebro-visceral connectives, one on each side of the body. The cerebral ganglia supply nerves to the anterior and posterior labial palps and to the anterior part of the mantle. Nerves are given off to the foot and its musculature by the pedal ganglion.

The vis­ceral ganglion gives out on each side, a dorsal and a posterior pallial nerve to the mantle, a renal nerve to the posterior end of the kidney, a branchial nerve to the gill and a nerve to muscles of the posterior adductor and a nerve to the anterior part of the ali­mentary canal.

Sense Organs of Unio:

The main sense organs are the Osphradium and the Statocyst. The osphradium an organ for tasting water and is present in connection with the visceral ganglion. It consists of a patch of sensory epithelium. The other sense organ is the organ of balance (statocyst), located near the pedal ganglion and has the nerve supply from the cerebco-pedal connective. There are two statocysts in Unio.

Each statocyst is a hollow sac lined by sensory cells and a centrally placed statolith (Fig. 16.36). Besides osphradium and statocysts, the margin of the mantle lobes are provided with sensory cells which are probably tactile organs and photoreceptors.

10. Reproductive System of Unio:

The sexes are separate. The male gonad or testis is white in colour and the female gonad or ovary is red. Both the gonads are large paired organs occupying the major portion of the visceral mass among the intes­tinal coils.

But Morton (1967) has claimed that the gonad is a single median organ in both the sexes. The gonads have a short duct that opens to the exterior by genital aperture located just in front of the renal apertures.

11. Development of Unio:

Development of the zygote starts within the external gill laminae which dilates to accommodate the developing embryo. Seg­mentation is complete and equal. After the formation of blastula and gastrula, a larval form emerges out. It is called Trochophore larva (see Fig. 17.12).

The trochophore larva has the same structural plan as in annelids. During development, this larval form subse­quently passes on to the characteristic Glochidium larva. These larvae are expelled from the body of the female to water (Fig. 16.37A).

Structure of Glochidium:

It has got a long bivalved shell. The valves are porous and united dorsally, but free ventrally. In a related genus Anodonta the free ventral ends of the valves become curved like hooks beset with many spines. Such structures are not observed in Unio. The body consists of an undivided mass which is differentiated into a single dorsally placed body proper and two mantle lobes, the right and the left (Fig. 16.37B).

A glandular pouch is present on the ventral side of the body which secretes a very long thread called the provisional byssus. A single large adductor muscle, which arises from the mesoderm, is present to connect the two valves. The byssus moves to and fro and attaches itself with the gills of freshwater fishes.

The glochidium penetrates the host tissue and becomes enclosed by gill epithe­lium. It lives there as an ectoparasite for about two and half months (Fig. 16.37C). The provisional byssus and sense organs disap­pear and metamorphosis starts.

The stomodaeum is formed by invagination of the ectoderm. Foot originates as a ventral eleva­tion. After such development, the young glochidia detach themselves from the gills of fish and sink to the bottom of the water (Fig. 16.37D) and attain adult morphology.