B. T.S. of Osphradium
2. Filibranchia (Arca)
1
. Protobranchia ( Nucula)foot
Inner gill lamina mantle
perforations
branchial septum Inner lamellar junctions
supra branchial chamber Outer gill lamina shell
Visceral mass
Outer gill filament Inner gill filament
shell
Fig.61 Unio: Gill lamina in section
Outer lamella Inner lamella
Water tubes Chitinous
rods
Inter lamellar junction
blood vessels
ostia Vertical gill filament
CLASS CEPHALOPODA:
The members of this class possess simple, bipectinate gills on either side of the anus. The ctenidia consists of a central axis with delicate leaf-like lamellae arranged on either side of the axis in a linear manner. The cilia on the gills are absent in them and the water is pumped in and out by the coordinated activities of the mantle, funnel and the inlet valves. A typical ctenidium and its functioning has already been described in the type study of Sepia.
The number of gills may vary, e.g. dibranchiates possess two gills (Sepia, Octopus etc.) while the tetrabranchiates (Nautilus) have four.
MANTLE
Mantle or pallium is a thin delicate covering formed by the fold of the body wall which secretes the shell. The active secretory centre is the mantle edge. Mantle encloses a cavity called the mantle cavity which contains the visceral organs. The mantle performs three functions:
1. Protects the visceral mass and the head.
2. Secretes the shell.
3. Serves as the additional respiratory organ in some molluscs.
A brief description of the mantle in various classes of mollusca is given below:
MONOPLACOPHORA:
The mantle lies just within the edge of the shell. A mantle groove is present between the foot and the mantle edge.
APLACOPHORA:
The dorsal body wall extends laterally to cover the whole animal except for the foot (when present) and is called the mantle. The epidermis of the mantle secretes the cuticle. The cuticle is made up of glycoprotein and contains calcareous spicules of different shapes.
POLYPLACOPHORA:
The mantle covers the dorsal surface. Like the aplacophorans, the mantle of the polyplacophorans also secretes the cuticle. The cuticle may be smooth, or bear scales, bristles, or calcareous spicules. The mantle is a thick and heavy dorsal layer extending right upto the lateral margins of the body, covering partially or entirely the shell valves. The mantle extends well beyond the lateral margins of the shell valves. The outer thick and fleshy mantle edge is sometimes called as the girdle.
SCAPHOPODA:
The mantle has two large lateral folds that extend the entire length of the body. The lateral folds reach the ventral midline and fuse to form a tube, open at both ends. The tubular mantle encloses the whole body. The mantle extends posteriorly on the dorsolateral side and is equipped with sensory receptors. The mantle cavity is large and ventral, extending from one
deep longitudinal slit which serves to eliminate the faecal matter. In Haliotis, the mantle bears a series of marginal slits while in Fissurella an apical hole is present in the mantle. In Pila, the free border of the mantle is attached to the margin of the shell and is in contact with the region above the head. The mantle is produced into highly contractile process called the nuchal lobes or pseudepipodium, the left being longer than the right and forms a respiratory siphon during aerial breathing. The mantle lining is thickened and differentiated into a glandular structure called the pallial mucus gland present between the gills and the rectum.
BIVALVIA:
The mantle consists of two lobes corresponding to the two valves of the shell and encloses the whole body. The two mantle lobes are fused dorsally as the mantle isthmus underlying the hinge ligament, but their margins are not fused. The two lobes are formed from the dorsal part of the body-wall. Each mantle lobe is attached to the corresponding shell valve by pallial muscles.
The bivalves differ in the degree of fusion of the two mantle lobes which plays an important role in the evolution of the adult body form and its habit. The simplest fusion is by the temporary apposition of the mantle edges of the two lobes. However, in bivalves where permanent fusion of the mantle lobes occur, it involves first the union of the inner folds, followed by the middle and finally by the inner surface of the outer fold of the mantle edges.
This fusion is brought about by the cilia and the tissues.
In the fresh water mussel like Unio, the mantle lobes are free ventrally and anteriorly, but are fused posteriorly in the middle to form a dorsal and ventral siphon. Water current is constantly passing in by the ventral siphon and going out by the dorsal siphon, so the siphons are called as the inhalant and the exhalant siphon respectively. The inhalant siphon is wider, with papillated margin. It is formed by the coming together of the two mantle lobes. On the contrary, the exhalent siphon is narrower, smooth and is formed by the fusion of the two mantle lobes. This is an adaptation for burrowing deep in the soil. Some fresh water mussels like Anodonta have an additional dorsal opening called the supra-anal aperture above and anterior to the exhalent siphon. However, the function of this aperture is obscure.
In some bivalves, like Nucula, Arca, Anomia and Trigonia, the edges of the mantle are free from each other, so that the siphons are absent in them. There is only a temporary fusion of the mantle edges, leaving two gaps: one anteriorly and the other posteriorly. The incurrent water enters through the anterior gap and traverses up in between the gill filaments into the dorsal exhalant chamber and finally leaves the mantle cavity through the posterior gap. In others, the edges of the two mantle folds are fused with each other at one or two places.
Many bivalves like oyster Ostrea and the scallop Pecten, the inner fold of the mantle edge is well developed and is known as the velum or pallial curtain. Velum bears marginal sensory tentacles. These sensory structures are a common feature of the mantle border of bivalves.
In the sedentary bivalves like Mytilus, the inhalant siphon is not properly differentiated. The bilobed mantle forms the exhalant siphon posteriorly. This lack of fusion of the mantle edge is because of the presence of a well developed byssus apparatus.
In Solen, the mantle lobes are united ventrally to form an elongated tubular mantle cavity.
Teredo has very long siphon fused together, and the mantle forms a closed tube. The siphon is non-retractile.
In Mya, the elongate siphon is covered with chitinous plates and is incompletely retractile.
In a few bivalves, in addition to the formation of the siphon, the ventral edges are fused to a greater or lesser extent leaving an opening for the foot to protrude.
CEPHALOPODA:
In cephalopods the thick, muscular mantle covers the trunk. The free oral edge of the mantle fits loosely around the neck to form a collar, and thus leaves a circular opening. A muscular, conical tube called the siphon or funnel projects beyond the collar just beneath the head. The funnel represents the molluscan foot. It is through this funnel that the water is expelled out of the mantle cavity. One end of the funnel opens into the mantle cavity by a wide aperture, while the other end opens to the outside by a narrow aperture. The mantle encloses the large mantle cavity posteriorly and ventrally. The visceral mass occupies most of the space of the mantle cavity. Like any other mollusc, the epithelial lining of the mantle secretes the shell which in most of them is internal.
TORSION IN MOLLUSCA
The most significant feature of the prosobranchiate gastropods is torsion or twisting of the internal organs of the body.
What is torsion?
Torsion is a process in larval gastropods whereby the visceropallium is rotated anticlockwise through 180° from its initial position on the head foot complex bringing the posterior mantle cavity with its pallial complex, anus, rectum etc to the front of the body behind the head.
Torsion takes place because of the retardation of growth on one side and active extension on the other. As has been already stated that it is the right side where the growth gets retarded and therefore the right side begins to move forward (Fig.62).
It is a developmental event that occurs in larva and not in adult.
Before torsion the larva is:
• Bilaterally symmetrical.
• The mantle cavity opens ventrally and posteriorly.
• It possesses a simple straight gut with mouth placed anteriorly and anus at the posterior end.
• A pair of ctenidia, osphradia and nephridiopore are posteriorly situated.
• Auricle is situated behind the ventricle.
• The nervous system is bilaterally symmetrical.
• The bilaterally symmetrical larva undergoes torsion as a result of differential growth.
After torsion:
• The mantle cavity and its associated parts shift forward to take the anterior position.
• Ctenidia, osphradia, and the two nephridiopores come to lie in the anterior part of the body behind the head.
• Digestive system becomes U-shaped so that anus comes to lie in front near the mouth.
• Auricle now becomes anterior to the ventricle.
pleuro- visceral nerve connective. The right pleuro-visceral connective with its ganglion passes over the intestine and becomes the supra-intestinal nerve while the left connective passes under the intestine and becomes infra-intestinal nerve. This is known as chiastoneury.
However, the head and foot retained the original bilateral symmetry and the shell retained the symmetrical spiral (Fig.63).
In the majority of gastropods, torsion takes place in two steps:
Step I:
The contraction of the larval retractor muscles account for 90°rotation of the visceral hump.
The mantle cavity at the end of this stage comes to lie on the right side with the foot projecting on the left side. This stage lasts for only a few hours.
Step II:
The torsion now is the result of only differential growth and is longer in duration.
However, the actual mechanism of torsion is not known. Thomson in 1958 has suggested five possible ways in which torsion has taken place in gastropods.
1. The rotation of visceral hump by 180°is achieved by muscular contraction only. He regarded this as the original way of torsion. This mechanism was seen in Acmaea.
2. The rotation of the visceral hump by 180°is achieved in two steps:
• The initial 90°rotation is caused by the larval retractor muscle.
• The remaining 90°is by differential growth.
The first step is however faster than the second. This is the most common way of torsion and is encountered in Haliotis, Patella etc.
3. In some gastropods like Vivipara, the complete rotation of 180°is achieved by differential growth alone.
4. In Aplysia, the differential growth is responsible for torsion, and the position of anus is halted at a region appropriate to the adult stage.
5. In Adalaria, torsion of the visceropallium is not recognisable. The different organs appear as in the post-torsional position.
shell
right mesodermal band midgut cavity
left mesodermal band
Fig. 62 T.S. of veliger larva showing disproportionate growth of mesodermal cells
2'
4
6 11
10
8 12
9 14 5 1
3 2
7
4
1
2
8 3
13 5 9 7
14 16 12 10
Fig.63 Torsion in Gastropods. 1. Mouth; 2. Right tentacle; 2‘. Left tentacle; 3.
Right visceral loop; 4. left visceral loop; 5. Right visceral ganglion; 6. Left visceral ganglion; 7. Right ctenidium; 8. Left ctenidium; 9. Right osphradium;
10. Left osphradium; 11. Mantle; 12. Anus; 13. Ventricle; 14. Visceral loop; 15.
Right auricle; 16. Left auricle.
13 15 16
15
Hypothetical ancestral stage before torsion Displacement of the mantle cavity to the right side of the body
Intermediate stage showing 90°torsion
Complete torsion
SIGNIFICANCE OF TORSION:
It is not clear whether torsion is of an advantage or not to the animal, or if it has any evolutionary significance, but it does take place during embryological development of gastropods. Many zoologists have postulated that that torsion represents a larval adaptation for the protection of the head. This view was first putforth by Garstang in 1928 and was supported by Ghiselin in 1966. Before torsion, the larva was an easy victim of the predators because of the posterior position of the mantle cavity in which case the foot would be withdrawn first and them the delicate vital head. The anterior position of the mantle cavity in the larva, after torsion, resulted in greater protection of the head and its associated parts by providing them with a cavity into which first the vulnerable parts and then the foot can be withdrawn at the time of danger.
Ghiselin (1966) suggested some evolutionary changes to account for torsion:
The shield like shell of the ancestral mollusc was of little protective value for the planktonic larva. Thus, a conical shell with a small aperture evolved which exposed only a small part of the body and at the same time served as a retreat into which the larva could withdraw its body at the time of danger. A long, straight conical shell was difficult to carry while swimming and thus the shell became spirally coiled. This planospiral shell was advantageous for the free swimming larval life but was of disadvantage for the adult crawling mode of existence.
According to him, torsion was an adaptation to correct this difficulty. Rotation of the visceral mass by 180°displaced the coils of the shell to a trailing position behind the animal, thus preparing the animal for settling down for its adult mode of existence.
Crofts (1937,1955), Youge (1947) and Eales (1949, 1950) also supported Garstang view.
However, there were many objections to this theory:
1. In case of Heliotis, the pelagic larva undergoes torsion only through 90°, so that the head cannot be retracted first at the time of danger. Complete torsion of 180°
occurs only when the larva settles down at the bottom.
2. Many pelagic larvae of lamellibranchs donot undergo torsion, but still they survive.
3. The velar cilia on some gastropod larva are under nervous control and they can be stopped even without being forcibly withdrawn into the mantle cavity.
4. The anus and nephridiopore are also anteriorly placed after torsion, thus discharging their waste on the animals head.
If the larva is not benefitted from torsion, then the adult must have been benefitted as has been proposed by Morton in 1958.
• The anterior position of the mantle cavity after torsion might facilitate the ventilation of the mantle cavity and the gills. Also, the anterior position provides the mantle cavity with water free of sediments.
drawn from this side and after bathing the gill, it leaves by exhalant current on the other side. Also, the anus and renal aperture are located far back in the mantle cavity. The excretory waste matter and the feces are carried away by the exhalant current.
DETORSION
Detorsion is the complete reversal of the changes that take place during torsion. It is a characteristic feature of opisthobranchiate gastropods. A very good example where detorsion occurs is Aplysia. Detorsion occurs when the shell is reduced or lost. The visceral hump during detorsion gets completely untwisted and the pallial complex shifts back to its posterior position. The ctenidia once again take a posterior position. Their anterior position is of no advantage to them. The visceral loop of the nervous system becomes untwisted and symmetrical. However, the organs lost during torsion are not replaced as a result of which the opisthobranchs also possess only one ctenidium, one kidney, and one auricle. In some opisthobranch mollusc like Acteon, and Bulla, detorsion is partial. The visceral loop remains partly twisted in them and the ctenidium and anus are laterally directed.
COILING
Coiling is a post larval development. It is believed that the early gastropods had planospiral shell where each coil is located completely outside the coil of the preceding one but in the same plane. The shell is exogastrically coiled, i.e. the direction of coiling is anterior over the head. The centre of gravity of exogastric shell would cause the shell to fall forward in front of the head so that the animal would have to push the shell forward over the vegetation. Torsion and endogastric coiling (direction of coiling is posterior over the foot) solved this problem.
PEARL FORMATION
Pearls are formed in some bivalves. It is a natural secretion deposited by the mantle epithelium as a protection against foreign bodies such as sand grain, debris, any microscopic organism or even a parasite. The foreign body enters the bivalve and occupies a position in between the shell and mantle thereby causing irritation to the animal (Fig.64, 65). This induces the epithelial cells of the mantle to secrete concentric layers of nacre around the foreign particle in defence and finally pearl is formed (Fig.65). The foreign particle inside the pearl is known as nucleus and the concentric layers of nacre around it as mother of pearl. If the foreign particle is enfolded within the mantle and moved about during secretion, the pearl becomes spherical or ovoid.
Finest natural pearls are produced by marine pearl oysters of Eastern Asia, of the genus Pinctada namely Pinctada margaritifera, Pinctada mertensi and Pinctada vulgaris. Unio and Anodonta also produce pearl but of inferior quality and rarely of any use.
shell mantle gill
pearl hinge
Fig.64 Site of pearl formation in Mytilus
FINAL STAGE PROGRESSIVE STAGE PRIMARY STAGE
pearl
ciliated epithelium of mantle nacre secreting cells of mantle foreign body
nacreous layer of shell
Fig. 65 Stages of pearl formation
connective tissue
nacreous layer of shell
foreign body
nacre secreting cells of mantle
ciliated epithelium of mantle layers of nacre
connective tissue
layers of nacre
PEARL CULTURE
Today pearls are artificially produced and Japan is one of the largest countries in the world to produce the bulk of pearls by pearl culture technique. Mr. Mikimoto of Japan has discovered a method of stimulating the pearl oysters to form pearls by artificially introducing foreign particles between the shell and mantle. Small windows are made on the shell through which sand grains, shell pieces etc. are introduced. The treated oysters are kept in perforated cages and lowered in the sea till pearls are formed. The main difficulty encountered in this method was that the partially exposed pallial membrane was prone to infection and damage.
The latest technique used is the grafting method or nuclear insertion method. In this method, a calcareous substance (nucleus) is grafted with a piece of mantle into the body of pearl oyster. The entire technique is performed very systematically which can be summarised in steps: