An Example of Phylum Mollusca: Chiton

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

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

1. Habit and Habitat of Chiton:

Chitons are very slow moving molluscs. They usually live in shallow water and are distributed in the rocky shores. Some of the members belonging to the family Lepidopleuridae live in deeper zones. They are nocturnal animals and remain concealed under rocks during daytime. They have the habit of rolling up their bodies like that of Diplopods. They are herbivorous.

2. External Structures of Chiton:

Chiton has an oval and dorso-ventrally flattened body. The dorsal side is convex and the ventral side is flattened. The middle portion of the dorsal side is covered over by eight overlapping pieces of calcareous shells arranged antero-posteriorly (Fig. 16.5A). These pieces are called the valves or shell plates. The valves are arranged in an imbri­cate fashion and are movably articulated with one another.

All the eight shell plates are encircled by the thick lateral mantle margin, called girdle. The width of the mantle girdle varies in different species. This girdle is hard and contains numerous calcareous spicules, scales and spines. The ventral side of the mantle girdle is comparatively soft and con­tains less calcareous elements. The ventral surface of the body is occupied by a broad elliptical foot (Fig. 16.5B).

The foot may be long and narrow in certain species as in Cryptoplax. The head is inconspicuous and is situated in front of the foot. The head is separated from the foot by a groove. The head bears a centrally placed mouth, but the eyes and tentacles are absent. A deep groove, present on the ventral side between the foot and the mantle, is called mantle groove.

This groove contains external gills of variable number. The genital ducts and the excretory ducts open into the postero-lateral part of the mantle groove. The mouth and the anus are placed at the two extremities of the body. The anus is situated posterior to the foot and occupies a median position.

Structure of shell plates:

As described earlier the shell of Chiton consists of a longi­tudinal series of eight pieces of shell plates. In majority of the species the valves remain in contact with each other but in Cryptoplax the valves remain separated. The valves in accordance with the dorsal convex side of the body, are curved.

The first and the last valves are smaller and hemispherical in outline. The rest of the valves (median valves) are almost triangular in outline. The surface of the me­dian valves is differentiated into three trian­gular areas—a median and two laterals.

The shell plates are made up of two layers—tegmentum and articulamentum. The tegmentum is the outer layer and is composed of organic materials and the articulamentum is a calcareous layer. The tegmentum is absent in Cryptochiton. It has been suggested by many workers that an organic layer called periostracum is present over the tegmentum.

The articulamentum is comparatively thick. The outer portions of the articulamentum covering the tegmen­tum of the lateral triangular regions of the shell plates, are called insertion plates. These insertion plates actually help in the attach­ment of the valves with mantle. The inser­tion plates usually possess insertion teeth.

3. Coelom of Chiton:

The primary body cavity in Chiton is not coelomic in nature. This is represented by extensive lacunae and they are filled up with blood. This cavity may best be regarded as a haemocoel. The true coelom or secondary body cavity is restricted to the pericardial cavity, lumen of gonad and kidneys.

4. Digestive System of Chiton:

The mouth is a round or oval aperture, situated at the centre of the head. The mouth leads into a very short buccal tube which is lined by epithelium thrown into longitudi­nal folds. The epithelium is composed of columnar cells with interspersed mucous cells. The buccal tube is continued into a wide buccal cavity.

The buccal cavity is lined by circular and longitudinal muscle fibres. It bears a pair of simple buccal glands (Fig. 16.6) at the anterior end, and the posterior end is prolonged into a blind sac called subradular pouch, which bears a subradular organ situ­ated ventral to the radular sac.

This organ is regarded as chemoreceptor and can be pro­truded into the mouth to test the substratum for food. There are no jaws in Chiton. The buccal cavity contains a long ribbon-like radula. The radula is enclosed by connective tissue sheath and contains transverse rows of teeth.

Each transverse row contains 17 teeth having a central tooth, flanked by eight teeth on each side. The teeth, are very hard and composed of dentine and instead of enamel are covered by iron. The buccal cav­ity leads into a short pharynx which produces two glandular diverticula, one on each side.

These glandular diverticula are called pha­ryngeal glands and have the same histologi­cal picture as found in the buccal glands. The pharynx continues posteriorly as a narrow oesophagus and receives ducts from two large salivary glands. The salivary glands are des­ignated now as the sugar glands which discharge amylase (carbohydrate splitting enzyme) into the oesophagus.

The oesophagus leads into a large stomach. The stomach assumes various shape in dif­ferent species due to the pressure of the voluminous digestive diverticula or liver. These glands have lobulated structures and they surround the stomach (Fig. 16.6). They occupy the spaces between the coils of the intestine.

These diverticula secrete both pro­teolytic as well as cellulose-splitting enzymes. They open into the stomach by ducts. Diges­tion is largely extracellular in nature. The next part of the alimentary canal comprises of a long and coiled tube called intestine which opens to the exterior through the anus.

The intestine is divided into two portions with a sphincter valve in between. The valve per­mits retention of food in the stomach, diges­tion and absorption at the anterior portion of the intestine and also the regulation of entry of waste into the posterior portion of the intestine in the form of faecal pellets. The anus discharges these pellets exterior through exhalant respiratory current.

5. Locomotion of Chiton:

Chiton creeps very slowly by the ventral foot (see Fig. 16.5B). The foot is modified for creeping movement. The movement is caused by muscular waves produced along the foot from the anterior to the posterior direction.

About 15-30 seconds are required for a wave to cover the entire length of the foot, and Chiton proceeds 4-8 mm by such a complete wave. Chitons can also adhere to the substra­tum very firmly to tide over the strong wave action of the sea.

6. Respiratory System of Chiton:

The respiratory system of Chiton includes small external gills of variable number. The number varies from 6 to 88 pairs. The number of gills are not a constant feature of a species but the number increases with the increase of size of the body.

The gills are arranged in the mantle groove (Fig. 16.7A). The mantle groove is divided into two narrow chambers by a continuous curtain of gills on either side. One chamber is designated as the inhalant cham­ber while the other is called exhalant cham­ber.

The inhalant chamber is located between the gill row and girdle and the exhalant chamber is located between the edge of the foot and the inner side of the gill row. A single bipectinate gill has two series of flat and oval lamellae arranged on a central axis. The lamellae are lined by ciliated epithelium.

The gills may extend the whole length of the body (holobranchiate type) or may be restricted towards the posterior end of the body (merobranchiate type).

The holobranchiate types of gills are present in most cases, but merobranchiate types are present in a few forms like Schizochiton, Lepidopleurus. The flow of blood between the gill lamellae is opposite to the direction of the water flow, forming the counter-current exchange system. This helps the gas exchange between the gills and water.

7. Circulatory System of Chiton:

The circulatory system consists of the pumping organ heart, arteries, sinuses and blood. The blood of some Chitons contains a soluble respiratory pigment (haemocyanin) and corpuscular elements.

The heart consists of a median tubular ventricle and a pair of lateral auricles. They are located in the peri­cardial cavity which is filled with a proteinaceous liquid. The pericardium is situ­ated beneath the last two shell plates and its dorsal wall is fused with the dorsal body wall.

The ventricle gives off a median dorsal aorta which supplies blood to the different parts of the body. The head contains a blood sinus of arterial nature. This sinus is called the head sinus.

From this sinus an arterial channel, called visceral artery, proceeds posteriorly and ramifies in the viscera. From the viscera venous blood is collected in the visceral sinus. The head sinus also gives off several longitudinal channels on the posterior side.

Of these channels, the important ones are the paired pedal sinuses and paired neuropedal sinuses. Besides these, there are a few more sinuses called the neurolateral sinus and the afferent and efferent branchial sinuses. The median longitudinal sinuses of the foot primarily receive most of the venous blood.

This blood is communicated to the afferent branchial sinus by large transverse sinus. After oxygenation the blood is col­lected by efferent branchial sinus. From the efferent branchial sinus most of the blood returns to the auricle by a number of aper­tures called auricular pores.

The arrangement and number of the lon­gitudinal sinuses vary in different forms of Chiton. The important variation is the ab­sence of visceral artery in most cases.

8. Excretory System of Chiton:

The excretory system consists of two slen­der symmetrically placed kidneys (Fig. 16.7B). Each kidney is more or less a Y-shaped tube. The single long limb of the ‘Y’ constitutes the main body and ends blindly at the anterior side.

This tube receives numerous minute tu­bules that ramify in the viscera. The two shorter limbs have openings, one is the nephridiopore situated near the genital pore and the other opens into the pericardium through a ciliated aperture called reno-pericardial aperture.

9. Nervous System of Chiton:

The nervous system of Chiton is very simple and consists of a circumenteric nerve ring which is divided into an anterior cerebral half (cer­ebral commissure) and a lower thin buccal part which gives rise to the buccal and subradular connectives. The buccal ganglia are connected by buccal commissure on the dorsal surface of the buccal cavity.

The cerebral half is thicker, lacks ganglia and gives off posteriorly two pairs of descending nerve cords, the pedal and pleural (or palliovisceral) nerve cords. The pedal cords innervate the musculature of the foot and the palliovisceral cords send nerves to mantle and visceral organs. Both the nerve cords are connected by transverse nerves.

Pedal nerve cords are free posteriorly whereas the pleural nerve cords are joined posteriorly to form suprarectal commissure. Such ladder­-like arrangement of the nervous system reflects primitive organisation.

Sense Organs of Chiton:

Specialised sense organs are lacking in Chiton. Elongated neurosensory cells with terminal bristles are quite abundant.

Gustatory organs:

They are situated in the floor of the buccal cavity. These organs are composed of sensory and supporting cells.

Subradular organ:

The subradular organ is regarded as chemoreceptor. It is one in number and situated on the posterior end of the subradular pouch.

Sensory epithelium:

There are patches of sensory epithelium in the pallial groove and these are said to be homologous with osphradia.

Aesthetes:

The tegmentum of shell plates is perforated by numerous canals which are provided with many light sensitive (photoreceptors) and tactile organs (tactoreceptors). These organs are known as aesthetes. Aesthetes are divided into two types according to size—megalaesthetes and micraesthetes.

The dorsal surface of Chiton is sensitive to light and tactile stimuli. They are innervated by the lateral nerve cords. A megalaesthete has a slender core of elongated sensory cells surrounded by glandular cells. These two components are surrounded by an epidermal layer which is continuous with the nerve cords.

The non-nucleated strands emerg­ing from the megalaesthetes are called the micraesthetes. Both of them ascend vertically to the surface through tegmentum and termi­nate in a hard cap. The caps of the megalaesthetes are called megalopores and those of micraesthetes are called the micro-pores.

In some forms of Chitons, few micraesthetes assume the form of eyes with cornea, lens, pigmented layer and retina (Fig. 16.8). The number of eyes in a few species are enormous. In Acanthopleura echinata the total number of eyes present on the valves are estimated to be 11,500. The number of micraesthetes present on a single megalaesthete vary from 20 to 25. In some cases micraesthetes may be absent.

10. Reproductive System of Chiton:

The sexes are separate. The testis and ovary are more or less similar in appearance. The gonad is placed on the dorsal side in front of the pericardium and above the intes­tine. The gonoducts are two and each opens into the mantle groove in front of the excre­tory aperture (Fig. 16.7B). In almost all the forms excepting Nuttalochiton hyadesi and Notochiton mirandus, the gonad is single and is medially placed.

11. Development of Chiton:

Fertilization occurs in the mantle cavity. In Callistochiton viviparus, a viviparous spe­cies, entire development of the young takes place in the female gonoducts. Segmentation is complete and occurs typically in spiral fashion. At the eight-cell stage, four small upper tiers of cells called micromeres are produced and the lower four large cells are called macromeres.

Each macromere then gives origin to three micromeres which trans­form into ectoderm. The macromeres gives rise to the endoderm. The mesoderm emerges from the cells originating from the posterior macromeres.

Both micromeres and macro­meres arrange themselves in such a way that a flattened blastula with blastocoel is produced. Gastrulation occurs typically on an embolic fashion, i.e., by the inpushing of cells.

By subsequent embryological changes a fully-formed larva is formed which comes out of the egg envelope. This larval form is called Trochophore larva. It resemble closely the trochopore larva of Annelida (Fig. 17.2). It has an oval body with prototroch, apical sensory tuft of cilia, a pair of eyes below the prototroch and primitive archenteron.

The trochophore larva after coming out from the egg envelope swims in water for a period varying from 15 minutes to a few days according to species.

The larva then sinks to the bottom and undergoes metamorphosis. The first noticeable change is the dorso­ventral flattening of the body of larva. The prototroch, eyes and the apical tuft of cilia are lost and a young Chiton is formed which attains adult morphology in due course.