An Example of Phylum Mollusca: Achatina

In this article we will discuss about Achatina:- 1. Distribution of Achatina 2. Habit and Habitat of Achatina 3. External Structures 4. Coelom 5. Digestive System 6. Locomotion 7. Respiratory System 8. Circulatory System 9. Excretory System 10. Nervous System 11. Reproductive System 12. Development.

Contents:

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

1. Distribution of Achatina:

Achatina (Lissachatina) fulica is a common giant land snail of India and the Indopacific islands. The anatomical organisation of Achatina is more or less similar to that of Pila although they belong different orders of the class Gastropoda. The anatomical peculiari­ties in Achatina are due to adaptation to land life.

Achatina fulica is a native of Africa. They are found today in some parts of India, Pakistan, Bangladesh and the Indopacific islands. In India, they are abundant in the northern and eastern parts of West Bengal and the Balasore district of Orissa. They are rare in the western districts of the state of West Bengal, Orissa and some districts of Bihar.

2. Habit and Habitat of Achatina:

Achatina is a terrestrial pulmonate and inhabits open grasslands, gardens and simi­lar habitats. They prefer humid areas. They are mostly nocturnal and spend the daytime under stones, wooden logs and plants. They are found to climb vertical walls and trees up to the height of about 5.8 m. They aestivate during summer to react in pending desicca­tion. They exhibit divergent food habits.

They are herbivorous and eat the vegetation avail­able in the area. The presence of well-devel­oped two-lobed crop in Achatina is an adap­tation to devour maximum quantity of food and to store them for future use. Although Achatina is primarily a herbivorous mollusc, it equally prefers to take dead insects and snails. It has been reported by Ghose (1963) that Achatina takes pieces of meat in labora­tory.

3. External Structures of Achatina:

The shell covering the body is elongated and the operculum is absent. The shell is of light horny colour. The columella and whorl of the shell is whitish in adults. The head is comparatively long (Fig. 16.21A) and can be projected to a great extent (Fig. 16.21B). Eyes are two in number and are placed at the tip of the longer tentacles.

The longer tentacles are designated as the ocular tentacles which can regenerate fully with all their component parts. Besides these long tentacles, there are a pair of small ventral tentacles in the snout. Genital aperture is located on the right side posterior to head.

The foot is flat and is modified for creeping by the large ventral sole (Fig. 16.21C). The foot is provided with a large tubular slime gland which produces profuse quantity of slime. The mantle cavity is closed excepting the right-hand side near the pulmonary aperture.

Removal of mantle shows the disposition of the internal organs (Fig. 16.22).

4. Coelom of Achatina:

In Achatina, the internal spaces are haemocoelic. The true coelom is restricted within the pericardial cavity and the cavity around the gonad.

5. Digestive System of Achatina:

The digestive system in Achatina is re­markably simple. This simplicity is corre­lated to terrestrial life. The digestive system consists of alimentary canal and digestive glands. The alimentary canal comprises of buccal mass, oesophagus, crop, stomach, intestine and rectum. The digestive glands are the salivary and digestive glands (Fig. 16.23).

(i) Mouth:

It is a semicircular opening placed ventrally to the anterior part of the snout. It encloses a small cavity (vestibule) posteriorly. The labial palps are placed dorsal to the corners of the mouth.

(ii) Buccal mass:

The anterior part of the alimentary canal becomes modified into the buccal mass. The buccal mass is thick-walled and muscular. It is broader at the posterior end and narrower anteriorly. The buccal mass can be protracted and retracted by sets of protractor and retractor muscles respectively. The buccal mass has a complicated struc­tural construction.

The mouth opens into the buccal cavity and is guarded dorsally by semicircular jaw on the dorsal surface. The jaw is composed of a homogeneous carti­lage. The jaw is operated by powerful sphinc­ter muscle. The ventral surface of buccal mass is formed by the sphincter muscle.

The radula is present on the floor of the buccal cavity. It is a ribbon-like structure with a blunt posterior end and a narrow anterior end. It contains 140 rows of teeth, each row having 129 teeth. The radula is operated by well-developed extrinsic muscles. The for­ward movement of the radula is caused by the anterior and antero-ventral radular pro­tractor muscles.

The backward pull is ef­fected by the radular retractor muscles. The radular sac starts from the posteroventral side of the buccal cavity while the oesopha­gus and the salivary glands run from the dorsal surface. The radular sac contains a central rod, called collostyle. This rod is oval in shape and small in size. It is composed of white dense connective tissue elements. The buccal cavity is lined with a thin cuticular layer.

The cavity is divisible into two cham­bers. The anterior chamber is tubular and the posterior chamber is dorsally compressed and arched. The peculiar configuration of the posterior chamber is due to the presence of an odontophore. The main body of the odontophore is formed by the buccal carti­lage. It is horseshoe-shaped and supports the radula.

(iii) Oesophagus:

The buccal cavity leads into a narrow thick-walled tube, called oesophagus. The inner wall is ciliated and bears longitudinal folds and is covered exter­nally by flattened epithelium. The oesopha­geal glands are present in the wall.

(iv) Crop:

The crop is a spacious thin-walled sac. The junction of the oesophagus and crop is marked by the large lumen of the latter. The crop is divided into an anterior and a posterior chamber.

A constriction is present between the two chambers which acts as sphincter and regulates the passage of food materials. The posterior chamber opens into the stomach by a round aperture. A short and wide duct from the digestive gland opens into the posterior chamber.

Histologi­cal picture of the crop shows the presence of the following layers:

(i) Innermost layer—composed of co­lumnar epithelium. Often ciliated and interspersed goblet cells are present.

(ii) The outermost layer consists of mus­cles which are differentiated into an outer circular muscle layer and an inner longitudinal muscle layer.

(v) Stomach:

The crop opens into the stom­ach which remains embedded in the diges­tive gland. The stomach is a thick-walled heart-shaped structure. The crop, stomach and intestine form an ‘U’-like configuration.

The base is formed by the stomach and the limbs are formed by the crop and intestine. The wall of the stomach is muscular. The epithelium is ciliated with numerous gastric glands. A digestive duct opens into the pos­terior part of the stomach.

(vi) Intestine:

The intestine is an elongated coiled tube. It ends in the rectum. The wall is thin with patches of longitudinal fibres and intestinal glands.

(vii) Rectum:

The rectum is a straight tube. It runs along the posterior wall of the mantle cavity and terminates in anus. The anal re­gion of the rectum bears strong muscular folds which serve as sphincter. The wall is thin and the longitudinal and circular mus­cles are present. The epithelium is ciliated with rectal glands.

Two salivary glands and one digestive gland are associated with the alimentary canal.

(viii) Salivary glands:

The salivary glands are elongated paired structures. These glands are cream-white in colour. They are united posteriorly but are free anteriorly. The sali­vary ducts open into the buccal cavity. These glands are many-lobed structures and each lobe is composed of mucous and serous alveoli. The secretion of the salivary glands is poured into the buccal cavity.

(ix) Digestive gland:

The digestive gland is a large spirally twisted gland. It is bilobed and divided into an anterior lobe and a posterior lobe. The duct from the anterior lobe opens into the crop while the duct from the posterior lobe opens into the stomach. The digestive gland is composed of numer­ous lobules. Each lobule is composed of four types of cells.

Storage, calciferous, vesicular and pseudopodial cells are claimed to be present. All the cells excepting the calcifer­ous cells are digestive in function and they are of columnar type. The calciferous or lime cells are roughly triangular in shape with granules of calcium phosphate.

6. Locomotion of Achatina:

Achatina moves by the foot. The foot is very much elongated and has a flat ventral sole. The secretion of the slime gland and the musculature of the foot help in progression. It moves on the substratum by producing waves of contraction passing from the pos­terior to the anterior end along the sole.

The sole is lifted from the substratum at the sites of contraction waves and proceeds forward as the areas between the waves again restore contact with the surface. By such alternate process of lifting from surface and making contact with the substratum Achatina moves very slowly from one place to another.

7. Respiratory System of Achatina:

Achatina is exclusively terrestrial and re­spires by the mantle cavity which transforms into pulmonary sac or the so-called lung. The wall of the mantle cavity is highly vas­cular.

8. Circulatory System of Achatina:

The heart of Achatina is composed of one auricle and one ventricle (Fig. 16.24A). The heart is enclosed by pericardium. The peri­cardial chamber is situated on the left side of the mantle and lies parallel to the kidney. The pericardial chamber has an elongated oval appearance. The pericardium is a closed sac except for the renopericardial opening.

The pericardial chamber is narrower towards the ventricular end and broader at the au­ricular end. The pericardium is fused with the apex of the ventricle near the base of the aortic ampulla. The auricle is also fused with the pericardium. The pericardium is a tough membrane consisting of interwoven muscle fibres.

The auricle is a thin-walled wine-glass- shaped chamber. The narrow auricular apex is continued with the pulmonary vein. The wall is provided with longitudinal, trans­verse and oblique-branched muscle strands. These strands form a coarse meshwork and reduce the cavity of the auricle (Fig. 16.24B).

The ventricle is a conical chamber with a narrow apex and a broad base. The wall of the ventricle is thick and spongy. The muscle fibres form prominent ridges on the inner side of the ventricle. The myocardium is made up of non-striated, ramified and syn­cytial muscle fibres.

The auricle communi­cates with the ventricle by the auriculo­ventricuiar aperture. This aperture is guarded by two semilunar valves. The ventricle gives off an inconspicuous aortic ampulla. The walls of the ventriculo-ampullar and pulmo-auricular apertures are provided with concentric sphincter muscles which act like valves.

The aortic ampulla is formed by the bases of the aortae. Two arteries, viz., a cephalic artery and a visceral artery arise from the apex of the ampulla. These arteries supply blood to the different parts of the body. The blood from the different parts of the body is collected in sinuses and from these sinuses blood returns to auricle by veins.

9. Excretory System of Achatina:

The excretory system consists of kidney. Originating from the anterior end of the kidney, the ureter runs along a groove to the posterior end. The kidney is a single-lobed body adherent to the mantle. It is a spongy, elongated organ of dull grayish colour. It is situated at the postero-lateral part of the mantle cavity and lies parallel to the heart.

The kidney has a peculiar configuration (Fig. 16.25). It has a narrow and rounded anterior end while the posterior part is broader and crescent-shaped. The kidney is made up of numerous narrow folds enclosing an incons­picuous central lumen. The ureter is divided into two portions—the adrenal ureter and the adrectal ureter.

The adrectal ureter pro­ceeds anteriorly in close contact with the rectum while the adrenal ureter runs by the postero-lateral side of the kidney. The adrectal ureter opens to the exterior by the renal aperture located close to the anus.

The kidney is bounded by a thin capsule. This capsule is fused with the ventricular end of the pericardial chamber leaving a small rounded renopericardial aperture. This aperture leads into a short and ciliated slen­der passage. The passage becomes exten­sively branched to open into the cavity of the kidney.

The kidney gets blood supply from the renointestinal artery of the cephalic aorta. The urine is stored in the lumen and the ureter. The urine also helps to keep the surface of the mantle moist. Normally the rate of discharge of urine is slow but during aestivation this rate is slightly higher. Uric acid excretion increases during aestivation in Achatina.

10. Nervous System of Achatina:

The nervous system of Achatina consists of cerebral, pleural, parietal, visceral, pedal and buccal ganglia. The parietal, pleural, visceral and pedal ganglia form a ring. The cerebral, pleural, pedal and visceral ganglia are concentrated in the head region due to shortening of the nerves between them. All these ganglia are paired excepting the visceral which is unpaired and shows the ten­dency towards fusion with the pleural.

The pleural and pedal ganglia are separate and are connected with the cerebral ganglia by connectives. All these ganglia are located around the oesophagus and thus form a circumenteric nerve ring. The circumenteric nerve ring on the ventral side of the oesopha­gus forms a visceral nerve chain. Based on the disposition of ganglia on the visceral nerve chain—the nervous system is desig­nated as the Zonitoid type.

This chain has five ganglia—one visceral, two parietals and two pedals, but they are very closely set. The cerebral ganglia are rounded bodies situated dorsal to the oesophagus and behind the buccal mass.

From each ganglion several nerves, viz., ocular nerve, ommatophoral nerve, superior frontal nerve, inferior frontal nerve, posterior oesophageal nerve, cerebrobuccal con­nective, nuchal nerves and labiotentacular nerve originate to supply the head and the anterior region of the visceral stalk (Fig. 16.26).

The buccal ganglia are paired and connected by a narrow transverse commissure. The buccal ganglia supply nerves to the buccal mass.

The pleural ganglia supply nerves to the middle portion of the visceral stalk while the parietal and the visceral ganglia supply nerves to the base of the visceral stalk. The pedal ganglia are massive and give off nine pairs of nerves to the foot. The nervous system becomes condensed due to reduction of all commissures and connectives between the different ganglia and consequent fusion and approximation of ganglia.

Untwisting and gradual shorting of the visceral chain have caused the cephalic concentration of the ganglia. Distinct chiastoneury is not ob­served in Achatina. The pallio-parietal nerve arising from the left side of the visceral ganglion turns to the right and proceeds along the right medio-parietal nerve. This complication is due to head-foot torsion.

Sense Organs of Achatina:

Achatina is provided with well-developed sense organs.

The sense organs are:

(a) Eyes:

Two eyes, one at the tip of each large tentacle (ocular tentacle) are present. Each eye appears as small black dot and consists of an outer cornea, an inner cornea, a structure-less lens and a retina.

(b) Olfactory organs:

Four olfactory organs, two at each tentacle, are present. An olfactory organ consists of a club-shaped olfactory ganglion.

(c) Statocysts:

Two statocysts are the organs for balance. A statocyst is placed on the dorso-lateral side of the pedal ganglion. The statocysts are spherical sac-like structures lined by fibrous tissue. The cavity is filled with elliptical statoconia.

(d) Tactile organs:

The ventral tenta­cles, foot and snout act as tactile organs.

11. Reproductive System of Achatina:

Achatina is a hermaphroditic animal.

The reproductive system consists of the follow­ing structures:

(i) An ovotestis (hermaphroditic gland),

(ii) An ovotestis duct,

(iii) A spermoviduct with prostatic acini,

(iv) A vas deferens with penis,

(v) An oviduct with vagina and

(vi) A genital atrium opening to the exte­rior through the genital aperture (Fig. 16.27).

(i) Ovotestis:

The ovotestis is a multilobate body embedded in the posterior lobe of the digestive gland. There are four or five finger­like lobes in the ovotestis. Each lobe has many lobules. Most of such lobules produce sperm cells while a few produce both ova and sperm cells.

(ii) Ovotestis duct:

Each lobe of the ovotestis produces a duct and these ducts of the lobes unite together to form a single ovotestis duct. This duct opens into the sperm-oviduct.

The ovotestis duct is divisible into three parts: (i) The first or the apical ovotestis duct is a narrow straight tube which opens into (ii) the ovisperm vesicle where the sperms remain stored. It is much folded and held together by a thin layer of connective tissue. The third part or (iii) basal ovotestis duct is a narrow and twisted duct.

(iii) Albumen gland:

The basal ovotestis duct is placed against the concave face of the albumen gland and enters into it. The albu­men gland is a cream-white gland which enlarges during breeding season. It is com­posed of many vesicles and enclosed by a capsule. The vesicles open into a ciliated albumen canal which leaves it as a short albumen duct.

(iv) Sperm-oviduct:

The ovotestis duct opens into the sperm-oviduct which receives secre­tion from the prostatic acini. Each prostatic acinus is composed of many alveoli which open into a central prostatic canal.

(v) Uterus:

The uterus is divided into an apical and a basal part. The walls of the apical and basal uteri are much folded. The basal uterus opens into a tubular oviduct. The spermathecal duct opens into the ante­rior part of the tubular oviduct. The tubular oviduct ends in the vagina. The vagina is divisible into a tubular apical part and an anterior expanded basal part. The basal part opens into the genital atrium.

(vi) Vas deferens:

This was deferens remains in continuation with the sperm groove or sulcus. The sulcus is a narrow incomplete passage running along the inner concave edge of the sperm-oviduct. The sulcus is separated from the uterus by two folds of the wall of the sperm-oviduct. The vas deferens, on reaching the posterior end of the basal vagina, turns left and enters the penis.

The penis is a curved muscular organ which continues anteriorly as the penial prepuce to open into the genital atrium. The genital atrium opens to the exterior through the common genital aperture. The common geni­tal aperture, as stated earlier, is situated on the right side and posterior to the head.

12. Development of Achatina:

The fertilized egg of Achatina divides into a number or cells by total and spiral cleav­ages. The first two cleavages are equal. The subsequent cleavages are unequal. The blastula is a coeloblastula and embolic type of gastrulation occurs. After passing through developmental stages, the embryo transforms into a modified veliger stage.

Gastrulation is completed by 36 hours and the embryo grows rapidly to assume an elongated configuration. Subsequently the blastopore is shifted dorsally and the ventral lip elongates to form the foot- rudiment. The visceral hump begins to differ­entiate antero-dorsally to the blasto-pore.

The prototroch becomes distinct which develops cilia to transform into the velum. The embryo at about 48 hours stage resembles a veliger larva. The development of Achatina beyond the veliger stage occurs in subsequent stages. The whole process is completed by 15 days.