Invertebrates Z202 and ZL203

Lecturer: Dr. Nagiba Shoker E-mail: [email protected] [email protected]

Phylum: This phylum is one of the three largest phyla of the kingdom. The Molluscs have always been of interest to man, as food, as sources of dyes, as intermediate hosts to many important parasitic helminthes , and as producers of pearls. General characteristics: 1. Habitat: molluscs inhabit marine, fresh-water and terrestrial habitats 2. Symmetry: The molluscs are either bilaterally symmetrical, or assymmetrical according to the species . 3. The coelom varies in development, being either reduced or complete . 4.Segmentation:The bodies are soft, unsegmented , without jointed appendage. 5. The Body regions are head, muscular foot, and visceral hump which is covered by a mantle that encloses a mantle cavity. 6. The Exoskeleton is represented by a calcareous shell which is secreted by the mantle and consists of one or more pieces. 7. The respiratory organs mainly consists of one to many comb-like gills in aquatic molluscs . However, the mantle is used for aerial respiration in the terrestrial forms . 8. The alimentary canal usually has a buccal mass with a grinding organ (radula), salivary glands and a stomach into which opens a digestive gland. 9. The circulatory system of the molluscs is open except in cephalopods which have a closed system. The heart dorsal and consists of ventricle and 2 auricles. 10. Excretion takes place by paired coelomic Kidneys. The kidneys with glandular epithelium that extracts nitrogenous wastes from the blood.

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Invertebrates Z202 and ZL203

11. The nervous system consists of paired cerebral, pleural, pedal and visceral ganglia. Eyes are either simple or developed. 12. The sexes are usually separate (unisexual), some are hermaphrodite. The development is either direct or indirect with metamorphosis through trochophore and veliger larvae. Subphylum (I): Placophora Class1: Polyplacophora This includes molluscs with an elongated bilaterally symmetrical body and a head without tentacles or eyes. The shell consists of 8 transverse plates. The nervous system is primitive, without definite ganglia. Order: Ischnochitonida Family: Chitonidae ex: Acanthochiton spinigera Acanthochiton spinigera is one of the large species, up to 10cm. in length, which is commonly found on sea coasts . It is seen adhering to rocks, but it can creep slowly from place to feed largely on algae . The sexes are separate. External features: The body is elongated and oval in shape convex above and flattened below. The mantle covers the whole of the dorsal side of the body. A calacareous shell formed of 8 overlapped transverse plates, covers the dorsal side of the mantle, while numerous calcareous spicules projects from the mantle edge. -The head: is small, flattened, lies anteriorly and bears no eyes or tentacles, but has the mouth opening as a transverse apereture. The foot: is large, broad, distinctly flattened and separated from the head in front by a narrow furrow. Besides affecting creeping movements, the foot acts as a sucker enabling the animal to cling to rough rocky surfaces. The mantle cavity is in the form of a narrow mantle groove extending all around the head and foot. One row of numerous short ctenidia or gills projects in the mantle

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Invertebrates Z202 and ZL203 groove on each side of the foot .Each of thegonopore, the excretory pore and the anus opens in the mantle cavity . Respiration is carried out by gills projecting in the mantle cavity. The circulatory system is an open system. The heart consists of two auricles and a median elongated ventricle. It lies dorsally in a large pericardium. Dorsal aorta passes anteriorly from the ventricle and carries blood to the various organs and a series of haemocelic sinuses. The blood is returned via a branchial circulation to the two auricles which join the ventricle laterally. The excretory system:The renal organs are a pair of kidneys. Each is usually in the form of a long U-shaped tube which opens internally into the pericardium (by a renopericardial canal), and externally in the mantle groove to discharge the wastes via the excretory pore. The Nervous system is primitive, without defenite ganglia . A nerve ring surrounds the oesophagus, anteriorly it sends nerves to the buccal region and posteriorly gives rise to two pairs of longitudinal pallio-visceral cords which innervate the mantle and visceral organs, and unite posteriorly via a supra-anal commissure. The pedal cords innervate the foot muscles. Both pairs are inter-connected by numerous transverse commissures. The Radula: It is the grinding appendage of some mollscus. It is formed of a long horny membrane on which a large number of recurved teeth are arranged in successive transverse rows.

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Invertebrates Z202 and ZL203

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Invertebrates Z202 and ZL203

Subphylum (II): Conchifera Class (1): - Class Gastropoda include marine, freshwater and terrestrial forms which are mostly bilaterally asymmetrical . They have a distinct head with eyes, tentacles and a radula . The foot is broad and muscular . The visceral hump is elongate and often coiled in a spiral, exhibiting torsion to various degrees. The mantle secretes a spirally coiled univalve shell. The excretory system includes one or two kidneys which drain from the pericardial cavity and discharge into the mantle cavity. The nervous system contains distinct cerebral and pleural ganglia beside pedal, and visceral ones. Subclass: Pulmonata Order : Family: Ex: Eremina desertorum (The desert snail) It is a common inhabitantant of the Egyptian deserts. It is a terrestrial herbivorous gastropod. The snail is generally inactive in the hot period of the year where the shell aperture is closed by a hard calcareous epiphragm . The external features : The body of the snail is mainly composed of 3 region , head, foot and visceral hump . The head and the foot are fused, forming the head- foot. The visceral hump is coiled in a spiral and is completely covered by a similarly coiled shell, which it never leaves during life. The shell: is milky-white to light brownish in colour, globose and dextral or right handed. The whorls are about 4 in number, the largest one is termed the body whorl. They are marked externally with transverse striations which represent the lines of growth. The apex is the top of the shell. The shell aperture is the peristome. The columella is the central axis of the shell formed by the coalescence of the inner walls of the whorl, ending with a ventral umbilicus.

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Invertebrates Z202 and ZL203

The head-foot: the head occupies the anterior region and has the mouth opening on its front surface. It carries dorsally two pairs of retractile tentacles, where the longer one bears a pair of simple eyes. The head also carries a single genital opening on the right side and an opening of the pedal gland below the mouth, which produces a slimy secretion. - The foot is a thick muscular mass which has a smooth, flat, ciliated ventral surface, on which the snail creeps. The visceral hump is a long, spirally coiled cone which includes the different body organs and is enveloped by the mantle. It occupies the cavity of the shell whorls in the living snail and attaches posteriorly to the columella of the shell. The anterior thick margin of the mantle, the collor, is the seat of secretion of the principal layers of the shell. The mantle covers over a spacious cavity known as the mantle cavity. There is a narrow orifices on the right side of the collar; these are: pulmonary opening, anal opening and excretory opening. Respiration: Eremina is a terrestrial mollusc which breathes atmospheric air. The mantle cavity acts as a lung or pulmonary cavity. The wall of the mantle cavity is provided with a network of blood capillaries, through which respiratory gaseous exchange takes place between the blood and the air entering the mantle cavity through the pulmonary opening. The circulatory system: The heart consists of an anterior auricle and a posterior ventricle , lying in the pericardial cavity . The ventricle gives rise to a single short aorta which divides into an anterior aorta and a posterior aorta which supply blood to the head foot complex and the visceral mass, respectively. Non-oxygenated blood is collected in the main haemocoelic sinus, from which blood passes into a circum pulmonary sinus for oxygenation. Afferent pulmonary vessels from this sinus ramify on the roof of the mantle and pass into efferent pulmonary vessels which carry the oxygenated blood to the large pulmonary vessel. This vessel runs backwards and opens into the auricle.

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Invertebrates Z202 and ZL203

The excretory system: This consists of a single triangular kidney that lies beside the heart. It drains wastes from the pericardial cavity by a renopericardial duct. A long ureter extends from the kidney and opens by the excretory pore on the mantle collar. The nervous system: The nervous system is typically ganglionated. It is concentrated into a circumoesphageal nerve ring on which the nerve ganglia are localized and send off nerves to all parts of the body. These consists of : 1.2cerebral ganglia innervate the head region. 2. 2 pedal ganglia innervate the foot. 3. 2 pleural ganglia innervate the mantle. 4. 2 parietal ganglia innervate the mantle. 5. A single visceral ganglion innervates the organs in the visceral mass 6. 2 small buccal ganglia innervate a considerable portion of the alimentary tract lie on the back of the buccal mass and are connected with the cerebral ganglia. Reproductive system:- The snail is hermaphrodite . The reproductive system is formed of a single small gland called the hermaphrodite gland or ovotestis. A single convoluted hermaphrodite duct arises from this gland and leads down till the base of a bean shaped albumen gland. There the hermaphrodite duct dilates into a small seminal receptacle, in which fertilization takes place, and the fertilized ova receive their albumen coat. This is followed by a large glandular sperm-oviduct which encloses two incompletely separated channels, one for the passage of sperms and the other for the ova. Anteriorly the two channels separate in to a narrow vas deferens and a wider oviduct. The vas deferens leads into a penis, while the oviduct leads into a short tubular vagina. Both of them open in a common genital atrium , which opens to the exterior through the genital opening . A number of accessory genital organs open in the oviduct. These are a small globular copulatory bursa (spermathecal) with a long duct, a club-shaped muscular dart sac and a pair of tubular mucucous gland.

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Invertebrates Z202 and ZL203

Reproduction:- Although the snail produces both sperms and ova, self fertilization is rare and cross fertilization is the common method . Copulation is carried out between two snails . Foreign sperms are stored in the spermatheca until needed. They pass down the genital tract, meet the ova in the seminal receptacle and fertilization takes place. The fertilized egg receive an albumen covering and a calcareous outer shell from the secrtions of the albumen gland, then passes down the sperm-oviduct where it receives a gelatinous coat from the glandular walls. Egg masses are then deposited in small holes in the ground. Development is direct, without larval stage.

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Invertebrates Z202 and ZL203

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Invertebrates Z202 and ZL203

ClassII: Cephalopoda These are well developed molluscs with a bilaterally symmetrical body. The head is well developed, bearing 2 large eyes and a radula, and surrounded by a crown of tentacles and arms, representing the front part of the foot. The posterior part of foot forms a muscular funnel. The mantle covers the visceral hump, and the shell is either well developed, reduced or absent and usually internal. They possess one or two pairs of gills , nerve ganglia are highly concentrated to form a brain. Sexes are separate and the development( metamorphosis) is direct. Subclass : Dibranchiata Order (1): Decapoda (Sepioida)

Family: Sepidae

EX. Sepia savignyi (The cuttlefish)

It is a marine carnivorous mollusc. It has a great power of changing its colour with great rapidity. They have 8 arms and 2 long tentacles, and an internal shell . It swims forwards, but when attacked it retreats after emitting a cloud of ink behind, helping the animal to escape .

General characteristics: The body is elongated , mainly divided into a distinct head separated by a neck region from a compressed visceral hump . The head carries a pair of large, highly developed eyes, similar to those of vertebrates. It bears anteriorly the mouth opening that is surrounded by a circular lip, two horny jaws and radula . Eight pointed arms and two much longer tentacles surround the head. Each provided with cup-like suckers on its inner surface. The foot: The arms and tentacles represent the highly modified anterior part of the foot , and hence the name cephalopoda, while the posterior part of the foot is modified to form the large muscular funnel . It assists in backwards swimming activity.

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Invertebrates Z202 and ZL203

The shell: The internal shell is found on the dorsal side of the visceral hump and make it hard. It is flattened and internal, buried just beneath the integument. The visceral hump is conical , dorsoventrally compressed and possesses 2 thin lateral fins. It is covered allover by the mantle. On the ventral side of the hump , the mantle forms a thick muscular wall that covers a large mantle cavity , with a wide anterior mantle opening . The funnel projects from the opening of the mantle cavity. The mantle cavity is a spacious cavity between the body and mantle fold. It encloses a pair of plume-like gills in addition to the viscera. The large ink sac also projects in the mantle cavity and open by a long duct in the rectum near the anus. Locomotion: The mantle is provided with strong circular and radial muscles fibres , that facilitates the rapid expansion and contraction of the mantle cavity , the water passes into the cavity through the mantle opening (inhalant opening) and out through the funnel (exhalant opening) creating a rapid movement. The funnel is highly mobile and can be directed anteriorly or posteriorly , resulting in either forward or backward movement. The fastest movement is achieved in backward escape movement. When at rest, the animal swims slowly in water with the help of undulated lateral fins which function as stabilizers , and the arms are pressed together and aid in steering. Respiration: The circulation of water through the mantle cavity not only produce the power for locomotion but also creates a respiratory water current to provide oxygen for the gills. There is one pair of gill filaments. The blood conducts through the gills in afferent and efferent vessels, increasing the speed of circulation.

The circulatory system of Sepia is primarily a closed system that includes arteries and veins. Therefore, it is regarded as more developed than those found in other molluscs.

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Invertebrates Z202 and ZL203

The systemic heart lies ventrally in a pericardium roughly between and above the bases of the gills. It consists of a large asymmetrical ventricle and two auricles all arranged in line. The ventricle gives rise to an anterior aorta which carries blood to the anterior parts of the body and a posterior aorta which supplies the posterior regions . Blood is returned to the heart region by veins . Those from the head end unite to form an anterior vena cava which bifurcates. Each branch traverse a kidney, is joined by veins from the viscera and mantle then leads to a contractile branchial heart located near the base of each gills. In addition, one of the two branchs joins a single genital vein and an ink sac vein . The contraction of the branchial hearts raises the blood pressure in the efferent branchial vessels which enter the gills, pass through the capillary networks , then emerge as afferent branchial vessels which enlarges as auricles, then enter the ventricle . The nervous system: The cephalopod nervous system is a well-developed system. There is a great cephalization. It’s most striking feature is the concentration of a large number of specific ganglia in the cephalic region to form a large and distinct brain, which is more or less enclosed in a cartilage-like capsule. These cephalic ganglia are: superior buccal ganglia, brachial ganglia, olfactory ganglia, cerebral ganglia, pallio-visceral ganglia, optic ganglia, and pedal ganglia . In addition, the visceral hump includes other ganglia: mantle ganglia, branchial ganglia and gastric ganglia.

***Compare between the main characteristics of phylum Mollusca and each of the following phyla: Annelida, Arthropoda and Echinodermata

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