LECTURE NOTES ON DENTITION IN MAMMALS

 Teeth are the dermal derivatives of integument They are developed as a result of calcification in the mucous membrane of the buccal cavity.. The teeth are present in almost all the mammals except in a few mammals In whale, the teeth are fused into plates and lost in the adult stage of Ant eaters. But in Echidna (spiny ant eater) the teeth are absent even in the embryo.

Structure of tooth: Each typical mammalian tooth is placed in the socket over the jaw bone. It is distinguished into three main parts.

1) Root - It is the basal part embedded in the bony socket of jaw.

2) Neck - it is the part above the root enclosed by the gum.

3) Crown - It is the upper part beyond the surface of the gum.

The toot is separated from the socket by a vascular pridontal membrane. The vertical section tooth consists of the following parts.

I) Pulp cavity: The entire tooth encloses a central pulp cavity surrounded by a layer of odontoblast cells filled with soft pulp. It is made up of connective tissue, blood vessels and nerve fibers.

II) Dentin: A substance chemically similar to bone- dentine forms the major part of the tooth. But the dentine is permeated by numerous thin canaliculi.

III) Enamel: It is present over the de in the crown and neck regions of the tooth. It is hardest and contains only traces of living matter.

IV) Cement: It surrounds the denting of the root portion of the tooth. It is bony in nature.

Types of Dentition

 Based on the attachment of teeth in the jaw bone, the teeth are classified into 3 types, namely Acrodont Pleurodont Thecodont

Based on the types of teeth, teeth are classified into two types, namely Homodont Heterdont .

Based on the number of sets of teeth that develops during the life time, the teeth are classified into 3 types. Monophyodont ,Diphyodont  and Polyphyodont

1. Acrodont :In this type of dentition, the teeth have no roots and they are fused to the edge of the jaw bone. Acrodont dentition is seen in lower vertebrates like amphibians and reptiles but not in mammals. 2. Pleurodont :In pleurodont dentition, the teeth are attached to the rim of jaw. This type of dentition is found in lizards and not in mammals.

3. Thecodont :In thecodont dentition, the teeth have roots and the roots are embedded in sockets of jaw bone. Thecodont dentition is seen in all mammals.

 4. Homodont :Homodont type of dentition is a condition where the teeth are similar in shape. Eg. Dolphins. 

5. Heterodont:In heterodont, the teeth are different in shape and function and are present in the same animal. In heterodont type, teeth are differentiated into four types, namely incisors, canines, premolars and molars.

 6. Monophyodont: In certain mammals, only one set of teeth develops during the life time and this condition is called monophyodont. Eg. Moles and squirrels.

7. Diphyodont:In diphyodont condition, two sets of teeth develop during the life time of an animal. The first set of dentition is called milk dentition or lacteal dentition. This milk dentition is later replaced by the permanent dentition. In bats and guinea pigs, the milk dentition is lost even before birth. In Homo sapiens (man), the milk teeth are replaced by the permanent teeth around the age of 7 years. 8. Polyphyodont In lower vertebrates, many sets of teeth develop during the life time. This kind of dentition is called polyphyodont.

 Types of Teeth In heterodont dentition, there are four different types of teeth:

1. Incisors They are flat chisel-shaped teeth having a single root. They are used for cutting.

2. Canines Canines are sharp pointed teeth having a single pointed crown and a single root. They are used for piercing and tearing the flesh of the prey.

3. Premolars and Molars (cheek teeth) They are collectively called grinders. They have two or more roots and the crowns bear complex cusps or tubercles. The cheek teeth are used for grinding the food.

 Cusp Patterns of Molars

 The molars show many cusps on their surface. Based on the number and shape of the cusps different types of teeth have been distinguished.

They are: 1. Monocuspid: This is characterized by the presence of only one cusp.

 2. Bicuspid: When two cusps are present, the tooth is called bicuspid.

3. Tricuspid: These are molars having three cusps. Tricuspid teeth are also called triconodont and trituberculate. 

4. Bunodont: In bunodont type, the crown has a number of blunt or pointed cusps. Eg. Man.

5. Lophodont: When the cusps are arranged in the form of ridges, it is called lophodont. Eg. Indian elephant.

6. Selenodont: These are molars with cusps in the form of crescent. Eg. Cow and sheep.

7. Secodont: In secodont type of molars, the crown has cusps having sharp cutting edges. Eg. Carnivorous mammals.

8. Hypsodont: These are molars with high crowns and short roots. The cusps are crescent-shaped.

9.Brachydont:Molar teeth with short crown and with neck at the surface of the gum are called brachydont.They have also crescent shaped cusps.

Mostly the number of teeth is fixed in each mammalian species. Mammalian heterodont dentition is expressed by a ‘dental formula’. The number and arrangement of teeth in each half of the upper and lower jaws is constant and identical. Hence the teeth can be expressed by using the initials - I, C, Pm and P4. The number of teeth differs in the various orders of mammals and is closely related to their ‘feeding habits. Typical number of teeth in the mammals is 44. This can be observed in Horse and pig and their dental formula is 

The numerator indicates the number of teeth on one side of upper jaw. The denominator indicates the number of teeth on one side of the lower jaw. As the two halves of each jaw have same number and kind of teeth, the number of teeth on one side alone is usually ex pressed in the formula.

The dental formulae of some common mammals are given below

 

LECTURE NOTES ON ACCESSORY RESPIRATORY ORGANS IN FISHES

Accessory Respiratory (Extrabranchial) Organs in fishes.
Fishes are aquatic, gill breathing vertebrates. They possess gills for branchial mode of respiration. However, in many fishes, either due to poorly developed branchial respiration or due to environmental stresses, additional respiratory structures have been developed. Such extrabranchial organs that supplement gills in respiration are collectively referred to as accessory respiratory organs.
Several reasons have been suggested for the existence of accessory respiratory organs:
1. To meet the environmental oxygen deficiency.
2. To compensate the degenerate gills.
3. To maintain life during aestivation (summer sleep).
4. To counter balance the intemal oxygen deficiency.
5. Air breathing may be obligatory (habitual) in some fishes.
These take different form and structure that suit the habit and habitat of the fish. Some accessory respiratory organs commonly found in fishes are skin, buccopharyngeal epithelium, gut epithelium, pelvic fins, pharyngeal diverticula, branchial diverticula and air bladder (swim bladder).
1.Skin:The skin of eels and many other fishes is richly supplied with blood vessels and serves as accessory respiratory organ. lt performs the respiratory function both in water and on land. Anguilla anguilla and Amphiopnous cuchia (eels) often move on land through wet vegetation. This feature is especially useful when the eels undertake extensive migratory journeys. In Periophthalmus (mud skipper) too skin functions as an accessory respiratory organ.
2.Bucco pharyngeal Epithelium:The buccal cavity and pharynx of Periophthalmus and Symbranchus are lined with highly vascular epithelium. Fresh air is gulped into these cavities and exchange of respiratory gases occurs through the epithelium.
3.Gut Epithelium: In a few fishes, certain regions of the alimentary canal are highly vascular and become modified for aerial respiration. ln Misgurus fossilis, for example, the region behind stomach serves as an accessory respiratory organ. The intestine of Lepidosiren and the rectum of Callichthyes also function as accessory respiratory organs. In these cases, fresh air enters the gut either through mouth or anus. The expiratory air goes out through anus.
3.Pelvic Fins:In the American lung fish Lepidosiren, during breeding season, the pelvic fins of the male fish enlarge in size, become highly vascular and form filamentous out growths. These fin; besides serving as accessory respiratory organs supply oxygen to the eggs guarded by them.
4.Pharyngeal Diverticula:The pharynx of the fishes Periophthalmus, Amphiopneus , Channa (Ophiocephalus), etc. forms sac-like outgrowth called diverticulum, which is lined by vascular epithelium. Sometimes the sac extends above the gill pouch.Air is drawn into the diverticulum, and oxygen and carbon dioxide are exchanged.Amphiopnous possesses a small and smooth diverticulum,which opens through midventral gill slits . The diverticulum of Channa is folded . In all these cases, the diverticula function as accessory respiratory organs during aestivation or when the oxygen content of the water is too low.


5.Branchial Diverticula : In many fishes the outgrowths or diverticula formed from gill  (branchial) chambers contain complex accessory respiratory organs for  aerial respiration. These include tubular diverticula, labyrinthine organs  and dendritic or arborescent organs.
a.Tubular Diverticula: in Heteropneustes (Sacchobranchus) a pair of  long tubular air sacs arises from the gill chamber one on either side and  extends up to the tail. Exchange of respiratory gases takes place  through the highly vascular wall of the diverticulum.
b. Labyrinthine organs: Anabas (lnclian climbing perch) often comes  to water gurface and gulp down fresh air for aerial respiration. lt has twospacious suprabranchial cavities as dorsal outgrowths of the gill chambers. Each of these cavities contains an accessory respiratory organ called labyrinthine organ . It is formed by much folded concentric bony plates, which develop from the first epibranchial bone. This organ is covered by vascular mucous membrane. Fresh air reaches the suprabranchial chamber through the mouth and after gas exchange the expiratory air is expelled through the opercular opening. InTrichogaster fasciates, a similar but simple a similar but simpler labyrinthine organ is present
c.Dendritic(Arborescent) Organs: In Clarias (Indian cat fish), the wall of the gill chamber on either side evaginates to form a pair of suprabranchial cavities. These cavities contain highly branched and tree-like accessory air breathing organs  called dendritic or arborescent organ.Exchange of respiratory gases occurs through the highly vascularized mucous membrane covering these organs.
6.Air bladder (Swim Bladder): Air bladder or swim bladder is found in all bony fishes. ln teleosts, it functions as a hydrostatic organ. However, in lower bony fishes such as Dipnoans, ganoids, etc. gills are poorly developed and the air bladder functions as an accessory respiratory organ. lt is vascular and contains many alveolus-like structures. ln Amia and Lepisosteus, single air bladder is present, which opens dorsally into pharynx. in Lepidosiren and Protopterus, the air bladder is bilobed, ventral and opens ventrally into pharynx.
7. Oral Papilla In electric eel (Electrophorus), the mucous membrane of the mouth is raised into oral papillae. They are well vascularized and they   help in aenal respiration.
8.Lungs:Lungs are present in Dipnoi. Eg.Protopterus, Lepidosiren, etc. They have apair of lungs. They open into the oesophagus by a common opening called glottis The lungs contain alveoli and are well vascularized. In Dipnoi, lungs are used for aerial respiration during aestivatiom.

LECTURE NOTES ON MIGRTION OF BIRDS

Migration:In a broad sense, 'migration', as defined by Cahn, "is a periodic passing of animals from one place to another, (L. migrare, to travel)". The bird migration is a two-way journey. It means a regular, periodic, to-and-fro movement of a population of some birds between their summer and winter homes, or from a breeding and nesting place to a feeding and resting place. Not all species of birds take part in the great pageant of migration. Bobwhite and the ruffled sand grouse, do not migrate at all. Birds which remain throughout the year in a country are known as residents.

I.Kinds of migration

Migration in birds takes place in a variety of manners, some of which are as follows :

1. Latitudinal migration: The most familiar migrations are latitudinal that is, north to south and vice versa. These are pronounced in the Northern Hemisphere, having larger land masses. Birds move during summer over the temperate and sub-arc tie regions of Northern Hemisphere, where there are facilities for feeding and nesting. The birds return to the south for shelter during winter, when north is covered with ice and snow. The American golden plover (Pluvialis dominica) passes the nine months of winter 8,000 miles south in the pampas of Argentina, thus enjoying two summers each year and knows not a hint of winter. Some birds of Siberia visit the plains of Himalayas in India.

2. Longitudinal migration: Some birds make migrations from east to west and vice versa. Thus, the starling moves from a breeding area in east Europe or Asia towards the Atlantic coast, to avoid the continental winter.

3. Altitudinal migration: Wherever large mountains are found in temperate regions, the birds migrate regularly up and down their slopes, as the weather changes. The birds pass the summer in the mountainous regions, but retum to the lowlands in winter. It is merely a dispersal or short journey from the bleaker slopes to the more protected valleys and has been called altitudinal or vertical migration. It ocurs in the grebes and coots of Andes in Argentina, violet green swallows of Great Britain, and the willow ptarmigan of Siberia

4. Partial migration:A part of bird population migrates and the major part  remains in the native land.Thus, barn owls (Tyto alba), blue-birds and many blue jays of Canada and Northern United States travel southwards to mingle with the sedentaiy populations of the southern states. Song thrush, redbreast, titmouse, finch, etc., seen throughout the year, actually represent partial migrants, as the birds seen in winter are not the same as seen in summer.

5.Irregular or vagrant migration: In some birds, such as herons, after breeding, the adults and the young may stray from their home to disperse in all directions over many or a few hundred miles in search of food and safety from enemies. Sometimes sea birds are taken by  hurricanes to as far as 2,000 miles away from home seas to drop exhausted or to die on unfamiliar shores.

6.Seasonal migration: Some birds migrates at different seasons of the year for food or breeding, called seasonal migration, e.g., cuckoos, swifts, swallows etc. In Britain swifts, swallows, nightingales and cuckoos are summer visitors, for they arrive in spring from the south, remain to breed and leave for the south in autumn. Some like fieldfare, snow bunting and redwing are winter visitors, as they arrive inautumn, chiefly from the north, stay throughout the winter and fly northwards again in spring.

II.Modes of flight in migration

1. Nocturnal. and diurnal flight. Many large birds fly in flocks mainly by day, such as the crows, swallows, robins, blackbirds, hawks,

bluebirds, jays, cranes, loons, pelicans, geese and other shore birds.  These are callec diurnal migrants .

Nocturnal migrants  include mostly small passerine birds, such as warblers, thrushes, sparrows, etc. They prefer to fly at night, under the protective cover of darkness, to escape their enemies.

2. Segregation during migration. Certain birds, such as night hawks, swifts and kingfishers, travel in separate companies, while swallows, turkeys, blue birds, etc., travel in mixed companies of several species, due to similarity in their size, method of search of food, etc.

3. Range of migration: The distances travelled by migratory birds depend upon local conditions and the species concerned. The Himalayan snow partridges descend a few hundred feet only and cover hardly a mile or two, while the chicades come down nearly 8,000 feet. The arctic tern spends the summer and breeds in the  Arctic circle. Then it travels a distance of 11,000 miles to reach its destination to the edges of Antarctica in winter.. European white stork winters in South Africa after a journey of about 8,000 miles.

4. Altitude of flight. Some birds fly quite close to the earth, while most routine migration probably takes place within 3,000 feet of the earth. Radar has shown that some small land birds, migrating at night, fly at 5,000 to 14.000 feet. Some species even cross the Andes and the Himalayas at altitudes of 20,000 feet or more.

5. Speed and duration of flight. Average flight velocity of most small birds seldom exceeds 30 miles per hour. The greatest speed, recorded in India, of two species of swifts by E.C. Stuart, is 171-200 miles per hour.

6. Regularity of migration. Several species of migratory birds show a striking regularity, year after year, in their timings of arrival and departure. In spite of long distances travelled or vagaries of weather, they are often punctual within a day or two in their time of arrival. Another remarkable feature, besides punctuality, is that they sometimes come back to the same breeding place year after year.

7. Routes of migration. :Sea,river, mountain,sea shore ,some land marls etc are major  routes of migrations

III. Causes of Migration:The major causes of migrations are

a. Instinct and Gonadal changes: The  impulse to migrate in birds is by instict and the migration towards the breeding grounds is associated with gonadal changes.

b. Scarcity of food - Scarcity of food is believed to produce endocrinal changes which initiate bird migration.

c.Shortening of daylight: Shortening of daylight is believed to produce endocrinal changes which initiate bird migration.

d. Photoperiodism:The increase of day length (Photoperiodism) induces bird’s migration. The day length affects pituitary and pineal glands and also caused growth of gonads which secret sex hormones that are the stimulus for migration.

e. Seasonal variation:The internal condition of the gonads which are affected by seasonal variation.

f. Light:Light plays an important role in the development of gonads, which has indirect role on migration.

IV.Navigation: The following factors determines the direction and course of migration.

(a) Visual landmarks. The sense of direction has been attributed to obvious topographical features or landmarks, such as great rivers, river valleys, coastal lines, chains of oceanic islands, mountain ranges, etc.

(b) Experience. A few naturalists have suggested that the birds learn by experience. Some older members, benefiting by a tradition following a path in past several years, become leaders to

guide the younger generations.

(c) Telluric currents. The air-currents, which would lead the birds straight to their destination.

(d) Homing instinct. Some have spoken of a homing instinct, enabling the birds to return to a goal

(f) Celestial bodies. The late Gustav Kramer, a German Ornithologist, claimed in 1949, that the birds which travel by day use the sun as compass for orientation . German, Franz Sauer, who experimented with night-flying old-world warblers, made the astounding discovery that the nocturnal, migrants navigate by the constellations of the stars

Advantages of migration:The advantages of migration are

1.Migration helps birds avoid harsh climatic extremes.

2,Migration from higher altitudes and latitudes (breeding areas in the north) during winter affords protection from cold and stormy weather.

3.The migrants get more food and better environmental conditions .

Disadvantages of Bird Migration:

i. Many youngs are not, able to reach the destination and die due to extreme factors on the way.
ii. Sudden changes in the climate such as storms and hurricanes, strong current of wind, fog are the causes for the death of a significant number of migrants.
iii. Sometimes man-made high towers and light houses cause the death of migratory birds.
iv. Man themselves are responsible by hunting  at these poor birds just for their own leisure and amusement.

SHORT NOTES ON PECTEN

In pigeon eye projecting into the vitreous body from the blind spot, a characteristic comb like structure  is present. It is called pecten. The pecten is highly variable in size and shape in different birds. In pigeon, it is composed of a thin dark pigmented plate which is folded fanwise and assumes the form of a comb.p
Pecten is made up of a vascular network supported by pigmented cells. The physiological role of pecten is not yet detailed. Pecten is claimed to perform a number of functions in  pigeon.

The functions of pecten are:
(1) The irregular shad­ows cast by this organ provide many blind spots on the retina which enhances the on-and-off effects in the visual field and thus increases the keenness of visual perceptions.
(2) The pecten helps in the nutrition of vitreous body and retina.
(3) The pecten helps to protect the retina from strong light.
(4) Some scientists argued that it helps in the process of accommodation. 

LECTURE NOTES ON APOPTOSIS OR PROGRAMMED CELL DEATH

Apoptosis is otherwise called programmed cell death. It is an essential process for multicellular organism growth and development. It is a highly-organized process.
During  apoptosis classical apoptotic cell death, heterochromatin forms from chromatin and moves towards the nuclear periphery of the cell.The Cell contents of an apoptotic cell remain membrane-bound as apoptotic bodies, which are then phagocytized, causing no disruption such as inflammation.Apototic necrotic cells typically expell their contents into the extracellular space, eliciting an inflammatory response .
Steps in apoptosis. 
The major steps are:
Shrinking and rounding of the cell membrane.
DNA distruction or fragmentationt hrough chromatin condensation and DNA compaction.
Condensation of cytoplasm and  cell  organelles.
Vesicle formation by membrane blebbing and formation of apoptotic bodies.
Phagocytosis of the apoptotic BODIES. 

Intrinsic pathway of apoptosis:
AmApoptotic protease activating factor 1, Procapases 2, 3,aVnd 9, latent AlF, and cytochrome C exist in the mitochondriali ntermembrane space.
Upon release of these apoptosis-inducing factors into the cytplasm the cytochrome C forces the cleavage of caspase 9.
Caspase 9 is the initiator caspace, which then activatesc aspases 3, 6, and 7. Effector caspases are activated and thesee ventually cause cell death.
Extrinsic pathway of apoptisis:
The extrinsic pathway of apoptosis is activatedb inding of a death ligand to a death receptor on the cell membrane . Death receptors are FAS and TNF receptors anda ctivated by FAS or TNF ligands. Activation of these receptorscauses the cleavage of initiator caspases within thec ytoplasm. This then activates downstream effector caspases,ultimately leading to cell death.

Significance

1. Apoptosis is important in every day of our lives, as many processes depends

on apoptosis to occur. Furthermore, research has shown that the inhibition

or hyperactivity of apoptosis can cause disease.

2. Apoptosis is vital for the formation of digits, organs and Ilimbs in

embryogenesis

3. Apoptosis is needed in the strict process of tissue homeostasis in the

developed animal

4. Increased apoptosis causes the vascular calcification seen in end-stage

kidney failure. 

5. Increased apoptosis can Ilead to larger plaque formation in coronary heart

disease

6. Decreased apoptosis is seen in many aggressive cancers, chemotherapy

targets cancer cells and induces them to die

7.In arthrtitis, the increased levels of inflammation cause continual activation of cell death leading to the degradation and damage of affected joints

SHORT NOTES ON ONCOGENIC VIRUS

Oncogenic viruses
Oncoviruses  are viruses that produce tumors when they infect humans.
The more common oncogenic viruses are following
Human  papillomavirus  (HPV). Human papillomavirus causes common warts but also is believed to cause cervical cancer.
Epstein-Barr virus (EBV). Epstein-Barr virus causes Burkitt’s lymphoma, which is a tumor of the jaw. It is seen mainly in African children and causes a tumor in the nasopharyngeal (nasopharyngeal carcinoma).
Herpes simplex virus 2 (HSV-2). Herpes  simplex  virus  2  causes genital herpes, cervical cancer (cervical carcinoma), and oral lesions.
Human T-cell leukemia virus 1 (HTLV-1). Human T-cell leukemia virus 1 causes acute T-cell lymphocytic leukemia, which is a cancer that affects Tcell– forming tissues.
Human T-cell leukemia virus 2 (HTLV-2). Human T-cell leukemia virus 2 causes atypical hairy cell leukemia.

BUTTERFLIES IMAGES

Lecture notes on Neurogenesis in frog /Development of Neural tube

Development of Neural tube 

In the gastrula, the presumptive material for nervous system, lies on the mid-dorsal line as a plate called neural plate or medullmy plate. It extends from the dorsal lip of the blastopore to the anterior end. Soon, the edges of the neural plate become thickened and raised above as neural folds or medullary folds. The neural folds of the two sides are continuous anteriorly to form the transverse neural folds. The neural folds enclose a shallow groove called neural groove. The neural folds increase their elevation and bend towards one another until their edges meet and fuse. Thus a tube is formed called neural tube. It encloses at canal  called neurocoel.The fusion first starts just behind  (in the anterior end regio  of future medulla and fmm here the fusion  both ameriorly and postenorly.Anteriorly, the neural tube opens to the exterior for some time by anterior neuropore. It becomes closed soon. Posteriorly the neural folds enclose the blastopore in such a way that the neurocoel communicates with the archenteron through the blastopore. The short narrow canal connecting the archenterons and neurocoel is neurenteric canal. Later it also disappears.After the neural folds have fused in the median line. theneural tube separates itself completely from the overlying epidermis. The free edges of the epidermis fuse together, so that the epidermis becomes continuous over neural tube. As the neural tube is separated from the ectoderm, a certain number of loose cells are liberated from the neural folds in the space between the ectodemt and the neural tube. These cells arrange themselves as two longitudinal bands on the dorso-lateral wall of the neural tube. These cells constitute neural crest. Later the neural crest cells differentiate into the ganglia of the cranial and spinal nerves, melanophores (chromatophores), adrenal medulla and visceral skeleton. The anterior portion ofthe neural tube differentiates into the brain and the posterior part into the spinal cord. The embryonic stage which is having the neural plate or the neural tube, is called neurula.

Lecture notes on Sub-phylum Trilobita

In Greek tria means  three, lobose means  lobe.Trilobita includes the extinct three-lobed anthropods. Trilobites are the oldest of all known arthropods, They are entirely extinct and exclusively marine forms, abundant and widely distributed in the Palacozoic seas. Most trilobites were bottom-dwelling crawling forms. Their creeping adaptations include flat body, leg-like endopodite and dorsal eyes. Some forms were burrowing, some were free-swimming, and still others were planktonic. Burrowing forms had darting, wedged and plough-like carapace. Swimming forms had flat, oar-like endopodites, lean body and lateral eyes. Planktonic forms had floating devices, such as long and radiating marginal spines.They flourished during the Cambrian and Ordovician periods, and extinct  towards the close of the Palacozic, nearly 240-250 million years ago.

The diagnostic features of trilobites are the following:

(i) Body is 3 to 10 cm long, segmented, dorsoventrally flattened and covered by an exoskeleton of calcite .

(ii) Body has three divisions, namely head or cephalon, thorax or trunk and tail or pygidium

(iii) Body is marked out into three longitudinal lobes by two longitudinal grooves. The

three lobes include a median or axial lobe and two lateral or pleural lobes. The name

"trilobita" refers to this trilobed division of the body.

(iv) Head is formed of five fused segments and an acron , the anterior non-segmented part. It is covered by a broad and five-segmented carapace or head-shield.Carapace has an elevated median lobe,called glabella, and two depressed lateral lobes,called genae or cheecks.

(v) Head bears dorsálly a pair of compound eyes dorsally on the genae

.Ventrally it bears the mouth and five pairs of cephalic appendages. Mouth is guarded by the labrum or hypostome and the labium or metastome.

(vi)Cephalic appendages include a pair of antennae and four pairs of maxillipeds. Antennae are preoral, many-jointed, sensory and uniramous. Maxillipeds are jointed and biramous, with exopodite and endopodite (telopodite). Exopodite is respiratory, and endopodite walking in function. Coxa of each maxilliped bears a jaw-like lobe for passing the food into the mouth.

(vii)Thorax is formed of several articulated segments, covered dorsally by hinged exoskeletal plates. The movable articulation of the exosketal plates enables some trilobites to

roll to a ball, when alarmed or attacked (as some wood lice do)

(viii) Thoracic appendages are typically biramous, with leg-like endopodite (telopodite),

respiratory exopodite, and jaw-like coxal lobe.

(ix) Pygidium is formed of a few fused segments, cach with a pair of biramous caudol

appendages. Its exoskeletal plates fuse to form a dorsal tail shield and a terminal telson.

(x) A mid-ventral food groove for the passage of food from behind forwards to the mouth.

(xi) J-shaped alimentary canal, with its crop in the head region,

(xii) Haemocoel is in the axial lobe of the body.

(xiii) Sexes are separate. No sexual dimorphism. Development involves three larval stages

namely protapsis, merapsis and holapsis.

(xiv)Development includes Protapsis ,Merapsis and  holapsis larval stages. Holapsis has head,

thorax and pygidium. It grows to the adult

Evolutionary  significance     

Trilobites are biologically interesting in that they stand in the ancestral line of arthropod

evolution. They are believed to have descended from a precambrian ancestral stock.Trilobites and the extant arthropods have had a common ancestor in the pre-cambrian. It is also believed that trilobites represent the ancestral stock from which the present day arthropods have evolved. The nearest arthropod relatives of trilobites are crustaceans. The crustacean form Apus is regarded as a connecting link between trilobites and crustaceans. However, the current view is that there are probably four lines of arthropod evolution from different annelidan and near annelidan ancestors, and also the evolution of chitinous exoskeleton and jointed appendages ,occurred independently four times in evolution.

Lecture notes on penaeus reproductive system

Penaeus Female Reproductive System:

The female has a pair of long ovaries extending the whole length of the thorax and the abdomen along the median line. The two ovaries are fused together posteriorly, but free anteriorly. Near the anterior end the ovaries produce finger-like outgrowths called diverticula. From each ovary arises an oviduct.The oviduct opens to the third walking leg.

Penaeus_Male Reproductive System:The male has a pair of tubular testes located in the thorax on either side of the middle line. The two testes are fused together anteriorly.Each testis has many finger-like outgrowts called diverticula. Posteriorly, the testis leads into a vas deferens. The terminal end of the vas deferens becomes dilated into an ejaculatory bulb. The ejaculatory bulb opensbto the outside by the male genital pore at thevbase of the last walking leg.

Excretory system of Prawn (Pinaeus) ppt

Excretory system of Prawn (Pinaeus)