Triploblastic condition is referred to as the three-layered condition. The outer layer is called ectoderm and differentiates into the body scales, integument, sense organs, nervous system and the like. The middle layer is called mesoderm and splits to create coelom. It also differentiates into muscles, internal skeleton, reproductive organs etc. The inner layer is called gastroderm or entoderm and is responsible for digestion and nutrition absorption (gut). The three layers greatly influence each other in their functioning and development. Each respective layer induces the organization of the other layer and its development. It may also cause or inhibit regression of other layers thus leading to controlled and coordinated development of the organism. Mesoderm emerges as an internal tissue between the gastrodermis and epidermis and it's not in contact with the external fluids. Owing to this factor, it proves to be very important as far as storage of valuable materials is concerned. It plays a very vital role in fattening an organism and cutting down on its surface-to-volume-ratio. In this regard, the surface offers the needed exchange areas with the environment while volume is necessary for storage. In terrestrial platyhelmiths, mesoderm helps them to store water especially terrestrial flatworms which are confined to living in very moist habitats like wetlands and rainforests. In higher animals, mesoderm is later specialized after being established to execute numerous functions especially in higher animals. In platyhelmiths, mesoderm is obtained from endoderm cells after or during gastrulation. This forms the true mesoderm or endomesoderm. The Platyhelmiths have a longitudinal antero-posterior axis which makes them exhibit bilateral symmetry. This means that their left and right side are each other's exact mirror image.
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The life history of Fasciola hepatica starts with the miraciduim which is a larva from the Fasciola hepatica egg. The miracidia are able to swim rapidly due to the presence of long cilia which cover them. In order for the development stage to proceed and completion of life history, the miraciduim must enter the body of an apt snail species. If it fails to locate such a host within a period of 24 hours after its emergence egg, it definitely dies. When it succeeds to locate the body of an appropriate host, it enters and chooses the spot with soft tissue. Once inside, it moves through the blood stream to the liver or digestive gland of the snail. Subsequent developments make it to loose its cilia and eventual formation of a cyst. This is called the sporocyst which later grows in size primarily by multiplication or germinal cells and growth. The growth and division is important because it gives rise to germinal ball of cells where each of the balls is bound to produce the following larval stage. The growth and multiplication is very significant because as they lead cause the sporocyst to distend thus leading to the rapture of the sporocyst into the snail digestive glands. The next stage is the redia which is a cylindrical larva. Its body is filled with fragile larval parenchyma packed with germinal cells. These cells multiply just like in the previous stage to form the germinal ball. These are the balls which form the cercariae which is the next larval stage. The cercariae move out of the snail and swims till they encyst themselves on vegetation where they form metacercariae. This is the infective stage of this organism. This complete life history spans for a duration of 2-3 months provided there are favorable conditions to facilitate the same. When the metacercariae are ingested by cattle sheep or other appropriate host they excyst in the hosts' small intestine and eventually to the abdominal cavity. These are the young flukes which then migrate to the liver's bile ducts where they mature to adult flukes. The onset of their maturity comes after 8-10 weeks after the initial infection. One of the strategies of survival employed by this parasite is the laying of very many eggs which increase the probability of landing a host. It's estimated that a single fluke can lay up to 25,000 in a single day and a single sheep can deposit 500,000 eggs in a day.
The life history of Diphyllobothrium Latum starts at the point where the final host eats infected or undercooked fish. The ingested parasite (plerocercoid) is not digested in the normal digestive system together with the fish tissue. This parasite matures into an adult in the hosts small intestines. Here, it is attached to the mucosa. After two weeks it starts to produce eggs. The eggs are released prematurely from the gravid proglottids and passed out in the feaces in extremely huge numbers. Once in the water, the mature eggs develop and hatch into larva. The larva is then consumed by copepod and undergoes further development. The second intermediate host (fish) then eats the copepod giving rise to a new larva and the life history continues. One survival tactic employed by this parasite is the laying of very many eggs with one worm having a potential to lay more than 1 million eggs. The sexual reproducing stages in the lifecycle of Diphyllobothrium latum are very important because they lead to the next stage in the life history.
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Polychaete has a very short life cycle and they produce sexually. The life history starts with swarming where Nereis virens worm disperses sperms or ova to the water surface (swarming). This stage may either be initiated by the male or female Nereis. Fertilization then occurs where the male wraps around the female tightly, then inserts his anus into the mouth of the female and inserts the sperms. After fertilization, the development stage starts in three distinct periods. These are pre-larval period, larval period and metamorphosis or post-larval period where larva develops into adult. It's during the post larval stage metamerism occurs as the larva continues to add new segments. This session sees the development of pre-orial apical portion into the prostomium. The first segment turns into peristomium while the last segment becomes pygidium. New segments continue to grow in front of the pygidium. Different polychaetes have different feeding mechanism depending on their classifications. They also live differently where some are crawling, pelagic, burrowing, tunicolous and yet others are parasitic. Nereis virens are carnivorous though at times they feed on sea lettuce in absence of other food. Metamorphosis plays a very vital role in ensuring that the larva of this organism develops to the next stage in the life history.
Danaus Plexippus exhibits a very complicated reproduction process. After mating, the female lays her eggs on plants mostly the milkweed plants. The eggs then hatch into larvae after three to fifteen days. The newly hatched larvae depend on the milkweed for two weeks. Later, they change into chrysalis a change that takes a relatively very short time period. The whole process takes two weeks. The larvae of this organism feed on milkweed while adults feed on flower nectar. Gaseous exchange takes place through holes located in the abdomen and thorax sides (spiracles). The nitrogenous waste is excreted through an internal excretory system through the anus. The excreted materials are mainly liquids. Danaus plexippus just like any other butterfly has three segments. The first segment is the head where the eyes and antennae are located. The second is the thorax and possesses the true legs (three pair). The wings also extend from the region above the true legs. The last segment is the abdomen which is the largest. It mainly consists of the digestive system. These segments are formed during the larval stage with wings extending later on as the larva matures into an adult fly. The Metamorphosis to this organism is very important because it the helps it in each stage to adapt the demands of that particular stage. For example during the first stage, the growing butterfly is covered by a hard shell which protects the larvae. The shell is waxy in order to prevent desiccation. Comparatively Danaus Plexippus has a shorter life history compared to Nereis in reference to time span. This is further evidenced by the number of stages followed by each to complete the whole cycle with the later having more stages. Basically, the two organisms are different in many ways as it has been demonstrated in the discussion above.
The life cycle of the Red-backed Jumping Spider (Phidippus Johnsoni) just like other spiders follows three stages. These are embryonic stage, larval stage and the nympho-imaginal stage. Before mating, the male spider approaches the female and then retreats. This is followed by a zigzag dance with some twitching sounds being produced by some males. This is termed as the courtship display (dancing). However, it's imperative to note that failing to dance does not lead to rejection by the female; it only minimizes the chances of acceptance. Virgin females are indifferent to courtship dancing while non virgins are known to be choosy. Again, after the successful mating the female may discriminate against the subsequent male in mating. However, she is free to replace the sperms from the first mating with those from the second mating. The eggs are laid in a ball shaped egg sac (silken) and either borne by the female or hidden. Each egg sac may contain hundreds of eggs. The eggs hatch some weeks later and the period between fertilization of the egg and maturation is called embryonic stage. During the larval stage the growing spider is already similar to full grown spider in terms of looks. There is the pre-larval stage from where it molts to get to the larval stage. The larva undergoes numerous more molts which lead to body differentiation. This is coupled with complete development of the organ system of the young Red-backed Jumping Spider. This stage is called the nympho-imaginal stage and is divided into two mini-stages. These are the nymph and the imago (adult stage). Sexual maturity of a spider is achieved after the imago stage is reached where it remains till it dies. This marks the end of molting.
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Of all terrestrial animas on earth, insects are said to be the most successful. This is so because of their high specialization. Insects have economy of structure, high success in reproduction, small size, undergo metamorphosis, have an exoskeleton and can fly. One of their most important adaptations is their exoskeleton. This is one adaptation that has assured their success. Exoskeleton provides protection and support and it's both a flexible and lightweight armor that has made insects to be dominant on land. Wings are strong due to exoskeleton strength and structure. The epidermis layer of exoskeleton has gland cells which secrete water resistant oils and waxes which help to reduce water loss through evaporation. It's a stable structure that they shed through ecdysis process in order to grow. It's hard for many organisms to attack them except for several micro-organisms. The outer layer (integument) comprises cuticle and the epidermis. Cuticle has high specialization since it's comprised of protein and chitin. It's a non-living secretion of epidermal cells. An attachment layer called Schmidt's located between living epidermis and the exoskeleton allows for exoskeleton attachment to the living soft tissue. Though the exoskeleton cannot be stretched, it is flexible enough to be molded into different shapes. Shedding of exoskeleton is a process that is controlled by hormones. It's only adult insects that possess functional and fully developed wings. In insects, wings develop in the early juvenile stages as outgrowths of naiads and nymphs. Wings allow insects to migrate to new habitats as well as freedom for locomotion. Wings may be used for other purposes other than just flight. Patterned and colored wings allow communication among member species through semaphores. Cryptic coloration or warning is an adaptation to defend against predators. Wings of insects are also used as communication means to members of the opposite sex. They also absorb sunlight and then transfer that heat to the insects' body. For wings to be drawn up, they use a pair of muscles located at their base where one set contracts to draw the wings up while the other contracts to draw them down.
The compound eye of insects is a set of photosensitive units referred to as ommatidia. The ommatidia function both as integrated and independent units in the compound eye. There is only one corneal lens in each ommatidium which is a thickening of the cuticle. The crystalline cone is a corneal lens under corneal lens and is secretion of simper cells. Under Simper cells is the retinula cells which are vertical cartridges where each cell contains microvilli. Light sensitive part of the compound eye is called rhabdom. This is formed by a composite of microvilli projecting inwardly. Retinula cells have their inner membrane extended as microvilli and this increases the surface area. As a result, the photosensitive area of the eye increases. Insects have two kinds of eyes: superposition and apposition eyes. Diurnal (day-flying) insects have apposition eyes such as bees and flies whole nocturnal (night-flying) have superposition eyes e.g., moths. For superposition eyes, their lenses are far from central rhabdom and retinula cells. Since these eyes are separated, their lenses have a cytoplasmic filament acting as a light guide connecting them to the rhabdom. Ommatidium in superposition eye is taller and this makes its outer layer to be thicker and more lenses in a unit area or ommatidia may be of the same number but lens have larger diameters. Despite this mechanism, superposition eyes possess a better ability of gathering light. Superposition eyes adjust to changing levels of light. Axons from retinula cells having similar field are taken as a set of optic cartridges in the lamina. There are around 6-8 axons from every optic cartridge. This means that the number of ommatidia is equal to that of optic cartridges. Optic cartridge cluster of neurons stimulates interneuron leading to medulla. Passing through lamina is the central eccentric cells which stimulate interneurons in medulla. Whenever, there is comparison of signals from eccentric cells and optic cartridges, color vision is allowed.
Visual images are integrated in the neuropil layers. For a damselfly, lamina carries the optic cartridges. Leading to the medulla are interneurons from the optic cartridges. Passing through lamina is the axons from eccentric cells going directly to medulla. On the position of the eye, position of neurons from medulla retains a similar geographic position like the ommatidia. This has good relationship with the image falling on eye's surface. Then, this is what maps an image in the brain. This is called a retinotopic map. Neural impulses are used to project the image into the brain of an insect. The work of the lobula is to interpret received information from different regions of medulla and then create sensations interpreting shape, objects color, size and movement in visual field.
Gradual metamorphosis also called incomplete metamorphosis has three life stages: egg, nymph and adult. Insects with such metamorphosis are called hemimetabolous. Growth is experienced in the nymph stage and an insect in this stage resembles the adult especially through appearance. A nymph also shares similar food, habitat and behaviors with the adult. In a winged insect, a nymph externally develops wings while growing and molting. Fully-grown and functional wings mark the attainment of adult stage. For complete metamorphosis, there are four stages: egg, larva, pupa ad adult. Each stage is quite different from the other. Such insects are termed as holometabolous. In this case, a larva has no resemblance to adults. Also food and habitats may be totally different. A larva of this kind of metamorphosis molts severally. The major advantage of complete metamorphosis is that since the larvae and the adult don't share similar habitats, food and behaviors, there is no competition for food as in gradual metamorphosis. There is also a wide separation of growth phase from dispersal to reproductive phase and thus allowing for division of labor.
Gastropoda (sea hare): It mainly feeds on red algae and green alga, Ulva lactuca. Sea hares obtain mycosporine-like amino acids (MAAs) from algae and then pass it to the spawn that absorbs Ultra Violet radiation in development. Adults with rich MAAs amount are capable of producing double as an adult spawn that has lower levels of MAAs. They accumulate metabolites of red algae and this show that the main food for sea hares is red algae which is obtained from sea grass beds. Sand makes around 25% of its intake volume. Swimming is related to searching for food since a well fed sea hare rarely swims. It feeds at night and mostly hides in crevices during the day.
Bivalve (clam): They have a pair of large gills contained in the pallial cavity. These are for capturing food particles from inhalant water current. These food particles are suspended in mucus carried by cilia through food grooves on gills edges to its mouth. Before particles are allowed into the mouth, the ciliated labial palps sort out the particles first. Its stomach is complex and large and the moment food reaches here, it encounters sorting mechanisms and a hyaline rod that liberates enzymes in its stomach. Digestion takes place in the stomach.
Cephalopod (squid): Feeding method is dependent on growth stage. The more they mature, the more they feed and even the size of the prey increases. Increase in size of food reduces energy expenditure on foraging. Its diet depends on prey availability. If Sandeels are adequate in size and prominent in a given period, squid will fed on them. Since prey availability has to do with time of the year, thus, the feeding habits of squid have seasonal variation. All cephalopods have a combination of muscular arms, intricate designed mouth and tentacles. These arms have suckers that assist the squid to envelop and seize the prey. It's mouth in inside buccal mass which is a complex and a large structure that has salivary glands, radula and a beak. Prior to ingestion of food, the chitinous beak chops the food particles. Radula assists this process as it is the tongue of the squid. Radula assists in passing food to esophagus and tearing its prey.
In mollusks, the shell is a product of secretion from the glandular cells found in mantle. Its shell is comprised of prismatic layer with packed calcareous cells materials which are secreted by the tip of the mantle together with thin inner layer of calcareous material. When the shell is thin, its nacreous lining is iridescent and pearly. Studies show that shell growth is a neurosecretory stimulated phenomenon. This is arrived at after studies on mantle which is a tongue-like protrusion. It's the mantle that sheds new pigment and shell material. The shell has calcium carbonate crystal structures and these have proteins and are pigmented. The cavity of the mantle also functions in gaseous exchange, release d elimination of reproductive products and in excretion. The shell of a mollusk is secreted by three layers: outer layer (periostracum) which is secreted by outer margin cells of mantle, middle layer (prismatic) which consists of a mixture of organic matter and calcium carbonate and this is also secreted by outer margin cells of mantle and nacreous layer (inner layer) which is a layer with alternating mixture of organic matter and calcium carbonate. Mollusks which dwell in the bottom of the sea like cuttlefish have no hydrostatic organ but it however hovers and swims and rests on bottom or above. It has capability of adjusting its buoyancy by using the amount of gases in its porous cuttlebone. Color change occurs due to the presence of chromatophores. These are cells with pigments and they secrete color due to hormonal and neuronal control mechanism.
Abalone shells are produced from nucleation sites in membranes rich in proteins. As a crystal grows, proteins are secreted. The extra proteins produced are used to direct growth of crystal. Every crystal will nucleate in one point and will grow outward and upward to meet all surrounding crystals to form a solid layer.
Octopus: They remain at the bottom and hunt there and thus does not need to fight to float. Actually, it doesn't have a mechanism to overcome density of its body tissues.
Loligo: It hunts close to bottom and in mid water. By adjusting the coleonic fluid salinity it obtains neutral buoyancy.
Nautilus: It fills its rear chamber of its shell with enough gas and these gases are secreted by siphuncle extending through every tissue.
Sepia: Achieves buoyancy by secretion of nitrogen gas in the internal shell, or cuttlebone which is under mantle surface.
Change of color in squid is due to change in platelets thickness. In this case, platelets become thinner and the optical properties of iridophores change to iridescent from non-iridescent. As this continues, platelets become even thinner and the iridescent color changes to that of shorter wavelength.