Zoology and Animal Diversity


Chordates belong to the kingdom Chordata. They are deuterostomecoelomates with their closest relation in the animal kingdom beingthe echinoderms. Chordates have five unique and necessarycharacteristics. They have a notochord that provides skeletalsupport. It develops into the vertebral column (Rychel et al., 2006).It is a rod-shaped structure that starts developing in the embryonicstage of all the chordates. In some chordates, it acts as the mainaxial support of the body throughout the animals’ lifetime. Theyalso have a dorsal hollow nerve that evolves into the central nervoussystem including the brain and the spine (Ryche et al., 2006). Theyalso have a pharyngeal slits located at the opening of the pharynxthat develops into jaws and the inner ear in the terrestrial animals.Also, they have a post-anal tail extension located at the posteriorend of the body. Although it is absent in human and apes, it ispresent during their embryonic development (Ryche et al., 2006).

The feeding mechanism of lancelets depends on microscopic protists.They filter feeders with ineffective sensory systems. Lancelets haveno eyes, hands, head, eyes and the nose (Riisgård &amp Svane,1999). To detect chemicals in their environment, they use sensorycells. They feed on microscopic protists caught, and they filter themthrough their cilia and the gills located in the pharyngeal area.Feeding in Jamoytius, an agnatha belonging to the phylum Chordata isdifferent from the one observed in Lancelet. Jamoytius is a jawlessorganism. The organism has a slow metabolism since it stays in thecold water and they eat less in such an environment. Jamoytius doesnot have a distinct stomach, but they have a long gut (Riisgård &ampSvane, 1999). They feed on other fish and small mammals. To crush adshred their prey, they use a set of sharp teeth. The inability oftheir teeth to move up and down and it, therefore, limit the type offoods (Riisgård &amp Svane, 1999).

The historical development if the jaw dates to the simpletransformation of an ancestral rostral gill arch. The cartilage ofthe vertebrate jaw comes from the cranial neural crest that forms thefirst pharyngeal arch, and it forms the maxillary and the ventralmandibular (Crompton &amp Jenkins, 1979). The maxillary gives riseto the palatoquadrate, and the mandibular gives rise to the meckel’scartilage. The condensation that took place to form these organs werebelieved to have formed the framework for the bones of the jaw. Inthe amniotes, the bones of the jaw prefigure the maxillary andprominent mandibular faces. The palatoquadrate and the Meckel’scartilage emanate from the ventral mandibular through condensation(Crompton &amp Jenkins, 1979).

The circulatory system in vertebrates evolved from the most primitiveto the complex system as observed in fish, amphibians, birds andmammals. The circulatory system primarily consists of the helm, theveins and the arteries (Kardong, 2006). The purpose of the arteriesis to transport the fluid to the other boy cells. The veins transportblood to the heart to receive oxygen before it is transported back tothe cells. Vertebrates exhibit both the open and closed circulatorysystem. The lancelet possesses one of the most primitive circulatorysystems. In its unique circulatory system, the vessels form from theoriginal coelomic cavity (Kardong, 2006). They are lined with asimple epithelium, but it lacks the basal lamina. Blood in lanceletmoves ventrally forward and backward dorsally. However, unlike inother vertebrates blood in lancelet lacks cellular elements and thepigment that carries oxygen. There is no heart in the lancelet, andthe Sinus Venosus acts the blood colleting organ. There is evidenceof evolutionary change in the vertebrates that shows cleardistinctions in the features of the heart between differentorganisms. The Branchiostoma lanceolatum also exhibits a primitivecirculatory system. The organism possesses a close hemal system(Moller &amp Philpott, 1973). The hemal fluid flows backwarddorsally and forwards ventrally just like in the lancelet. Like thelancelet, it does not have a proper heart. The hemal fluid circulatesthrough the contraction of several main vessels (Moller &ampPhilpott, 1973).

The sinus Venosus in the fish acts as the receiver of blood comingfrom the cardinal veins emanating from the side of the body and thehepatic veins emanating from the liver. When blood gets into theatrium, it balloons out of the muscular ventricle forcing blood toflow into the ventricle. A series of valves acts as the barriers toprevent the flow back of blood (Randall, 1970). The evolution of thecirculatory system shows an improvement when it comes to thelungfish. It has a double-chambered atrium. The left atrium receivesthe oxygenated blood that pumped to the rest of the body. The rightatrium receives deoxygenated blood that is transported to the lungsto receive oxygen before coming back to the heart. In amphibians, thetwo atriums are more separated than in the lungfish (Randall, 1970).Blood flows from the right atrium into the ventricle that contains afolding muscle to prevent the mixing of blood in the right and theleft ventricle the deoxygenated blood from the right ventricle flowsthrough the pulmonary artery to the lungs, gills, and the skin. Thepulmonary veins collect the oxygenated blood ad directs it into theleft atrium, and it then enters the systemic arteries to proceed tothe cells. In reptiles, the two atriums become even more separatethan in the amphibians (Randall, 1970). However, there is a slightmixing of blood between the two chambers. The evolutionarydifferences continue to show differences in birds that have two atriaand two ventricles asking their hearts to have four completechambers. Birds are the first to exhibit a double circulation unlikein amphibians and fish. Mammals exhibit the most complex and advancedcirculatory system. Their hearts have four chambers with completelydivided atria (Randall, 1970).

Fish have a two-chambered heart composing of a single atrium andventricle. They contain an enlarged area referred to as the sinusthat collects and stores blood. As the blood leaves the ventricle, itpasses through bulbous arteries that have muscular thickened wallsthat help in absorbing blood from the ventricle (Randall, 1970).Blood from the ventricle passes through the conus arteriosus. Itcontains valves to prevent the backflow of blood. Blood from theheart goes to the gills to collect oxygen before collecting into thearteries that distribute into the body cells. In fish wit swimbladder, the organ serves the purpose of buoyancy and bloodcirculation. When a fish wants to increase buoyancy and go to theshallow waters, it introduces gas into the swim bladder (Randall,1970). The glands in the bladder secrete lactic acid that introducescarbon dioxide. The resultant environment causes the blood to losesome of its hemoglobin and oxygen. Blood flowing back to the boyenters the rete mirabile in the swim bladder and virtually loses allthe carbon dioxide and oxygen emanating from the gas gland beforediffusing back to the arteries (Randall, 1970).

A tadpole gradually develops into a frog. The most visible thatdisappear from a tadpole is the tail. A tadpole breathes by taking inoxygen through its thin skin that contains a rich supply of vascularcells. Before metamorphosis, the tadpoles start developing the lungs(Benbassat, 1974). While the tail shorter during metamorphosis, theskin thickens. In addition, it becomes less able to take in air. Dueto the reduced capability of the skin, the tadpoles develop lungs toact as the main respiratory organ. The circulatory system has anintricate relationship with the respiratory system. The blood in thetadpole becomes thick and it accumulates more serum proteins. Thelarval blood cells that form in the kidney and the liver die as morecomplex and numerous red blood cells. Blood develops from the stemcells in the bone marrow. The different type of hemoglobin that formscan in more red blood cells than the ones formed during the larvalstage (Benbassat, 1974).

The circulatory system in reptiles is adapted in several ways. First,its ventricle is not completely divided. There is, therefore, therisk of oxygenated and deoxygenated blood mixing. However, the heartprevents the mixing by contracting of the valves, the movement if theseptum and the ventricular walls (Randall, 1970). Secondly, pumpingblood through the lungs and back to the heart requires energy.However, to conserve energy, reptiles use little oxygen when try areinactive, and they can say for some time without breathing. A good,example is when they are under water where there is low amountoxygen. Reptiles may also require raising the temperature of somebody organs (Randall, 1970). To do so, the warm blood from the skinmay fail to pass through the lungs and directly go to the internalorgans, bypassing the lungs prevents the transfer of warmth to myorgans.

Birds also have a circulatory system that suits their high metabolicactivities. They have larger hearts than mammals that are efficientin pumping blood at a fast rate to provide oxygen required for flightand diving. In addition, a bird’s heart pumps blood in a highercapacity than mammals. Their cardiac output exceeds that of mammals(Randall, 1970). The increased output is necessary to for thehigh-speed activities that birds undertake. The mammaliancirculatory system also has various adaptations. The heart pumpsblood under high pressure to make sure it reaches all the organs inthe body (Randall, 1970). The heart has completely separatedventricles, and this makes it impossible for oxygenated anddeoxygenated blood t o mix. Mammals, therefore, have the capacity tomaintain optimum functioning cells all the time. The doublecirculatory system in mammals keeps blood in a closed systematiccircuit. It helps mammals to remain active all the time since bloodcan reach the respiring tissues at a consistent rate (Randall, 1970).

Fish have different receptor organs that detect changes in theexternal stimuli. They include the chemicals, pressure, and electricreceptors. The pressure detection takes place in the organ of Weberthat contains three appendages that help in changing the shape of thegas bladder (Fay &amp Tavolga, 2012). The receptors help the fish tomaintain buoyancy. Chemoreception helps the fish to detect theirprey. The cells are located in the anterior and the posterior nasalopening. The shark uses these cells to detect the presence of bloodin the water (Jorgensen, 2005). They can detect one part of a millionof blood in seawater. They can also identify the direction f thescent by the timing in each nostril (Fay &amp Tavolga, 2012). Theelectric sensory organs are also imperative in aiding fish indetecting electric current in their environment. The sensory cellsoccur in clustered groups connecting to different parts of the skin,and they are visible as dark pores on the skins (Jorgensen, 2005).Their numerous number assists in the effective detection of thecurrent. Sharks use the technique to detect their prey since allliving organisms produce electromagnetic currents (Fay &amp Tavolga,2012).

Fish also contain a lateral line that is a sensory organ for aidingthem in identifying the position and movements of objects. It appearsas a faint line one each side of the fish and extends to the gill andthe tail. They use it to locate predators and the prey as well asavoid currents. In murky fish, the lateral line is adapted forelectrolocation (Fay &amp Tavolga, 2012). The electro-receptiveanimals employ this technique to locate objects in their environment.For example, objects buried in murky water. Fish also use specializedcells called neuromats (lateral line organs) that are in a finelyinterconnected network proceeding from the head and the body. Inmurky water, fish produce small charges that can discriminate objectswith resistance and capacitance (Fay &amp Tavolga, 2012).

A good number of animals lay eggs. The amniotes egg is unique and hasvarious adaptive features. Animas that lay these kinds of eggsinclude turtles, lizards, birds, and dinosaurs. The inside of the eggcontains membranes containing a fluid. They include the amnion, theallantois, the York sac and the chorion. The amnion surrounds theembryo, and it contains the amniotic fluid. It provides the embryowith a stable environment. The Allantois provides a space for gasdiffusion and the removal of the waste materials. The York sac holdsthe food require for the development of the embryo. The chorionencloses the young embryo as it develops. I the placental mammals,the membranes in the egg have been modified (Brawand et al., 2008).The amnion still surrounds the embryo, and it contains the amnioticfluid that gives stability to the embryo. The allantois and the Yorksac turn out to be the umbilical cord, and they provide a medium forfood exchange between the mother and the embryo and for wasteremoval. The chorion and the other membranes make up the placenta(Brawand et al., 2008).

The calcareous shell in the egg acts as its basic unit. Thecharacteristics of the egg are genetically controlled. The shellcontains calcium carbonate that gives the shell its structure,rigidity and resistance to breakage. Also, several adaptationrequirements lead to the evolution egg lying reptile to placentalmammals (Brawand et al., 2008). The mammals expanded their awarenessof their surrounding, and they exhibited an improved sense of light,smell and touch. There was therefore and increased the need forcoordination that demanded a constant cerebral palsy (Moffett &ampLoke, 2006). Placental development suited this function of themammals. The need for brain development requires the embryo to remainprotected in the uterus while receiving nutrients for the entiregestation period.


Benbassat, J.(1974). The Transition from Tadpole to Frog Haemoglobin DuringNatural Amphibian Metamorphosis I. Protein Synthesis by PeripheralBlood Cells In Vitro. Journal of cell science, 15(2),347-357.

Brawand, D., Wahli,W., &amp Kaessmann, H. (2008). Loss of egg yolk genes in mammals andthe origin of lactation and placentation. PLoS Biol, 6(3),e63.

Crompton, A. W., &ampJenkins, F. A. (1979). Origin of mammals. JA Lillegraven, Z.Kielan− Jaworowska, and WA Clemens, Jr.(eds.), Me− sozoicMammals: The First Two− thirds of Mammalian History, 59-73.

Fay, R. R., &ampTavolga, W. N. (Eds.). (2012). Sensory biology of aquatic animals.New York N.Y: Springer Science &amp Business Media.

Jorgensen, J. M.(2005). Morphology of electroreceptive sensory organs. SpringerHandbook of Auditory Research, 21, 47.

Kardong, K. V.(2006). Vertebrates: comparative anatomy, function, evolution(pp. 455-461). Boston: McGraw-Hill.

Moffett, A., &ampLoke, C. (2006). Immunology of placentation in eutherian mammals.Nature Reviews Immunology, 6(8), 584-594.

Moller, P. C., &ampPhilpott, C. W. (1973). The circulatory system of amphioxus(Branchiostoma floridae) I. Morphology of the major vessels of thepharyngeal area. Journal of morphology, 139(4),389-406.Randall, D. J. (1970). The circulatory system. Fishphysiology, 4, 133-172.

Riisgård, H. U., &ampSvane, I. (1999). Filter feeding in lancelets (amphioxus),Branchiostoma lanceolatum. Invertebrate Biology, 423-432.

Rychel, A. L.,Smith, S. E., Shimamoto, H. T., &amp Swalla, B. J. (2006). Evolutionand development of the chordates: collagen and pharyngeal cartilage.Molecular biology and evolution, 23(3), 541-549.