3. Transformation of arterial (gills) arches of vertebrates.
Most of largest arterious vessels of human develops from gills arteries. For embryo from heart go on ventral aorta and 6 pairs of arteriousus arches more away from ventral aorta.
In all vertebrates (fishes, amphibians, reptiles, mammals) first two pairs of gills arches are disappeared, because two gill septums or visceral arches included in composition of fascial skull. Other four pairs of gills arches undergoes to transformation due to air breathing.
3rd pair with section of route of the spinal aorta transformed to carotid arteries which pumps oxygenated blood to the head.
Vessels of 4th pair with segment of spinal (dorsal) aorta transformed to aortic arches, which are symmetrical in amphibians and reptiles. But in mammals this is only left arch is developed and right arch is disappeared. It is asymmetric development.
5th pair of gills arches is reduced.
6th pair transfers to pulmonary artery and lost connection with other arches and routes of aorta.
Duct which joins pulmonary arteries with aortic arches (4th pair), is called ductus arteriosus.
4. Excretory and Osmoregulatory organs of Protozoans and Invertebrates.
Protozoans lose waste matter by simple diffusion through the cell membrane into the surrounding water where solute concentration is lower. Some excretion occurs by way of contractile vacuole also.
Sponges. The main excretory matter in sponges is also ammonia. It leaves the cells by diffusion into the water filling the canals and surrounding the sponge. A few sponges live in hypotonic fresh water and have contractile vacuoles in most of their cells.
Coelenterates. E.g. Hydra, also get rid of waste matter by diffusion from all the cells.
Flatworms (planarians, flukes and tapeworms) have flame cells with bundles of vibratile cilia for excretion. The flame cells receive the solutes and water from the surrounding tissue fluid by ultra filtration and discharge the filtrate into the nephridial ducts. The final urine so formed is eliminated to the exterior by way of excretory pores.
Roundworms have a system of intracellular canals and canaliculi opening out at the excretory pore.
Annelids (sandworms, earthworm, leech) have for excretion and osmoregulation many coiled tubules, called nephridia.
Arthropods (insects, arachnids) have numerous blind Malpighian tubules, which float in the haemolymph and open into the gut.
Crustaceans (prawn) have a pair of antennary, or green gland and a median renal sac for excretion and osmoregulation.
Mollusks have 1 or 2 pairs of kidneys for excretion. The kidney opens into the pericardium (coelom) at one end by renopericardial aperture (nephrostome) and into mantle cavity at the other end by nephridiopore.
5. Kidneys are the major excretory and osmoregulatory organs of Vertebrates.
Embryonic laying of the kidneys occurs by way of constriction of coelomic bladder. In the region of constriction form the thin isthmus, which is called as stem of somite.
Upper part is somite and lower part is lateral plate.
During embryogenesis the stem of somite becomes thinner and break off. Stem of somite also become longer and began turn up. An anterior wall connected with subsequent and the posterior walls formed the wall of common duct. The end of stem of somite open into the coelom cavity by aperture (nephrostome) and many tubules, which form the stem of somite are called as nephridic uriniferous tubules. It is formation of pronephros or head kidney.
There are 6 to 12 nephridic tubules for pronephros. In embryonic period pronephros lays in the head part of an embryo and called as head kidney.
In lowest vertebrates consequently excretory organs lays at the first in head part, than in trunk part of an embryo (it is primary kidney or mesonephros). In highest Vertebrates form next 3rd lining in pelvic part of the body (it is secondary kidney or metanephros). Early lining of kidney or early stages is reduced after appearance of next stage. This change of excretory organs during ontogenesis demonstrates the stages of phylogenesis. In modern Vertebrates pronephros exist only in embryonic period.
Next stage of evolution development of kidney is primary kidney, or trunk kidney, or mesonephros. Mesonephros operates in fishes, amphibians. It has following features:
1. Number of nephridic tubules is increase and becomes more than 10-12 and tubules are elongated;
2. Common duct of mesonephros is separated into an anterior Wolfov’s duct and second Muller’s duct;
3. Structure of nephron is complicated.
Mesonephros operates during all life in lower Vertebrates (fishes, amphibians – Anamnia), but for higher vertebrates (Amniota – reptiles, birds and mammals) present only in embryonic period.
pronephros mesonephros metanephros
In mesonephros an anterior tubules have nephrostome, and posterior will formed the nephros body. It is a direct connection between circulatory and excretory system. Waste substances are secreted from blood into kidney, not into coelom. Connection with coelom is partly lost.
Next stage is secondary kidney, or pelvic kidney, or metanephros, which develops in pelvic region of an embryo from nephrogene tissues. The nephrostome is absent and the connection with coelom is completely lost. A metanephros contains about a million nephrons. A nephron is a unit of structure and function in a kidney.
Uriniferous tubules becomes longer and blind-ended. A nephron is a long tubule differentiated into four regions: Bowman’s capsule, proximal convoluted tubule (PST), loop of Henle, and distal convoluted tubule (DST). Length all is considerably augmented. The secretory function of cells of tubules walls strengthens. Very much amount of nephrons.
Metanephros operates in highest Vertebrates and in human.
Excretory system of Vertebrates connected with organs of reproductive system. Reproductive glands of Vertebrates lays in form of paired folds on the ventral sides of mesonephros. Lining of gonads consist of thickening of epithelium with much amount of connective tissues. In the first male and female gonads have similar structure, later its differentiated.