Excretory system


Physical Health
Archived from the original on May 30, Although the endocrine system in fishes is similar to that of higher vertebrates, there are numerous differences in detail. Help us improve this article! Some important reference ranges in medicine are reference ranges for blood tests and reference ranges for urine tests. Further evidence is needed to solve the problem of their classification and relationships. Common Conditions Treated by Urologists benign prostatic hyperplasia BPH or enlarged prostate cancers of the urinary tract such as kidney cancer , bladder cancer , prostate cancer , and testicular cancers infertility in women or men interstitial cystitis kidney stones urinary incontinence ; overactive bladder prostatitis sexual dysfunction such as erectile dysfunction urinary tract infections. Consequently, measuring the total amount of VOCs in exhaled air, a kind of metabolomics also referred to as breathomics.

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Urinary System

Its functions in the domestic fowl are to:. The two reddish-brown kidneys of the domestic fowl, each generally with three lobes, are found immediately behind the lungs on each side of the vertebral column and closely associated with it.

The ureter connects to many funnel shaped structures from each lobe of the kidney. The kidney, on close inspection consists of the renal cortex and the renal medulla. A microscopic examination of a section of kidney will show that it consists of a large number of renal tubules, or nephrons , each divided into cortical and medullary parts.

Birds have two types of nephrons. The renal tubules extract the constituents of the urine from the blood that flows through the kidney. Like almost all birds, the domestic fowl does not have a bladder as is found in most mammals and amphibians.

This re-absorbed water is available for use by the bird and, to some extent, offsets the limited ability of birds to concentrate their urine as efficiently as mammals. The urine is in a thick pasty form with a very low water content but high in uric acid from nitrogen metabolism.

It is usually passed as a paste and is deposited as a whitish or cream cap on some faecal stools. Several of these centres or parts are primarily associated with one type of sensory perception, such as sight, hearing, or smell olfaction. The sense of smell is important in almost all fishes. Certain eels with tiny eyes depend mostly on smell for location of food. The olfactory, or nasal, organ of fishes is located on the dorsal surface of the snout. The lining of the nasal organ has special sensory cells that perceive chemicals dissolved in the water, such as substances from food material, and send sensory information to the brain by way of the first cranial nerve.

Odour also serves as an alarm system. Many fishes, especially various species of freshwater minnows, react with alarm to a chemical released from the skin of an injured member of their own species.

Many fishes have a well-developed sense of taste, and tiny pitlike taste buds or organs are located not only within their mouth cavities but also over their heads and parts of their body. Some species of naturally blind cave fishes are especially well supplied with taste buds, which often cover most of their body surface.

Sight is extremely important in most fishes. The eye of a fish is basically like that of all other vertebrates, but the eyes of fishes are extremely varied in structure and adaptation. In general, fishes living in dark and dim water habitats have large eyes, unless they have specialized in some compensatory way so that another sense such as smell is dominant, in which case the eyes will often be reduced.



Most fishes see well, despite the restrictions imposed by frequent turbidity of the water and by light refraction. Fossil evidence suggests that colour vision evolved in fishes more than million years ago, but not all living fishes have retained this ability. Experimental evidence indicates that many shallow-water fishes, if not all, have colour vision and see some colours especially well, but some bottom-dwelling shore fishes live in areas where the water is sufficiently deep to filter out most if not all colours, and these fishes apparently never see colours.

When tested in shallow water, they apparently are unable to respond to colour differences. Sound perception and balance are intimately associated senses in a fish. The organs of hearing are entirely internal, located within the skull, on each side of the brain and somewhat behind the eyes.


Compared with humans, however, the range of sound frequencies heard by fishes is greatly restricted. Many fishes communicate with each other by producing sounds in their swim bladders, in their throats by rasping their teeth, and in other ways. A fish or other vertebrate seldom has to rely on a single type of sensory information to determine the nature of the environment around it. A catfish uses taste and touch when examining a food object with its oral barbels.


An important sensory system in fishes that is absent in other vertebrates except some amphibians is the lateral line system. This consists of a series of heavily innervated small canals located in the skin and bone around the eyes, along the lower jaw , over the head, and down the mid-side of the body, where it is associated with the scales.

Intermittently along these canals are located tiny sensory organs pit organs that apparently detect changes in pressure. The system allows a fish to sense changes in water currents and pressure, thereby helping the fish to orient itself to the various changes that occur in the physical environment. Although a great many fossil fishes have been found and described, they represent a tiny portion of the long and complex evolution of fishes, and knowledge of fish evolution remains relatively fragmentary.

In the classification presented in this article, fishlike vertebrates are divided into seven categories, the members of each having a different basic structural organization and different physical and physiological adaptations for the problems presented by the environment. The broad basic pattern has been one of successive replacement of older groups by newer, better-adapted groups. One or a few members of a group evolved a basically more efficient means of feeding, breathing, or swimming or several better ways of living.


The process was repeated until all or almost all members of the old group in a variety of habitats had been replaced by members of the newer evolutionary line. The earliest vertebrate fossils of certain relationships are fragments of dermal armour of jawless fishes superclass Agnatha , order Heterostraci from the Upper Ordovician Period in North America , about million years in age. Early Ordovician toothlike fragments from the former Soviet Union are less certainly remains of agnathans.


Jawless fishes probably arose from ancient, small, soft-bodied filter-feeding organisms much like and probably also ancestral to the modern sand-dwelling filter feeders, the Cephalochordata Amphioxus and its relatives. The body in the ancestral animals was probably stiffened by a notochord. Although a vertebrate origin in fresh water is much debated by paleontologists, it is possible that mobility of the body and protection provided by dermal armour arose in response to streamflow in the freshwater environment and to the need to escape from and resist the clawed invertebrate eurypterids that lived in the same waters.

Because of the marine distribution of the surviving primitive chordates, however, many paleontologists doubt that the vertebrates arose in fresh water.



No fossils are known of the more primitive ancestors of the agnathans and acanthodians. The extensive marine beds of the Silurian and those of the Ordovician are essentially void of vertebrate history. It is believed that the ancestors of fishlike vertebrates evolved in upland fresh waters, where whatever few and relatively small fossil beds were made probably have been long since eroded away.

Remains of the earliest vertebrates may never be found. By the close of the Silurian, all known orders of jawless vertebrates had evolved, except perhaps the modern cyclostomes , which are without the hard parts that ordinarily are preserved as fossils.

Cyclostomes were unknown as fossils until , when a lamprey of modern body structure was reported from the Middle Pennsylvanian of Illinois, in deposits more than million years old. Fossil evidence of the four orders of armoured jawless vertebrates is absent from deposits later than the Devonian. Presumably, these vertebrates became extinct at that time, being replaced by the more efficient and probably more aggressive placoderms , acanthodians, selachians sharks and relatives , and by early bony fishes.

Cyclostomes survived probably because early on they evolved from anaspid agnathans and developed a rasping tonguelike structure and a sucking mouth, enabling them to prey on other fishes. With this way of life they apparently had no competition from other fish groups.

Cyclostomes, the hagfishes and lampreys, were once thought to be closely related because of the similarity in their suctorial mouths, but it is now understood that the hagfishes, order Myxiniformes, are the most primitive living chordates, and they are classified separately from the lampreys, order Petromyzontiformes. Early jawless vertebrates probably fed on tiny organisms by filter feeding , as do the larvae of their descendants, the modern lampreys.


Small organisms then were strained out by the gill apparatus and directed to the food canal. The gill apparatus thus evolved as a feeding, as well as a breathing, structure. The head and gills in the agnathans were protected by a heavy dermal armour; the tail region was free, allowing motion for swimming.

Most important for the evolution of fishes and vertebrates in general was the early appearance of bone, cartilage, and enamel-like substance. These materials became modified in later fishes, enabling them to adapt to many aquatic environments and finally even to land. Other basic organs and tissues of the vertebrates—such as the central nervous system, heart , liver , digestive tract, kidney, and circulatory system— undoubtedly were present in the ancestors of the agnathans.

In many ways, bone, both external and internal, was the key to vertebrate evolution. The next class of fishes to appear was the Acanthodii , containing the earliest known jawed vertebrates, which arose in the Late Silurian, more than million years ago.

The acanthodians declined after the Devonian but lasted into the Early Permian, a little less than million years ago. The first complete specimens appear in Lower Devonian freshwater deposits, but later in the Devonian and Permian some members appear to have been marine. Most were small fishes, not more than 75 cm approximately 30 inches in length. We know nothing of the ancestors of the acanthodians. They must have arisen from some jawless vertebrate, probably in fresh water.

They appear to have been active swimmers with almost no head armour but with large eyes, indicating that they depended heavily on vision. Perhaps they preyed on invertebrates. The relationships of the acanthodians to other jawed vertebrates are obscure. They possess features found in both sharks and bony fishes. They are like early bony fishes in possessing ganoidlike scales and a partially ossified internal skeleton. Certain aspects of the jaw appear to be more like those of bony fishes than sharks, but the bony fin spines and certain aspects of the gill apparatus would seem to favour relationships with early sharks.

Acanthodians do not seem particularly close to the Placodermi , although, like the placoderms, they apparently possessed less efficient tooth replacement and tooth structure than the sharks and the bony fishes, possibly one reason for their subsequent extinction. The first record of the jawed Placodermi is from the Early Devonian, about million years ago. The placoderms flourished for about 60 million years and were almost gone at the end of the Devonian.

Nothing is known of their ancestors, who must have existed in the Silurian. The evolution of several other, better-adapted fish groups soon followed the appearance of the placoderms, and this apparently led to their early extinction.

Their greatest period of success was approximately during the middle of the Devonian, when some of them became marine. The tail remained free and heterocercal that is, the upper lobe long, the lower one small or lacking. Most placoderms remained small, 30 cm 12 inches or less in length, but one group, the arthrodires , had a few marine members that reached 10 metres about 33 feet in length.

Important evolutionary advances of the placoderms were in the jaws which usually were amphistylic—that is, involving the hyoid and quadrate bones and development of fins, especially the paired fins with well-formed basal or radial elements. The jaws tended to be of single elements with strongly attached toothlike structures. These were too specialized to be considered ancestral to the more adaptable jaws of subsequent bony fish groups.

It has been proposed that sharks arose from some group of placoderms near the Stensioelliformes and that the chimaera line class Holocephali arose from certain arthrodires; this suggestion, however, is uncertain. A peculiar 5-cm 2-inch fossilized fish, Palaeospondylus , from Middle Devonian rocks in Scotland, is probably not a placoderm, although it is sometimes classed with placoderms. Various suggestions that its relationships are with the agnathans, placoderms, acanthodians, sharks, and even lungfishes and amphibians are unconvincing, and its relationships remain completely unknown.

The earliest sharks class Chondrichthyes first appeared in the Early Devonian about million years ago, became quite prominent by the end of the Devonian, and are still successful today.

Two Early Devonian orders of primitive sharklike fishes, the Cladoselachiformes and the Cladodontiformes, became extinct by the end of the Permian, about million years ago, while the freshwater order Xenacanthiformes lasted until the end of the Triassic, about million years ago. The final Devonian order, Heterodontiformes, still has surviving members.

Modern sharks and rays arose during the Jurassic Period , about million to Presumably marine cladoselachians gave rise to the hybodont Heterodontiformes during the close of the Devonian. These had the placoderm amphystylic jaws but had paired fins of a more efficient type. In turn the hybodonts are thought to have given rise to the living but archaic mollusk-eating Port Jackson sharks heterodonts. The relationships of the surviving but archaic hexanchiform sharks are unknown.

The three main orders of modern Selachii—the Carcharhiniformes ground sharks and Lamniformes mackerel sharks and Rajiformes skates and rays —appeared during the Jurassic Period. They are characterized by a hyostylic jaw in which articulation involves only the hyoid bone , an improvement allowing greater mobility of the jaws and an important feature in the methods of predation used by modern selachians.

Skates and rays evolved from some bottom-living sharklike ancestor during the Jurassic. The primary evolution and diversification of modern sharks, skates, and rays took place in the Cretaceous Period and Cenozoic Era. Thus, along with the teleost fishes discussed below , most surviving sharks, skates, and rays are essentially of relatively recent origin, their main evolutionary radiation having taken place since Jurassic times.

The class Holocephali—the chimaeras or ratfishes, as their modern survivors are called—first appeared in the Late Devonian but were most common and diversified during the Mesozoic Era. Only one of the seven known orders survived beyond the close of the Cretaceous Period Although not many modern species of chimaeras are known, they are sometimes relatively abundant in their deep-sea habitat. The relationships of these fishes are in question.

It has been proposed that they are related to the Devonian ptyctodont arthrodires, which had a chimaera-like shape and pelvic claspers. It has also been suggested that they are closely related to the Selachii because both selachians and holocephalians have many characters in common, such as placoid scales, pelvic claspers, and the absence of true bone. It has been suggested that both holocephalians and selachians are related to the acanthodians on the basis of the gill arch structures.

Further evidence is needed to solve the problem of their classification and relationships. Fishes of the class Sarcopterygii are extremely ancient in origin, their first remains appearing in Lower Devonian strata of Germany.

Some authorities contend that the rhipidistians , one of the three groups of sarcopterygians, gave rise to the amphibians by the end of the Devonian; however, other authorities believe that tetrapods evolved from one of two other groups, the coelacanths and the dipnoans lungfish.

The rhipidistians became extinct about million years later, near the beginning of the Permian, but the coelacanths and the dipnoans have survived, albeit in small numbers. The primitive sarcopterygians show several similarities, supporting the view that they had a common ancestor.

The nature of the ancestor remains a mystery. The sarcopterygians probably evolved from unknown Silurian jawed freshwater fishes that may also have been ancestral to the actinopterygians.


Paradoxically, terrestrial amphibians first arose through the need to survive in water. The early coelacanths of the Late Devonian were small freshwater and inshore fishes, and it was not until the Late Permian and Triassic that they became marine and grew larger and more diverse.

They are not known as fossils later than the Cretaceous, and it was therefore a great surprise when in a live cm inch specimen was taken at metres approximately feet depth off the coast of eastern South Africa. A second living coelacanth species was discovered off the Indonesian island of Sulawesi in The dipnoans first appeared in the Early Devonian and were fully differentiated at that time.


The Actinopterygii , or ray-finned fishes, are the largest class of fishes. In existence for about million years, since the Early Devonian, it consists of some 42 orders containing more than families, at least 80 of which are known only from fossils.

The class contains the great majority of known living and fossil fishes, with about 26, living species. The history of actinopterygians can be divided into three basic stages or evolutionary radiations, each representing a different level of structural organization and efficiency. The Chondrostei may have first arisen as early as the Early Devonian, increased in numbers and complexity until about the Permian, and thereafter declined, becoming almost extinct by the middle of the Cretaceous, million years ago.


They gave rise to a great variety of types, with elongate bodies and jaws, bottom-living types that fed on microorganisms, deep-bodied marine reef fishes, and coral-eating reef fishes.

Anatomy of the Endocrine System