......... KINGDOM METAZOA III. VERTEBRATES I (Fishes and Amphibians)

Introduction. The fishes are the oldest vertebrates. The first appeared about 500 million years ago, were small, had a bony shell and had no mandible. Its mouth was circular and adapted to filter the food. Then the armor-plated fishes with mandible appeared, then the sharks that have barely changed ever since, and finally, the fishes with bones. Because of respiring through the gills they have not been able to colonize the terrestrial environment. A little group called Lungfish, thanks to having its swim bladder connected to the outside, can live out of the water, crawling and jumping on wet mud with its pectoral fins. When it is dry and the mud begins to dry, these fish are sinking in the mud and survive by taking an advantage of the inferior moisture. The Lungfish give us a clue about how the lungs and limbs of animals that walk on land appeared.

The amphibians appeared about 370 million years ago and some even exceed three meters in length. Thanks to having pulmonary respiration system they were the first vertebrates to colonize the terrestrial environment, but as its lungs are very simple, they still need the cooperation with the cutaneous respiration system. This means that its skin is naked and humid, and this means that they can only live in very humid environments. Moreover, by having an external fertilization, they need water to be able to reproduce. In conclusion, amphibians can live out of the water but only in humid environments and in those where from time to time heavy rains occur so they can be able to reproduce, therefore, they are not fully independent from the aquatic environment.

Activities to do. Read the explanations on the general characteristics and classification of Fishes and perform Multiple Choice Test 18.1 and Relate pictures with names 18.1. Then read the text on the amphibians and perform Multiple Choice Test 18.2, Relate pictures with names 18.2 and Crossword 18.

1. The vertebrates. Are the metazoans that have the spinal column, meaning the articulated bone structure that protects nerve cord or spinal cord. The name "vertebral column" is most appropriate for bipedal vertebrates like us humans and birds, but for the fishes and quadrupedal vertebrates, it is also customary to use the name of dorsal spine.

All vertebrates have a closed circulation, which means that the blood is always flowing inside the ducts called vessels (arteries, veins and capillaries). There are known around 50,000. They differ into 5 groups of vertebrates are: fishes, amphibians, reptiles, birds and mammals. This chapter discusses the first two.

• Fishes. They are the vertebrates that have skin with dermic scales and which swim in the water using fins.

• Amphibians. They are vertebrates the that have wet and uncovered skin (without scales, feathers nor hair) and have four legs in a lateral position for walking and swimming.

• Reptiles. They are the vertebrates that have skin with epidermic scales and have four legs in a lateral position (except snakes, which does not have legs) so they usually move by crawling on the ground.

• Birds. They are the vertebrates that have skin with feathers, the extremities that form two wings, which allow most of them to fly, and the back extremities that constitute two legs that allow them to walk.

• Mammals. They are the vertebrates that have skin with hair and suckle their youngs. Most have four legs to walk, a few have fins for swimming, like whales, and a few have wings to fly like bats.

2 . The fishes. They are the aquatic vertebrates that have fins to move, the skin provided with dermic scales (such as sardines) or dermic denticles (as in sharks) or without them (as in the lamprey), they breathe through gills, have a variable internal temperature and they usually reproduce by eggs which do not supporte the desiccation. Fish scales are dermic and are covered with a transparent and mucous skin that outside of the water dries. They are the earliest known vertebrates, appeared in the Paleozoic, around 500 million years ago.

1. External morphology. There are three parts: head, trunk and tail.

• The head. It has two nasal ofirices, mouth and two lidless eyes.

• The trunk. It has four paired fins (the two pectoral fins and the two ventral fins) and 2 unpaired fins (dorsal fin and anal fin). It also presents a line of nerve endings that pick up the vibrations of water (lateral line). The trunk goes from the head to the sewer that is a little open cavity inside which can be differentiated the genital opening, excretory orifice and anal opening or anus.

• The tail. It goes from the serwer to the end, which is where the tail fin is.

 

2. Internal morphology. They have a spinal column or dorsal column (bone or cartilage) that protects the spinal cord. They have a ventral heart with a single atrium and a single ventricle, so the circulation is simple, meaning that the blood giving a full circle around the body passes only once through the heart. Many have a swim bladder taht is able to inflate and deflate (flotation function).

3. Reproduction. The males have two testicles and the females have two ovaries. The majority of the species is oviparous. Both parents release gametes outside, the fertilization occurs in water (external fertilization). After some time and the embryonic development being completed, the egg emerges and the small fish called the fry comes out.

There are also the ovoviviparous species. They have the internal fertilization and the egg remains and hatches inside the mother. For example the gambusia and aquarium fishes "guppy". There are also viviparous species. In them the fertilization is internal and the embryo gets nourishment from the mother via blood. For example blue shark and great hammerhead.

4. Classification. There are three classes:

Classification of the Finshes
Cyclostomes. Are the eel shaped but without jaws, scales and paired fins. The mouth is round with teeth and the skeleton is cartilaginous. Example: the lampreys, are the animals that using mouth stick to other fishes and and bleed them to the death.	Cartilaginous fishes. They have jaws, boneless cartilaginous skeleton, dermic denticles in the skin and the caudal fin with the upper lobe well developed, but have no lids, that is why lateral gill fissures are visible. Examples: white shark, tiger shark and mantas.	Bony fishes. They have jaws, the bony skeleton with fish bones, dermic scales on the skin, caudal fin with two similar lobes and lids that cover gills. Many have an expandable swim bladder that allows them to go up and down buoyancy. Examples: hake, sardine, mackerel scad, carp and trout.

3. The Amphibians. Are the vertebrates that as an adult have four limbs (tetrapods), uncovered, thin and moist skin, that can live outside of the water as they breath through lungs, and which like fishes require water to reproduce because of fertilization process (external fertilization).

1. External morphology. The body of some amphibians such as frogs and toads have only two parts (head and trunk) and others, such as salamanders and newts, have three parts (head, trunk and tail).

• The head. It has two eyes with eyelids (one upper, one lower, which is mobile, and transparent nictitating membrane that covers the entire eyeball when the amphibian is submerged), two lateral tympanic membranes and the mouth with a forked, hurling tongue and very weak and equal teeth.

• The trunk. It has four limbs. The upper ones have four fingers and the lower one have five fingers, which in many species, are united by interdigital mambranes (adaptation for swimming).

2. Internal anatomy. They have two very simple and little effective lungs (pulmonary respiration), it is why they also need to capture the oxygen through its thin and wet skin (cutaneous respiration). In order to keep the skin moist they secrete mucous substances and sometimes, to repel predators, also a toxic one. The heart has two atrias, one that receives blood from the lungs and another that receives blood from the body, and only one ventricle where both are mixed, which means that they present the incomplete and double circulation (double means that the blood while giving a full circle around the body passes through the heart twice, and incomplete means that in the heart, the blood rich in oxygen and the blood poor in oxygen are not completely separated but mix).

3. Reproduction. The fertilization is external (1). The fertilized eggs are deposited in the water and are fixed with mucous substances (2). The eggs have the permeable cover that would dry in the aerial environment. After 2 or 3 months larvae called tadpoles are born, they breathe through gills and do not have limbs so they move using a tail (3). Then, they undergo a metamorphosis: the tail is reduced, the four legs shown, gills disappear (4) and the lungs are formed, which is why the individuals can leave the water (5).

4. Classification. The two main groups of amphibians are:

Classification of the amphibians
Anuras. They have no tail in the adult stage. The second pair of legs is adapted to jump and they are capable of emitting sounds. Are the frogs and toads (they are like frogs but with shorter front legs).	Caudatos. The adults have a long body and a tail. The limbs are short and are adapted to crawl. Examples are the salamandres (black colour with yellow spots) and newts (have a dorsal crest acting as teh fin)