................. KINGDOM METAZOA III. VERTEBRATES II (Reptiles and Birds)

Introduction. The Reptiles evolved about 300 million years ago from amphibians. Thanks to having a very effective pulmonary respiration system they were able to undergo the evolucion towards a fully waterproof skin that allowed them to colonize the dry areas. Internal fertilization and having the eggs with a waterproof shell that protects them from drying out completely, even allowed them to colonize the deserts. They dominated every ecosystem on the planet (sea, land and air) and get to have gigantic sizes, that in the terrestrial environment no other group has ever achieved. The majority of the species suddenly disappeared about 60 million years ago. This fact is related to the fall of a large meteorite that caused volcanic activity. This activity generated a layer of ash that kept arriving solar heat from arriving to the ground and causing a great temperature decrease that wiped out most of the reptiles, because they are animals that cannot control its internal temperature. Other authors have proposed that those reptiles, particularly the large dinosaurs, they could control its internal temperature and that the reason for the termination was the lack of food, as without enough light (as a result of the ash layer) most plants died.

The Birds appeared about 144 million years ago, long after the appearance of the first mammals, they evolved from the currently not existing reptiles, that were able to fly and walk with two legs. This modern vertebrate group has been very successful biologically, especially due to being able to fly which is why they can easily escape from predators and breed in places that are not accessible for non-flying animals. The proof of this is that they are currently the only wild vertebrates who dare to be seen in the wild and even live and breed in our cities without hiding, which the rats and mice of town or urban bats cannot do. In addition, the ability to maintain a constant internal temperature has allowed them to colonize the planet's coldest areas, such as Antarctica (-60ºC in winter), which cannot do variable temperature animals (reptiles, amphibians, insects , mollusks, etc.).

Activities to do. Read the explanations on the general characteristics and classification of reptiles and perform Multiple Choice Test 19.1 and Relate pictures with names 19.1. Then read the text about the birds and perform Multiple Choice Test 19.2, Relate pictures with names 19.2 and Crossword 19.

1 . The Reptiles. Are the vertebrates that have the body covered with corneus epidermic scales (the same substance that forms our nails and our hair), have pulmonary respiration system and reproduce by internal fertilization through eggs with a calcareous envelope (shell) that let them survive in very dry environments. They may have four legs, like lizards, or none of them like snakes. Reptiles scale, unlike the fishes are epidermic, dry and waterproof, allowing them to withstand very dry environments.

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

• The head. It has two nostrils, mouth with teeth (for example lizards and snakes) or with callous beak (as in turtles) and two eyes with lids. Some snakes have teeth with the venomous gland.

• The trunk. It has four legs ended with five seperated fingers (lizards), united by interdigital mambranes (european water turtle) or fins (for example the leatherback sea turtle). There are reptiles that have no limbs (snakes). The limbs are articulated laterally this is why they move through dragging their abdomen on the surface (crawling). They can walk without crawling only for a short period of time. Many ancient reptiles could walk on four legs (for example the diplodocus and the stegosaurus) or on two legs (tyrannosaurus).

• The tail. It goes from the gutter to the end.

1.2. Internal anatomy. The skeleton presents ribs attached to the spinal column and to the sternum, forming a ribcage, but not in snakes because of the lack of sternum. The circulation is similar to that of amphibians (double circulation but incomplete) in all reptiles except in crocodiles, which have two ventricles such as birds and mammals (complete double circulation).

1.3. Reproduction. The males have a copulatory organ or penis. Fertilization is internal. As the internal temperature is variable (poikilothermic) they do not incubate the eggs, but simply bury them. The development is direct, which means it occurs without larval stages.

1.4. Classification. There are four types:

2. The Birds. Are the vertebrates with a pair of wings and a pair of legs, mouth with corneus beak, body covered with feathers, pulmonary respiration system, reproduction through eggs (oviparous) and constant temperature (homeotherms). The feathers are epidermic structures that consist of a basal canyon, one elongated vertebral column and numerous barbs. The functions of feathers are to protect the individual from thermal variations, keep skin wet and enable to flight. We distinguish five types of feathers: rectrices features in the tail, the flight feathers on the wings, coverting feathers on the back and down feathers on the ventral side.

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

• The head. It has mouth like turtles with calloeus beak, ears covered with fearthers and eyes with two lids and a semitransparent, horizontal nictitating membrane. The peak has different forms depending on the type of feeding: carnivore (eagle), insectivores (sparrow), granivores (sparrow and pigeon), omnivore (goose), filter (flamingo) and frugivorous (toucan).

• The trunk. It has four limbs, the upper two are wigs usually adapted to the flight and the lower two are paws with four toes covered with scales like reptiles and adapted to walk (hen), to grasp (falcon) or to swim (duck).

• The tail. It is very short and supports the tail feathers which are responsible for the direction of flight.

2.2. Internal anatomy. The bones are internally empty in order to reduce its weight. The sternum or breastbone is very well developed as it serves as a point of insertion of the large muscles that move the wings. The alimentary canal presents esophagus, crop (it allows to store the food for the little ones, called chicks), muscular stomach or gizzard (inside the food is crushed by previously ingested small stones), glandular stomach, intestine and anus that empties into the cloaca (the cavity into which also flow the excretory system and the reproductive system). The respiratory system has the expansions called air sacs (that make breathing easier and constitute the air reserves). The circulation is double (thr body circuit and lung circuit) and complete (without the mix of oxygenated and deoxygenated blood) due to having two atrias and two ventricles as we have.

2.3. Reproduction. In many species there are acts of courtship prior to the copulation. This is why the males tend to present very bright colors. The females, however, have a little contrasted colors to stay unnoticed during the incubation of the eggs. The copulation is performed by the approach of cloacas. The internal fertilization by simple approximation of the cloacas. Only in a few species, such as ducks, the male has the copulatory organ present (penis). The fertilized egg (commonly known as yolk) is surrounded by a nutrient substance (white part, glair) and protective wrappers (membranes and calcareous shell). All of this form the whole egg. To produce full embryonic development the egg incubation is neccesary (heating), which generally does only the female. After hatching from the egg, in nidifugous species (as present for example, chickens, ducks and partridges) chicks leave the nest, while in the nidicolous species (as present for example the storks, swallows, eagles and sparrows) chicks remain for some time in the nest and are fed by the parents.

2.4 Classification. The main groups of birds are: