Jointed appendages and exoskeleton. Arthropods use jointed appendages as wings, legs, antennae and mouth parts. Arthropods also have a ridged external skeleton known as a exoskeleton. The exoskeleton, made of chitin, provides places for muscle attachments. There is one problem with chitin. It cannot support great weight and as a result it must be thicker in large insects so there is a limit to how big an arthropod can be. Most arthropods are about a millimeter in length, although larger ones may be seen.

Other characteristics of arthropods

There are numerous subphyla and classes of arthropods. One class includes the horseshoe crab, another includes the centipedes, another the millipedes, another their arachnids, and the insects. One should remember that the spiders which are classified as arachnids have four pairs of appendages. The class Insecta includes approximately a quintillion (a billion) members. There are two hundred million insects for each human being. Distinguishing characteristics of insects include the following:

1. The body is divided into a head, thorax, and abdomen.
2. Two pairs of wings are usually present.
3. The thorax usually bares three pairs of legs which may be modified for walking, jumping, swimming, digging, or grasping.
4. Presence of compound eyes composed of many individual units which provide a large visual field.
5. Presence of antennae.

One feature that sets the insects apart from all other invertebrates and most vertebrates is their ability to fly. This ability is made possible by the following characteristics of insects (Ref. Pechenik ):

1. Abundance of strained muscle specialized for rapid, strong contractions;
2. muscle antagonism by means of a lightweight joined skeleton, permitting a great amount of movement to be generated from relatively short changes in muscle length;
3. small body size;
4. water-impermeable outer body covering, preventing dehydration;
5. efficient systems for gas exchange, nutrient storage, and distribution of nutrients to the musculature; and
6. highly developed nervous and sensory systems for steering, navigating, and sensing wind direction".

The insect nervous system consists of a brain and a ventral nerve cord two which are attached numerous nerves throughout the body. Insects may undergo complete or incomplete metamorphosis.

Deuterostome development and endoskeleton. There are two major kinds of coelomate animals. The way the anus and mouth develop determines whether they are protostomes or deuterostomes. Without going into detail, in protostomes positional information is the key to the developmental fate of embryo cells while in deutoerstomes groups of cells move around during development to form new tissue associations. The term echinoderm means spiny skin and refers to an endoskeleton composed of hard plates just beneath the delicate skin. In adults the endoskeleton forms a hard shell around the animal.

Other characteristics of echinoderms:

It should be noted that all echinoderms are bilaterally symmetrical and larvae, but radially symmetrical as adults. Adult echinoderms have a five part body such as is seen in a starfish. The nervous system of echinoderms consists of a central ring of nerves from which five branches arise. Although there is no brain, echinoderms are capable of complex response patterns. Echinoderms have a water vascular system which aids in their movement. This system is composed of a central ring canal from which five radial canals extend into the arms. From each radial canal tiny vessels extend into thousands of hollow tube feet. At the base of each tube foot is a fluid filled muscular sack that acts as a valve. When a sack contracts its water is prevented from re-entering the radial canal and is forced into the tube foot, extending it and permitting it to attach to the ocean bottom, pull against the tube feet and move across the ocean floor.

References

1. Brusca, R. C. and Brusca, G. J.: Invertebrates, 1990. Sinauer Associates, Inc., Sunderland, MA.
2. Johnson, G. B.: The Living World, 2^nd Edition, 2000. McGraw-Hill Co., Inc., New York
3. Pechenik, J. A.: Biology of the Invertebrates, Ed. 3, 1996. McGraw-Hill Companies, Inc., New York.
4. Ruppert, E.E. and Barnes, R. D.: Invertebrate Zoology, 1994. Saunders College Publishing, New York.

Phylum Chordata

1. Notochord - a long, stiff rod beneath the nerve cord;
2. Nerve cord - Cord to which the nerves that reach the various parts of the body are attached
3. Pharyngeal gill slits - series of slits behind the mouth that extend into the pharynx
4. Post-anal Tail - Tail extending behind anus. It should be noted that all chordates have all of these characteristics at some time of their life. For many chordates, these characteristics are present only during early embryonic development.

Other Characteristics

Which chordates may be vertebrates or invertebrates the large majority are vertebrates. In these animals the notochord becomes replaced by a backbone. Basic characteristics of the various classes of vertebrates will be considered below.

Fishes

There are numerous classes of fishes. Of the nearly 20,000 living species of fishes 18,000 species are bony fishes which comprise the class Osteichthyes. They have a skeleton of real bone like humans. Goldfishes, guppies, tunas, trout, bluegills, bass, catfish, sailfish, eels, cods are all bony fishes. Other characteristics of this group of fishes include the following:

1. Skin with mucous glands and embedded dermal scales which toughen and protect the skin.
2. Possess fins which help a fish swim and steer: dorsal fin on top and anal fin on bottom help to stabilize fish. Fish also have 2 sets of paired fins on side. The paired fins behind the head are the pectoral fins and the fins in front of the anal fin are the pelvic fins. Pectoral and pelvic fins
3. Gills enable fish to breathe underwater by filtering oxygen from water.
4. Presence of a swim bladder which is a float that helps fish rise, sink or stay in place. Swim bladders are basically gas bags and work like hot-air balloons. These bladders are inflated with oxygen from the water or by gulping air at the surface and they help the fish rise in the water (expansion with gas) or sink in water (bag shrinks). (Note: fishes without swim bladders such as sharks must move continuously or they will shrink.)
5. Mouth is terminal with many teeth and jaws are present. Olfactory sacs are paired.
6. Circulation consists of a 2 chambered heart with arterial and venous systems and four pairs of aortic arches.
7. Nervous system consists of brain with small olfactory lobes and cerebellum and 10 pairs of cranial nerves.
8. Sexes are separate and fertilization of eggs occurs externally in water and temperature of the water stimulates reproduction.

Shape - is one of those adaptations of fish that have evolved that determine where fish live and what their life type entails. For example, streamlined fish like torpedo-shaped fish ship through water more easily than bulky fish. These fish live in open water far out at see and are long-distance travelers, swimming many thousands of miles a year. They are usually predators and chase down their prey. Butterfly fish have thin, flat bodies that let them slide between aquatic plants.

What do fish eat? Depending on the species, they eat phytoplankton or other animals (other fish, insects, crustaceans, clams, and worms). Adaptation - teeth of fish are adapted to their diet. Sharks (not a bony fish but a cartilaginous fish) are described as super predators because of their keen senses. They can smell for more than one mile in distance, hear for 1000 feet, detect objects 100 feet away and see objects 50 feet away.

Predators for fishes are jellyfish, garter snakes, herons, eagles, otters, sea lions, bears, and humans. Adaptation such as color, shape and behavior keep fish from being eaten.


Class Amphibia

There are 3 orders of amphibians. Frogs and toads comprise the order with the largest number of living species. Other well known amphibians - salamanders and newts-are found in another order. The term amphibia means "both lives" and refers to the fact that these animals straddle the boundary between an aquatic and terrestrial existence. The larval forms live in water and the adult live on land. The change from a larval form to an adult is known as metamorphosis and entails loss of gills and development of lungs. (Some amphibians such as certain salamanders keep their gills and do not develop lungs). The lungs of many amphibians are poorly developed and they "supplement" breathing with their lungs with "breathing" through their skin which is coated with mucus. Most amphibians need to keep their skin from drying out and therefore live in damp places or have adaptations to prevent this. For example, some frogs that live in dry places and spend six months hiding more than three feet beneath the ground.

Other characteristics of amphibians include:

1. Sexes are separate and while fertilization is mostly internal in salamanders, it is mostly external in frogs and toads. External fertilization must occur in water so sperm can swim to eggs.
2. Skeleton mostly bony with various numbers of vertebrae.
3. Body forms vary greatly from the compact tailless bodies of frogs with large heads fused to the trunk and rear limbs specialized for jumping to the slender bodies of salamanders with a long tail and 4 limbs at right angles to the body.
4. Usually 4 limbs although some are legless.
5. Three-chambered (2 atria and one ventricle) and double circulation through the heart.
6. Exothermic
7. Nervous system includes ten pairs of cranial nerves.
8. Excretory system of paired kidneys with urea being the main nitrogenous waste.

What do amphibians eat and what eats them? Generally, they eat insects, other arthropods and earthworms, although larger ones eat larger animals. They, in turn, are generally eaten by reptiles or large birds.


Class Reptilia

Characteristics of reptiles include:

1. Tough scaly skin or hardened outer coat that resists water loss and protects body. Turtles and tortoises have shells, snakes and lizards have scales, and crocodiles and alligators have bony plates.
2. Internal fertilization
3. Amniotic egg (has 3 membranes that protect embryo) although some bear young alive.
4. Exothermic (body temperature dependent on how warm or cold surroundings are), although they have adaptations to deal with this.
5. Limbs paired usually with 5 toes although absent in snakes and some lizards. 6. Respiration is accomplished by lungs.
7. All but crocodiles have 3-chambered heart. There are separate systemic and pulmonary systems.
8. Paired metanephric kidneys with uric acid as the main nitrogenous waste. 9. Nervous system with 12 pairs of cranial nerves.

What do reptiles eat? In general, lizards eat insects or plants. They are eaten by larger reptiles, birds, and small mammals and destroyed by man. Snakes are carnivores and eat worms, insects, amphibians, birds and small mammals, except for larger ones which eat larger prey. Predators for snakes include a variety of birds like hawks, eagles and owls. Turtles, which usually live in or near water, eat both plants and animals (fish, clams, jellyfish), while land-dwelling tortoises eat mostly plants and some insects. Baby turtles may be eaten by birds and fish but adults are well protected against mose enemies, except for big sharks and humans. Predators for young tortoises and tortoise eggs include raccoons and foxes. Man is the predator for larger tortoises, crocodiles and alligators eat fish, frogs, birds and small mammals such as rodents. Larger ones may eat larger mammals which come to the water to drink. Man is the predator of crocodiles and alligators.


Class Aves

Characteristics of this class include:

1. Limbs are paired with the forelimbs modified for flying.
2. Body covered with feathers and leg scales.
3. Nervous system well developed with brain and 12 pairs of cranial nerves.
4. Four-chambered heart and red blood cells comprise part of circulatory system. 5. Endothermic
6. Respiration by lungs which contain tubelike parabronchi through which air flows continuously. Also have extensive system of air sacs which are converted to the lungs in such a way that a larger portion of the inspired air bypasses lungs and flows directly into air sacs in thorax, abdomen, and bones. This system helps birds meet the high metabolic demands of flight.
7. Excretory system of metanephric kidney, no bladder and ureters open into cloaca.
8. Fertilization is internal and amniotic eggs have yolk and hard calcerous shells.


Class Mammalia

Characteristics of this class include:

1. Warm blooded (endothermic)
2. Brains highly complex with neocerebrum, and 12 pairs of cranial nerves.
3. Possess molars which enable them to grind up their food before swallowing.
4. Young grow inside uterus
5. Possess mammary glands and integument with sebaceous, scent and sweat glands
6. Covered with body hair.
7. Respiratory system of lungs with alveoli; larynx and muscular diaphragm present.
8. Excretory system of kidneys and ureters that usually open into a bladder
9. Circulatory system of four-chambered heart and red blood cells

References

1. Hickman, C.P., Roberts, L.S., and Larson, A.: Animal Diversity, 2^nd Edition, 2000. McGraw-Hill Company, Inc., New York.

2. Johnson, G. B.: The Living World, 2^nd Edition, 2000. McGraw-Hill Co., Inc., New York.

Click here to go to the Fall 1999 Page!