I. Chordates/Vertebrates

For the most part, this chapter (and the rest of the year) deals with animals with a vertebral column ("backbone"). However, there are a few exceptions as the actual phylum is called the Chordata. These animals all have a central nervous system organized around a spinal cord (thus "Chordates"). All chordates meet the following qualifications in their embryonic stage:

1. Notochord (a stiff yet flexible rod running along the dorsal aspect of embryo)

2. Tubular dorsal nerve cord (spinal chord in humans)

3. Slits in walls of pharynx

4. Embryonic tail

II. Chordate Classification

Starr and Taggart mention that the chordates are often divided into three subphyla: Tunicates, Lancets, and Vertebrates. We will briefly cover the tunicates and lancets, and then spend most of the chapter on vertebrates.

A. Tunicates: An invertebrate chordate

As adults, these animals are my favorite things to see while scuba diving. They come in brilliant colors and look like thin-walled hearts. Surprisingly, as larvae they actually look like fish, complete with nerve cord and notochord. Thus, they are considered to be one of the first chordates. When they reach adult phase, they attach to a surface (becoming sessile) and begin breathing through pharyngeal gill slits and leading an life that seems more like that of a cnidarian.

B. Lancelets: The almost fish (another invert chordate)

Look at the picture on page 457. The lancelets have a knife-like appearance and meet all four characteristics of a chordate. They have a closed circulatory system, no lungs/gills as oxygen diffuses across their cell membranes and they are filter-feeding animals.

C. Vertebrates: Encased nerve cord

The rest of the chapter will deal with the major vertebrate groups: fishes, amphibians, reptiles, birds and mammals.

D. Evolutionary Trends

As we study these groups, keep evolutionary trends in mind. The reason a group (e.g. the amphibians) has done well is because of their unique adaptations to a given environment. Overall, we can observe a few key trends in vertebrate anatomy/physiology:

1. Circulation: The basic open circulatory system, closed single-loop/two-chambered heart, three-chambered/four-chambered heart as seen in crocodilians, and the double-loop/four-chambered heart seen in birds and mammals.

2. Feeding: The basic filter feeder (lancelets), to the developed jaws and teeth of later herbivores and carnivores.

3. Breathing: Simple diffusion (lancelets) to gills to lungs. All of these are means of using an organ with a great amount of surface area to increase the diffusion of oxygen into the body tissue.

 

III. The Fishes

Life began in the ocean, and fish represent the first animals with a true "backbone" (actually, many bones or cartilaginous pieces). The first fish wouldn’t look like any we see in the classroom. In fact, the fish we have in class are the most complex of all fish (bony fish). Four groups of fish are recognized: Jawless (currently represented by hagfish and lampreys), Jawed plated fish (placoderms), jawed fish with cartilaginous skeletons (sharks, skates, rays, paddlefish), and bony fish (most common today).

In fish, a number of unique adaptations are evident. First, distinct bone/cartilage segments evolved to protect the nerve cord. Second, to obtain oxygen fish use gills, which are connected to the two-chambered heart and closed circulatory system. In many fish, the "lung" tissue has been modified to become a swim bladder, used to adjust buoyancy. Finally, the jawed fish show modification of bones formally used to support gill structures to become bones of the jaw.

The existing jawed fish show the following traits: scales, jaws, bones or cartilage, swim bladder (often), paired fins, and gills.

IV.Amphibians

Amphibians are adapted to life in the water (early in life) and life on land (adult stage). Examples include salamanders, frogs, toads, and caecilians. They all go through amazing transformations, such as tadpole to frog. Tadpoles are fully adapted to life in the water: they breathe with gills and use flattened tails to swim. As adult frogs, they live a terrestrial lifestyle but are still reliant on water to keep skin moist (helps respiration) and for laying eggs. Adult frogs have lungs and more efficient circulatory systems (but still need moist skin to help respiration).

IV. Reptiles

Reptiles were the first vertebrates to be fully adapted to life on land. Reptiles, such as crocodiles, snakes, lizards and turtles, show four major adaptations for life on land:

1. Dry, scaly skin. Reptiles have a strong respiratory system that allows them to develop a skin expressly for the purpose of conserving water. Amphibians require moist skin to increase gas diffusion, but with efficient lungs and circulatory system, reptiles developed water-conserving skin.

2. Internal fertilization. Fish and some amphibians require standing water as their sperm swim only in an aquatic medium. As reptiles no longer live in water, they have internal fertilization to allow sperm to be delivered directly to the female without a water medium.

3. Fish produce ammonia-based urine. When organisms break down proteins (the amino group is NH3), nitrogenous waste is produced. Ammonia is very toxic, so fish dilute it with a great deal of water. For reptiles, this would involve excessive use of water. Their kidneys produce urea-based urine, which doesn’t completely break down the proteins but it does conserve water.

4. Amniotic Egg. Unlike the egg of a fish or amphibian, the amniotic egg contains all of the following: water-retentive shell, embryo, stored water and stored food. Bird eggs are also amniotic eggs. The advent of the amniotic egg allowed for eggs and nests away from the water.

Finally, while not a defining characteristic of the class, reptilian brains show great development, including the first defined cerebral cortex (area of human brain largely associated with higher-level thinking). I read an article once that suggested dreaming started with reptiles: the reason for dreams (or, according to some theories, the cause) is the complex brain, and this is first seen in reptiles. Reptiles, for example, display REM sleep.

The major groups of reptiles include snakes, crocodilians, turtles, and lizards. The crocodilians display remarkable adaptations, including maternal care of the young and a functional, four-chambered heart (when a valve between the atria is closed). Thus, a functional double-loop circulatory system is seen for the first time in the reptiles.

 

V. Birds (Aves)

The class Aves is made up of modern birds and, possibly, dinosaurs. At present, Aves are characterized by the presence of feathers. It should be noted that the dinosaurs, long considered the great reptiles, have been discovered with feathers, creating disagreement on how to classify the dinosaurs. Although feathers define birds, they also share the following characteristics:

1. Four-chambered heart with double-loop circulation. Flight is extremely taxing in terms of energy use and therefore birds need efficient respiration/circulation.

2. Usually, birds display hollow long bones.

3. The first true "warm-blooded" animal. Actually, biologists prefer the term endotherm (heated from within to maintain a constant temperature) as cold-blooded creatures reach the temperature of their surrounding (ectotherms) and can be warmer-blooded than mammals or birds.

VI. Mammals

This group includes humans along with an amazing variety of other organisms, such as whales, duckbilled platypus, bats, walruses, lions, tigers and bears. For the most part, you know this group well:

1. Hair. Mammals produce hair (sometimes only whiskers).

2. Mammary glands. Only mammals produce milk.

3. Extremely development of brain. The highest level of cerebral development is seen in mammals.

4. Complex behaviors. Along with complex brains, mammals display very complex behaviors.

Within the class Mammalia, there are three living orders: Monotremes (egg-laying mammals), Marsupials (young develop within a pouch) and Eutherians (placental mammals).

Monotremes – Only two species of monotremes exist, the spiny anteater and the platypus. Both live in Australia/New Guinea and only exist in that region because the area is an island community and therefore has had an isolated gene pool for 150 million years (geographic isolation, or allopatric speciation). These animals are unique in many ways: the females don’t have a nipple, but rather release milk from a skin gland as if they ‘sweat’ milk. Also, they have a leathery bill and webbed feet, making them look duck-like (convergent evolution, two distinct lines converging upon a common adaptation).

Marsupials – Some mammals help young develop by allowing them to develop within a pouch. This allows the embryo to develop with set temperatures and with considerable safety.

Eutherians – Placental mammals are the most successful group of mammals in recent times. The placenta is tissue linking the maternal and fetal organisms; oxygen/CO2, food, etc are exchanged between the two.

VII. Geologic Ages

       a. Paleozoic (Ancient Age): No/Little Oxygen; Inverts and Fish

 

       b. Mesozoic (Middle Age): Dinosaurs, Ferns, Few Mammals

 

       c. Cenozoic (Current Age): Age of mammals (and insects)