8.27.2 Lesson: Chordates

Chordates

Objective:

H.1 – Identify the characteristics of Kingdom Animalia and its various phyla.

Vocabulary

Phylum Chordata Characteristics

The purpose of taxonomic hierarchy is to distinguish organisms from one another. Recall that the levels of the system become more specific when moving down the hierarchy. Thus, the levels of domain and kingdom are the most inclusive groups, while the levels of genus and species are the most specific groups and are used to attribute a scientific name to a distinct organism.

When we look at Kingdom Animalia, we can begin distinguishing the animals from one another by moving down the taxonomic hierarchy. The next level below kingdom is phylum. There are 36 total recognized animal phyla. The animals within each of these groups share the same general body plan when observing internal and external appearances.

Today, we will focus on Phylum Chordata, which is defined by extreme diversity. Members range from the eel-like lancet, which is only a couple of inches long, to human beings. How can humans be grouped with eels? The first thing that all chordates share is the notochord. The notochord is a rod-shaped group of cells that extends down the dorsal side of an organism. It is located underneath the nerve cord. In some chordates, the notochord is present only in the embryonic forms; for other chordates, the notochord is present in the adult forms.

Another characteristic that chordates share is the presence of a dorsal nerve cord. The dorsal nerve cord is the early central nervous system, which is made up of the brain and spinal cord.

A third shared trait of chordates is called the pharyngeal slit. These slits, or pouches, are small openings in the pharynx that are present in embryonic forms and remain in only a few adult forms.

Before moving onto the next taxonomic rank of class, chordates can be separated into three sub-phyla: Cephalochordata, Urochordata and Vertebrata.

Subphylum Cephalochordata

All known organisms in this category live in the ocean and are rather simple in structure. They are typically small in size and retain their notochord and pharyngeal slits for their entire lives. The pharyngeal slits become gill slits and are used to gain nutrients through filter-feeding. One such member is the lancet (see photo below).

The genders of cephalochordates are separate. Males produce sperm and females produce eggs. When the sperm and eggs are released, fertilization occurs in the water, external from the two individuals.

Subphylum Urochordata

Urochordates look more like sponges in appearance but meet the shared characteristics of other chordates. Unlike the cephalochordates, members in this subphylum lose their notochord and dorsal nerve cord. Pharyngeal slits, however, are kept into adulthood. An example of one such animal is the bluebell tunicate, also known as the sea squirt.

Tunicates, like all animals, reproduce sexually. Eggs and sperm are released into the water. Upon fertilization, eggs develop into free-swimming larvae that resemble tadpoles in appearance. The adult form of the organism is sessile.

Subphylum Vertebrata

You are probably most familiar with subphylum vertebrata. All vertebrates share the same qualifying characteristics of other chordates: notochord, dorsal nerve cord, and pharyngeal slits; however, organisms in this subphylum are distinguished from other members in phylum Chordata by the development of a backbone. The notochord develops into the vertebral column or spine and is made up of a series of bony vertebrae that are separated by discs. Vertebrates share the same basic body plan: the vertebral column, which is much like a stiff rod, runs through the length of the animal alongside of a hollow tube of nervous tissue known as the spinal cord.

Vertebrates also have two informal classifications: endotherms and exotherms. Endothermic organisms are generally thought of as warm-blooded, but more specifically, they are animals capable of regulating their own body temperature. This requires much energy. Humans, mammals, and birds fall within this classification. Ectotherms, on the other hand, are considered cold-blooded. This means that the animal cannot regulate its own body temperature, and instead, depends on other sources to maintain its body heat. Ectothermic animals include reptiles, fish, and amphibians.

Movement

Vertebrates utilize two different divisions of support for movement: the axial skeleton and the appendicular skeleton. The axial skeleton includes the skull, thorax (sternum and ribs), and the vertebral column. It is the central core of the body and serves to house and protect vital organs. Additionally, the axial skeleton functions as a surface of attachment for muscles.

The appendicular skeleton is comprised of the remainder of bones in the body, which are girdles and limbs. The girdles are attachment points for the limbs. Bones in the appendicular system include those that make up the shoulder, arms, forearms, hands, pelvis, legs, and feet.

Circulation

The circulatory system of vertebrates is closed and includes a heart and system of vessels. Closed circulation involves a system that is essentially one loop. Blood is pumped by the heart and is contained within vessels at all times as it travels through the body. The blood does not fill body cavities. Circulation is unidirectional, meaning it travels only in one direction, which is from the heart and through the circulatory route before returning back to the heart.

The heart of vertebrates has two to four chambers depending on the organism. Humans have a four-chambered heart. The heart pumps oxygenated blood through a network of blood vessels, delivering oxygen and nutrients to the body’s cells. Deoxygenated blood then returns back to the heart.

The three main types of blood vessels include arteries, capillaries and veins. Arteries carry blood away from the heart and into the rest of the body. Capillaries carry blood from the arteries into tissues. Veins, on the other hand, carry blood from the tissues back to the heart.

Nutrition

Vertebrates can be differentiated based on the method used to obtain nutrients.

• Carnivorous
• Carnivores prey on other animals. They have a variety of methods for hunting: claws, sharp teeth. or even sticky tongues.
• Examples include lions, sharks, and frogs.
• Herbivorous
• Herbivores eat only plants. Recall that plants contain cellulose in their cell walls, which is very difficult to digest. The digestive systems of these organisms have adapted to breaking down cellulose with the presence of bacteria living in their digestive tracts.
• Examples include deer, cows, and horses.
• Omnivorous
• Omnivores eat a combination of plants and other animals. Teeth are mixed in design in order to chew plants and rip meat.
• Examples include bears, pigs, and rats.

Reproduction

The genders of vertebrates are separate. Male have two testes that produce sperm cells and females have two ovaries that produce eggs. Fertilization can occur externally when the eggs and sperm are released into water, or it can take place internally when the male inserts the sperm inside of the female’s body.

Development of Young

Animals can also be grouped based on birthing methods.

• Oviparous
• Oviparous organisms lay eggs. After internal fertilization, a protective coating or shell develops. The female can then lay the egg(s) and the offspring will continue to develop outside of the mother’s body and within the egg before hatching occurs.
• Birds are an example of this group.
• Viviparous
• Viviparous organisms give birth to live young that have developed in the mother’s uterus or equivalent structure. They receive nutrients from the mother during development.
• Humans are an example of this group.
• Ovoviviparous
• Ovoviviparous organisms give birth to live young, however, the offspring does not receive nutrients from the mother during development. Rather, the shelled fertilized egg stays in the mother, where it hatches, and then the young are born.
• Garter snakes are an example of this group.

Nervous System

The nervous system of vertebrates is relatively complex. It is responsible for controlling and coordinating the body’s actions through the transmission of signals. The brain, spinal cord, nerve cells, and sensory organs are all involved in making this happen. The typical brain has five lobes that each serve a specific function: olfactory (smell), cerebrum (voluntary movement- movement that the organism controls), optic (sight), cerebellum (muscle movement and involuntary movement- movement that occurs naturally, such as breathing), and the medulla oblongata (reflexes).