Phylum Chordata

1.    INTRODUCTION TO PHYLUM

Phylum Chordata (Refer to Classification at end of Lecture)

This phylum is the most advanced phylum in the animal kingdom and includes man. The phylum is characterised by possessing a simple skeletal rod down the back of the individual known as the notochord. Members of this phylum also possess a single hollow dorsal nerve cord and a solid segmented portion of the body posterior to the anus called the tail.

• THE DIVISIONS OF CHORDATA

The chordata can be divided into two sub-phyla:

• The Acraniata – these are those chordates possessing no skull, jaws or backbone; and
  • The Craniata – these possess a skull, jaw and a backbone.

SUB PHYLUM ACRANIATA

This sub-phylum contains only small marine animals they are all species of Amphioxus. These organisms have no brains, skull or cartilaginous or bony skeleton.

SUB PHYLUM CRANIATA

This sub-phylum is also known as the vertebrata or vertebrates. This is the more important sub- phylum of the two, and contains fish amphibians, reptiles, birds and mammals. The vertebrates differ from other chordates in that they posses a well developed brain encased in well-developed skull. They also possess a vertebral column or backbone in most cases. They also possess a skeleton of cartilage or bone. In some texts, the word vertebrate is used as a synonym for chordate, however, this is incorrect.

In most vertebrates, the backbone is made up of a series of bones called vertebrae; these are hollow and form a long hollow rod attached to the skull. The Craniata can be divided into two branches:

• Agnartha; and
  • Gnarthostomata.

The former possesse skulls but no jaws, while the latter possesse skulls and jaws.

The Agnartha are a very small group of aquatic organisms. Most important vertebrates belong to the branch Gnarthostomata.

GNARTHOSTOMATA

The animals in this branch can be further divided into two branches. Firstly those animals in which the embryo develops in an amniotic sac and secondly those animals which do not undergo embryological development in an amniotic sac. The former division is known as Amniota and the latter is known as Anamnia.

DIVISION – ANAMNIA

This division is divided into two classes:

• Pisces – these are the fish; and
  • Amphibia which include frogs.

CLASS PISCES

These are the fish – they are all aquatic and their bodies are covered in scales. They all possess gills which filter oxygen out of the water in which they live. The gills are situated on either side of the neck just behind the head. For the purpose of this study, Pisces can be accepted as a class on its own. However, zoologists now believe that the fish are divided up into five distinct classes of vertebrates. Fish do not possess lungs.

CLASS AMPHIBIA

These organisms are the frogs, toads, newts and salamanders. They possess limbs and these are pentadactylate (5 fingers or toes). Fish only possess fins. The amphibia are egg-laying, but the eggs are fertilized out of the body and are not covered by a shell. The amphibia dwell on land but have to return to the water for spawning, i.e. egg-laying.

DIVISION AMNIOTA

This division is divided into three classes:

• Reptilia;
  • Mammalia; and
  • Aves.

CLASS REPTILIA

It is believed that the reptilia are direct descendents from the amphibia. Reptiles differ from the amphibia in that the skin of reptiles is horny and the eggs are enclosed in a shell. Reptiles together with the amphibia are cold-blooded animals. They rely on their environment to maintain a viable body temperature. Their activity also depends upon ambient temperature. Most reptiles live on the land, although some have adapted to an aquatic environment. This class includes snakes, lizards, crocodiles and some extinct species.

CLASS AVES

These organisms are commonly known as birds. They are warm blooded, i.e. they maintain their  own body temperature and they possess feathers. The forelimbs are developed into wings and most birds fly; there are, however, notable exceptions, such as ostriches.

CLASS MAMMALIA

These organisms are the most advanced of all vertebrates. Mammals are characterised by a number of features, including that:

• they possess mammary glands – these are glands specialised for the secretion of milk, for feeding their young;
  • they maintain their body temperature at +-37° C – the exact temperature depends on the species concerned; and
  • they are also covered with hair and possess an advanced nervous system. Mammals can be divided into three sub-classes:
  • Monotremata;
  • Metatheria; and
  • Eutheria.

Monotremata

This sub-class of mammals shows very apparent similarities with reptiles. They possess beaks like birds and they lay eggs. They feed their young on a milk-like fluid secreted by cells on the underside of the body. There are only two species; namely, the duck (billed Platypus) and the Spring ant eater, both of these are Australian.

Metatheria

These animals are commonly known as Marsupials; they have a very primitive uterus. The young are born very prematurely, and are then placed in an abdominal pouch, where they attach themselves to a teat until they are mature enough to venture away from the pouch. The majority of these animals are found in Australia, and they include the kangaroo, the Koala Bear and the Tasmanian Wolf.

Eutheria

The Eutheria are the higher mammals, they include the bulk of the mammals. They are characterised by having a well-developed uterus and the growing fetus is attached to the uterus wall by a placenta. Examples are cats, dogs, cattle, apes, man and numerous others.

• THE DEVELOPMENT OF MAMMALS

Mammals are descended from reptiles. Fossil records have revealed that millions of years ago, an animal evolved called the Cynagnathus. This animal is believed to have been the precursor for the mammals of today. Scientists believe this animal was warm-blooded. It has been described as a cross between a lizard and a dog. The Marsupial mammals are older than the Eutherian mammals, which may have descended from them. The Eutherian mammals have more chance of surviving than Marsupials, because they are better adapted to their environment.

• ANIMAL PARASITES

Parasites have already been discussed in the Botany Course, and specific animal parasites have been examined in this Course. Parasites live on or in another animal, causing a detrimental effect. They generally have poorly developed sensory and digestive systems: Many parasites survive by absorbing nutrients and oxygen through their skin. Thus their circulatory system is very simple or even absent.

Some parasites, however, live on the outside of the host and survive in most cases by sucking the

hosts blood or body fluids. These parasites have modified mouth parts for this purpose. They are called ectoparasites. Below are listed some examples of Endoparasites and Ectoparasites.

Table 1: Endoparasites and Ectoparasites

Endoparasites	Host	Ectoparasites	Host
Liver Fluke	Sheep / Cattle	Ticks	Mammals
Trypanosomes	Cattle	Mites	Mammals
Roundworms	Mammals	Leaches	Mammals
Tapeworms	Mammals & Birds	Flies	Mammals

Figure 1: Classification of the Animal Kingdom

PHYLUM                   CHORDATA

SUB-PHYLUM         ACRANIATA              CRANIATA

BRANCHES                                                     AGNARTHA              GNARTHOSTOMATA

DIVISIONS                                                                             ANAMNIA                              AMNIOTA

CLASS                                         PICES                       AMPHIBIA            AVES REPTILIA MAMMALIA

SUB-CLASS                                                                                                                                 MONOTREMATA

METATHERIA EUTHERIA