Invertebrate Classification and

Relationships

Introduction

• One million animal species have been described and named so far.

• 4 to 10 million animal species awaits discovery and description.

• First animals may have evolved 3 billion years ago. Earth is 4.5 billion years old

• First metazoan animal similar to the present day animals appear during the Cambrian Period

• Cambrian Explosion- the sudden appearance and diversification of complex animals over some 550 million years ago.

• Before we can consider the evolutionary interrelationships among different groups of organisms we must sort the millions of animals species into categories.

• Sorting out by means of their similarity and differences

Classification by Cell Number, Embryonic Patterns and Body Symmetry

• Single celled or unicellular• Multicellular or many celled- Metazoans• True metazoans are multicellular, diploid

organisms, develops into blastula • Widely agreed that the earliest

invertebrates are unicellular and multicellularity was an evolutionary innovation

General body Form

• Bilateral Symmetry-possessing right and left sides that are approximate mirror images of each other

• Bilateral symmetry is highly correlated with cephalization- concentration of the nervous and sensory tissues and organs at one end of the animal resulting in distinct anterior and posterior ends.

Radial and Bilateral Symmetry

General body Form ( cont )

• Radial symmetry- organism can be divided into two approximately equal halves by any cut that passes through the center of the animal.

• Assymetrical- possessing no symmetry at all, that is the animal cannot be equally divided

Classification by Developmental Pattern

• Based on number of Germ layers formed during embryogenesis

• Germ layers- group of cells behaving as a unit during the early stages of embryonic development and giving rise to distinctly different tissue and or organ systems in the adult.

• Diploblastic- 2 germ layers; ectoderm and endoderm

• Triploblastic- 3 germ layers; ectoderm, endoderm and mesoderm- always at the middle of ectoderm and endoderm

• Ectoderm outer layer- give rise to the skin• Endoderm – inner layer give rise to the

internal organs

Diploblastic and Triploblastic Layer

Based on presence or absence of Coelom

1. Acoelomic- without body cavity, region lying between the outer body wall and gut is solid • 2. Pseudocoelomic- not true body cavity; region

between the outer body wall and the gut is a fluid filled cavity

• 3. Eucoelomic- true body cavity; an internal fluid filled body cavity lying between the gut and outer body wall musculature and lined with tissue derived from embryonic mesoderm

Eucoelomic Animal

Classification of Animal Based From Coelom Formation

• Schizocoely-coelom formation occurs by gradual enlargement of a split in the mesoderm; present among protostomes

• Enterocoely- coelom is formed from the evagination of the archenteron into the blastocoel of the embryo; present among deuterostomes

• Whether the coelom is formed by enterocoely or schizocoely the end result is similar

Types of Coelom Formation

Depending on Mouth Formation( Stomes )

1. Protostomes ( first mouth )- mouth forms from the blastophore

- number of coelomic cavities formed is highly variable

2. Deuterostomes ( second mouth )- mouth arises away from the embryonic blastophore

- number of coelomic cavities divides into 3 coelomic pouches

Types of Cleavage

• Radial- the spindles of a given cell and the cleavage planes are oriented either parallel or perpendicular to the animal-vegetal axis

• Daughter cells derived from a division in which the cleavage plane is parallel to the animal –vegetal axis ends up lying in the same plane as the original mother cell

• Two daughter cell resulting from a division perpendicular to the animal-vegetal axis come to lie directly one atop the other with the center of the upper cell directly over the center of the underlying cell

Types of Cleavage

• Spiral- the spindle axes of cells are oriented at 45 degrees angles to the animal-vegetal axis; the division line may not pass through the center of the dividing cell

• As a result the eight cell stage consists of micromeres –group of smaller cells lying in the spaces between the underlying macromeres- larger cells

• Cell division continues in this fashion , with the cleavage planes oblique to the polar axis of the embryo

Radial and Spiral Cleavage

Indeterminate and Determinate Cleavage

Fate of Cells with Respect to Cleaving

Embryos • In Deuterostomes, one can separate the cells

of a two-cell or four cell embryo and each cell will typically develop into small but complete and fully functional animal- indeterminate or regulative cleavage.

• In Protostomes- developmental potential of each cell is irrevocably determined at the first cleavage- determinate or mosaic cleavage

• Protostomes never produce identical twins

• Protostomes –much of the mesodermal tissue derives from a single cell of the 64 –cell embryo, located at the edge of the blastopore

• Deuterostomes produces embryo from the walls of the archenteron

• Polar lobe- produced by some protostome only – conspicuous bulge of cytoplasm that forms prior to cell division.The lobe contains no nuclear material

Carolus Linnaeus, father of Taxonomy, gave the binomial system of naming organisms. The first part of the system is the genus where the species belongs and the second part refers to one species within the genus.

Hierarchical Classification

• Beyond the grouping of organism within the genera, taxonomy extends into broader categories. Beyond the genus is the family, order, classes, phyla kingdoms and domains.

Scientist classifies organisms by getting the general similarities of the organisms. Next, scientists gets more specific and identify more detailed similarities. This classifies the phylum. More detailed similarities are identified and so on.

Classification and Phylogeny

• Systematics has some other goals beyond classifying organisms. By comparing the similarities of various organism, Scientists manage to trace the evolutionary history of a species, which is phylogeny. Through hierarchical classification of several species, it can form a phylogenetic tree. This can be based on fossil records, homologous structure, comparison of DNA and cladistic analysis

Sorting through Homology

• This is classification by looking at the same structures of several species.

• Species of different evolutionary branch may have similar structures as it is the result of adaptation and natural selection. This is called convergent evolution. For example the wings of insect and birds.

Molecular Biology used in classification

• This is classification by comparing the genes and proteins of organisms. Scientists arranged similarly structured genes and inferred that the organisms have a common ancestry.

Cladistic Approach

• This method is based on derived similarities. Unlike other classification, this is based on the overall similarities, in other words it looks for several similarities in determining the evolutionary relationship.

• Willi Hennig is widely regarded as the founder of cladistics.

• The advantage of this approach is that all data that forms the basis of postulated relationships is shown, which often suggests new relationships, and can be more readily tested.

Example of a Cladogram

In 1969 Whittaker argued on a five kingdom system. Here are the characteristics of each kingdom.