An introduction to animal diversity

BY Carlos Paez

OVERWIEW

WE WILL BE LEARNING ABOUT THE ANIMAL KINGDOM THAT GOES BEYOND HUMANS AND WE WILL ALSO LEARN ABOUT ANIMAL DIVERSITY WHICH IS THE DIFFERENCE OR VARIETY OF ANIMALS.

NUTRITIONAL MODE

Animals differ from both plants and fungi in their mode of nutrition plants create food and unlike plants animals cannot construct all of their own organic molecules and so in most cases they ingest them either by eating other living organisms or by eating nonliving organic material.

REPRODUCTION AND DEVELOPMENT

Cleavage- a succession of mitotic cell divisions without cell growth between division cycles.

Blastula-During the development of most animals cleavage leads to the formation of a multicellular stage known as this

Gastrulation- in this process layers of embryonic tissues that will develop into adult body parts are produced.

CONTINUATION

Gastrula- after the gastrulation the resulting developmental stage is known as the gastrula.

Larva- a sexually immature form of an animal that is morphologically distinct from the adult stage

Metamorphosis- a resurgence of development that transforms the animal into an adult.

NEOPROTEROZOIC ERA

Despite the molecular data indicating a much earlier origin of animals the first generally accepted fossils of animals are only 575 million years old. These fossils are known collectively as the Ediacaran Fauna named for the Ediacara Hills of Australia where they first discovered the fossil in figure 32.5 and similar fossils have since been found on other continents

PALEZOIC ERA

Animal diversification appears to have accelerated dramatically between 542 and 525 million years ago, early in the Cambrian period of the Paleozoic Era a phenomenon often referred to as the Cambrian Explosion. In strata formed before the explosion only a few animal phyla can be recognized.

BODY PLANS AND GRADES

One way in which zoologists categorize the diversity of animals is according to general features of morphology and development. A group of animal species that share the same level of organizational complexity is known as a grade

The set of morphological and developmental traits that define a grade are generally integrated into a functional whole referred to as a body plan

SYMMETRY

Radial symmetry- is symmetry that radiates from the center and any imaginary slice through the central axes divides the object into mirror images.

Bilateral symmetry- only one imaginary cut divides the object into mirror-image halves.

BILATERAL ANIMALS

Dorsal- the top side of a bilateral animal

Ventral- the bottom side.

Anterior- the head of the animal

Posterior- is the tail of the animal

TISSUES

The germ layers form the various tissues and organs of the body and ECTODERM which is the germ layer covering the embryo which gives rise to the outer covering of the animal. The ENDODERM which is the intermost germ layer lines the developing digestive tube which is known as the archenteron and this gives rise to the lining of the digestive tract and also organs derived from it.

CONTINUATION

Animals that have only these two germ layers are said to be diploblastic which are usually jellies like jelly fish and coral.. A third layer is called a mesoderm which is between the endoderm and ectoderm these animals are known as triploblastic. In triploblasts the mesoderm forms the muscles and most of the other organs between the digestive tract and the outer covering of the animal.

BODY CAVITIES

Body cavities are fluid-filled space separating the digestive tract from the outer body wall. The body cavity is also known as the coelom the inner and outer layers of tissue that surround the cavity connect dorsally and ventrally and form structures called mesenteries that suspend the internal organs animals with a coelom are known as coelomates animals that have cavities made from blastocoels are known as pseudocoelomates. Animals who lack a coelom are known as acoelomates.

PROTOSTOME AND DEUTEROSTOME DEVELOPMENT

Based on certain features of early development many animals can be categorized as having one of two developmental modes protostome development and deuterostome development three features often distinguish these modes.

CLEAVAGE

A pattern in many animals with protostome development is a spiral cleavage in which the planes of cell divisions are diagonal to the vertical axis of the embryo. The determinate cleavage of some animals with this development pattern rigidly casts the developmental fate of each embryonic cell very early. A cell isolated at the four cell stage from a snail for example forms an inviable embryo that lacks many parts.

CONTINUATION

In contrast to the spiral cleavage pattern deuterostome development is predominantly characterized by radial cleavage. The cleavage planes are either parallel or perpendicular to the vertical axis of the egg. As seen in the eight-cell stage. Most animals with deuterostome development are further characterized by indeterminate cleavage meaning that each cell produced by the early cleavage divisions retains the capacity to develop into a complete embryo.

COELOM FORATION

As the archenteron forms in the protostome development initially solid masses of mesoderm split and form the coelomic cavity the pattern is known as schizocoelous development. In contrast formation of the body cavity in deuterostome development is described as enterocoelous.

POINTS OF AGREEMENT

The hypotheses agree on a number of major features of animal phylogeny.

All animals share a common ancestor, sponges are basal animals, eumetzoa is a clade of animals with true tissues, most animal phyla belong to the clade Bilateria, vertebrates and some other phyla belong to the clade Deuterostomia

DISAGREEMENT OF THE BILATERIANS

While these two phylogenic hypotheses agree on the overall structure of the animal tree they also disagree on some significant points. The most important are the relationships among the bilaterians. The morphology based tree in figure 32.10 divides the bilaterians into two clades deuterostomes and protostomes. The hypothesis assumes that these two developments reflect a genetic pattern. Within the protostomes the figure indicates that arthropods are grouped in annelids. Both groups have segmented bodies, earthworms which is an annelid and the underside of the tail of the lobster which is arthropod.

ECDYSOZOANS

In contrast several recent molecular studies as shown in figure 32.11 generally assign two sister taxa to the protostomes rather then one. Which are the ecdysozoans and the lophotrochozoans the ECD. include nematodes, arthropods and some other animals. These animals secrete external skeletons the cricket’s cover is an example. As the animal grows it molts squirming out of its old external skeleton and the having a new and larger one the shedding is known as ecdysis which is why this animal is classified as a ecdysozoans.

LOPHOTROCHOZOA

The name refers to two different structures observed in animals belonging to this clade. Some animals such as ectoprocts develop a structure called a lophophore which is a crown of tentacles that function in feeding. Which includes annelids and molluscs which go through a distinctive larva stage known as trochophore larva relating to the word lophotrochozoan

EARLY EMBRONIC DEVELOPMENT

EDIACARAN FOSSILS

CAMBRIAN SEASCAPE

RADIAL AND BILATERAL SYMMETRY

BODY PLANS OF TRIPLOBLASTIC ANIMALS

PROTOSTOME AND DEUTEROSTOME DEVELOPMENT

HYPOTHESES OF ANIMAL PHYLOGENY

ECDYSIS

LOPHOTROCHOZOANS