LiLiving ThingsLiLiving Things Living Things

The Three Domains of Life Figure 15.14B BACTERIAARCHAEAEUKARYA Earliest organisms  The newest system recognizes two distinctive groups or domains of prokaryotes: The domain Bacteria The domain Archaea  The third domain, the Eukarya, includes all kingdoms of eukaryotes

Prokaryotes are classified into two domains, based on nucleotide sequences and other features Table 16.8

(1) Animals are multicellular, heterotrophic Eukarya. –They must take in preformed organic molecules through ingestion, eating other organisms or organic material that is decomposing. Animals oxidize glucose, converting it to carbon dioxide, water plus energy. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings In Zoology, we study the Kingdom Animalia in the Domain Eukarya WHAT IS AN ANIMAL ?

(2) Animal cells lack cell walls that provide structural supports for plants and fungi. –The multicellular bodies of animals are held together with extracellular proteins (ie.Collagen). –In addition, other structural proteins create several types of intercellular junctions, including tight junctions, desmosomes, and gap junctions, that hold tissues together. Desmosomes use protein cables that span the cell membrane of two adjacent cells and bind them together. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

(3) Animals sometimes have two unique types of tissues: nervous tissue for impulse conduction and muscle tissue for movement.

(4) Most animals reproduce sexually, with the diploid stage usually dominating the life cycle. –In most species, a small flagellated sperm fertilizes a larger, nonmotile eggs. –The zygote undergoes cleavage  more mitotic cell divisions  blastula  gastrula: gastrulation – invagination producing two tissue layers, ectoderm and endoderm. Fig Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Single cell through blastula in sea urchin

gastrulation

(5) The transformation of a zygote to an animal of specific form depends on the controlled expression in the developing embryo of special regulatory genes called Hox genes. –These genes regulate the expression of other genes. –Many of these Hox genes contain common “modules” of DNA sequences, called homeoboxes. –Only animals possess genes that are both homeobox-containing in structure and homeotic in function. All animals, from sponges to the most complex insects and vertebrates have Hox genes, with the number of Hox genes correlated with complexity of the animal’s anatomy. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

The Phylogenetic Classification of Animalia Shows a Flagellated Protist Ancestor

PHYLUM The various phylum of animals can be organized into a Phylogenetic Tree of Animals

The major divisions are distinguished by structural changes at four deep branches. (1)The first branch point splits: the Parazoa - lack true tissues, from the the Eumetazoa - have true tissues. –The parazoans, phylum Porifera or sponges, represent an early branch of the animal kingdom. –Sponges have unique development and a structural simplicity. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings 1

(2) Second branch point -- The eumetazoans are divided into two major branches, partly based on body symmetry. –Radiata - radial symmetry. Cnidaria (hydras, jellies, sea anemones etc), Ctenophora (comb jellies). -- Bilateria – bilateral symmetry with a dorsal - ventral side, an anterior and posterior end, and a left and right side. 2 Linked with bilateral symmetry is cephalization, an evolutionary trend  anterior CNS, extending to the tail end as a longitudinal nerve chord.

Radiata and bilateria differ in the basic organization of germ layers (embryonic tissues), differs between. The Radiata are diploblastic - 2 germ layers. –The ectoderm,outer layer  integument, and in some phyla, the CNS. –The endoderm, the innermost layer  lines the developing digestive tube, or archenteron, and gives rise to the lining of the digestive tract and the organs derived from it, eg. liver and lungs of vertebrates. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings The Bilateria are triploblastic – 3 germ layers –The third germ layer, the mesoderm lies between the endoderm and ectoderm. –The mesoderm  the muscles and most other organs between the digestive tube and the outer covering of the animal.

(3) Third branch point -- Bilateria divided by the presence or absence of a body cavity (a fluid-filled space separating the digestive tract from the outer body wall) and by the structure the body cavity. Acoelomates (the phylum Platyhelminthes) have a solid body and lack a body cavity. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 32.6a 3

Pseudocoelomate - there is a body cavity, but it is not completely lined by mesoderm. – Pseudocoelomates include the rotifers (phylum Rotifera) and the roundworms (phylum Nematoda). Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 32.6b

Coelomates -- true coelom, a fluid-filled body cavity completely lined by mesoderm. –The inner and outer layers of tissue that surround the cavity connect dorsally and ventrally to form mesenteries, which suspend the internal organs for support. Allows for space for organs to grow Provides a cushion for internal organs Prevents twisting of organs from their position Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 32.6b

(4) Fourth branch point -- Coelomates are divided into two groups based on differences in their development. –Protostomes - Mollusks, annelids, arthropods, and several other phyla. –Deuterostomes - Echinoderms, chordates and several other phyla. –These differences center on cleavage pattern, coelom formation, and blastopore fate. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings 4

Fig Developmental Difference between Protostomes and Deuterostomes Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Many protostomes undergo spiral cleavage, in which planes of cell division are diagonal to the vertical axis of the embryo. –Some protostomes also show determinate cleavage where the fate of each embryonic cell is determined early in development. Thus, no identicle twins can occur!!!!! The zygotes of many deuterostomes undergo radial cleavage in which the cleavage planes are parallel or perpendicular to the vertical egg axis. –Most deuterostomes show indeterminate cleavage whereby each cell in the early embryo retains the capacity to develop into a complete embryo. This is the etiology of identical twins Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Coelom formation begins in the gastrula stage. –Prostostome: As the archenteron forms in a protostome, solid masses of mesoderm split to form the coelomic cavities, called schizocoelous development. –Deuterostomes: mesoderm buds off from the wall of the archenteron and hollows to become the coelomic cavities, called enterocoelous development. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

The fate of the blastopore, the opening of the archenteron is different in protostomes and deuterostomes. –In many protostomes, the blastopore develops into the mouth and a second opening at the opposite end of the gastrula develops into the anus. –In deuterostomes, the blastopore usually develops into the anus and the mouth is derived from the secondary opening. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Kingdom Animalia General Characteristics

More than a million extant species of animals are known, and at least as many more will probably be identified by future biologists. –Animals are grouped into about 35 phyla. Animals inhabit nearly all environments on Earth, but most phyla consist mainly of aquatic species. –Most live in the seas, where the first animals probably arose. Terrestrial habitats pose special problems for animals. –Only the vertebrates and arthropods have great diversity on land. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Our sense of animal diversity is biased in favor of vertebrates, the animals with backbones, which are well represented in terrestrial environments. –But vertebrates are just one subphylum within the phylum Chordata, less than 5% of all animal species. Most of the animals inhabiting a tidepool, a coral reef, or the rocks on a stream bottom are invertebrates, the animals without backbones. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Parazoa 1.Phylum Porifera: Sponges are sessile with porous bodies and choanocytes

Choanocytes resemble the choanoflagellates. Germ layers are loose federations of cells, relatively unspecialized, but 12 different types. No real tissues. Sessile animals that lack nerves or muscles. –But individual cells can sense and react to changes in the environment.. Phylum Porifera: Sponges are sessile with porous bodies and choanocytes Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

The 9,000 or so species of sponges (1 cm to 2 m in height). Mostly marine. –Only ~ 100 species live in fresh water. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 33.2

Giant sponges can provide sanctuary for other organisms

Fig Sponge Anatomy (structural fibers) suspension feeding Choanocyte

Most sponges are hermaphrodites. –Gametes arise from choanocytes or amoebocytes. –The eggs stay in mesohyl; sperms are carried out the osculum by water current. –Sperms drawn into neighboring individuals and fertilize eggs. –Zygotes develop into flagellated, swimming larvae that disperse from the parent. –Larva finds a suitable substratum, and develops into a sessile adult. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Sponge Life Cycle

Sponge aggregation Expt. If a sponge is forced through a small screen so that the cells are separated from each other and then put in a glass beaker, within two weeks the sponge will have reassembled itself into its native form. What does this experiment tell us? That cells communicate with each other and know their position relative to each other.

Most systematists now agree that the animal kingdom is monophyletic. If we could trace all the animals lineages back to their origin, they would converge on a common ancestor. That ancestor was most likely a colonial flagellated protist that lived over 700 million years ago in the Precambrian era. The animal kingdom probably evolved from a colonial, flagellated protist

This protist was probably related to choanoflagellates, a group that arose about a billion years ago. Modern choanoflagellates are tiny, stalked organisms inhabiting shallow ponds, lakes, and marine environments.

One hypothesis for origin of animals from a flagellated protist suggests that a colony of identical cells evolved into a hollow sphere. The cells of this sphere then specialized, creating two or more layers of cells. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 32.3