LiLiving ThingsLiLiving Things Living Things

A newer system recognizes two basically distinctive groups of prokaryotes –The domain Bacteria –The domain Archaea A third domain, the Eukarya, includes all kingdoms of eukaryotes Figure 15.14B BACTERIAARCHAEAEUKARYA Earliest organisms

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

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(ATP). Structure, nutrition and life history define animals Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings WHAT IS AN ANIMAL?

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

Single cell through blastula in sea urchin

Development of Mesoderm in Most Animals

PHYLUM

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig Traditional phylogenetic tree of animals - based mainly on grades in body “plans”, and characteristics of embryonic development Each major branch represents a grade, defined by certain body-plan features shared by the animals belonging to that branch. unresolved

The major grades are distinguished by structural changes at four deep branches. 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. –Multiple cells living together/specialized Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings 1

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  gives rise to the muscles and most other organs between the digestive tube and the outer covering of the animal.

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

Pseudocoelomate – presence of a body cavity, 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 membranes, which suspend the internal organs. –Include Phylum Mollusca and up….. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 32.6b

Fourth branch point -- Coelomates are divided into two grades 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

This phylogenetic tree is based on nucleotide sequences from the small subunit ribosomal RNA. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 32.8

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig

Choanocytes resemble the choanoflagellates. No Germ layers. Alternatively, 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