Extends far beyond humans and other animals we may encounter Figure 32.1
Animal are multicellular, heterotrophic eukaryotes with tissues that develop from embryonic layers Several characteristics of animals ◦ Sufficiently define the group
• Animals are heterotrophs – That ingest their food
• Animals are multicellular eukaryotes• Their cells lack cell walls
• Most animals reproduce sexually – With the diploid stage usually dominating the life cycle• After a sperm fertilizes an egg – The zygote undergoes cleavage, leading to the formation of a blastula• The blastula undergoes gastrulation – Resulting in the formation of embryonic tissue layers and a gastrula
• Early members of the animal fossil record – Include the Ediacaran fauna
• The Cambrian explosion – Marks the earliest fossil appearance of many major groups of living animals – Is described by several current hypotheses
• During the Mesozoic era – Dinosaurs were the dominant terrestrial vertebrates – Coral reefs emerged, becoming important marine ecological niches for other organisms
• The beginning of this era – Followed mass extinctions of both terrestrial and marine animals• Modern mammal orders and insects – Diversified during the Cenozoic
• Animals can be categorized – According to the symmetry of their bodies, or lack of it• Some animals have radial symmetry – Like in a flower potRadial symmetry. The parts of a radial animal, such as a sea anemone (phylumCnidaria), radiate from the center. Any imaginary slice through the central axis divides theanimal into mirror images.
Bilateral symmetry. A bilateral animal, such as a lobster (phylum Arthropoda), has a left side and a right side. Only one imaginary cut divides the animal into mirror- image halves.
• Based on certain features seen in early development – Many animals can be categorized as having one of two developmental modes: protostome development or deuterostome development
In protostome development ◦ Cleavage is spiral and determinate In deuterostome development ◦ Cleavage is radial and indeterminate Protostome development Deuterostome development (examples: molluscs, annelids, (examples: echinoderms, arthropods) chordates) (a) Cleavage. In general, protostome Eight-cell stage Eight-cell stage development begins with spiral, determinate cleavage. Deuterostome development is characterized by radial, indeterminate cleavage. Spiral and determinate Radial and indeterminate
In protostome development ◦ The splitting of the initially solid masses of mesoderm to form the coelomic cavity is called schizocoelous development In deuterostome development ◦ Formation of the body cavity is described as enterocoelous development Coelom (b) Coelom formation. Coelom formation begins in the gastrula stage. In protostome Archenteron development, the coelom forms from splits in the mesoderm Coelom (schizocoelous development). In Mesoderm Blastopore Mesoderm deuterostome development, the Blastopore coelom forms from mesodermal Schizocoelous: solid Enterocoelous: outpocketings of the archenteron masses of mesoderm folds of archenteron (enterocoelous development). split and form coelom form coelomFigure 32.9b
In protostome development ◦ The blastopore becomes the mouth In deuterostome development ◦ The blastopore becomes the anus Anus Mouth Digestive tube Mouth Anus Mouth develops Anus developsFigure 32.9c from blastopore from blastopore
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