AP Biology
Notes: Introduction to Animals

·        Animal life began in Precambrian seas with the evolution 
  of multicellular forms that lived by eating other organisms.

·        Early animals populated the seas, fresh waters, and 
  eventually the land.

 

Animal characteristics

 Structure, nutrition and life history define animals

            While there are exceptions to nearly every criterion for distinguishing
   an animal from other life forms, five criteria, when taken together,
   create a reasonable definition.

 1)  Animals are multicellular, heterotrophic eukaryotes.

·        They must take in preformed organic molecules through ingestion,
  eating other organisms or organic material that is decomposing.

        2)    Animal cells lack cell walls that provide structural supports for plants and fungi.

·        The multicellular bodies of animals are held together with the
  extracellular proteins, especially collagen.

·        In addition, other structural proteins create several types of intercellular
   junctions, including tight junctions, desmosomes, and gap junctions,
  that hold tissues together.

        3)    Animals 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, a succession mitotic cell divisions, leading
  to the formation of a multicellular, hollow ball of cells called the blastula.

·        During gastrulation, part of the embryo folds inward, forming the
  blind pouch characteristic of the gastrula.
This produces two tissue layers:
  the endoderm as the inner layer and the ectoderm as the outer layer.

·        Some animals develop directly through transient stages into adults,
  but others have distinct larval stages.
  The larva is a sexually immature stage
  that is morphologically distinct from the adult, usually eats different foods, and
   may live in a different habitat from the adult.

·        Animal larvae eventually undergo metamorphosis, transforming the animal into an adult.

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 the complexity of
 the animal’s anatomy.

 

Evolution of animal Kingdon

·     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.

·        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.

 Animal diversity:

 There are three main hypotheses for what caused the diversification of animals.

1) Ecological Causes: The emergence of predator-prey relationships led to a diversity of evolutionary adaptations, such as various kinds of protective shells and diverse modes of locomotion.

2) Geological Causes: Atmospheric oxygen may have finally reached high enough concentrations to support more active metabolism.

3) Genetic causes: Much of the diversity in body form among animal phyla is associated with variations in the spatial and temporal expression of Hox genes within the embryo.

·        A reasonable hypothesis is that the diversification of animals was associated 
  with the evolution of the Hox regulatory genes, which led to variation in morphology 
  during development.  

  The major branches are distinguished by structural changes at four deep branches.

1)     The first branch point splits the Parazoa which lack true tissues
from the Eumetazoa which 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.

 2)    The eumetazoans are divided into two major branches, 
        partly based on body symmetry.

·        Members of the phylum Cnidaria (hydras, jellies, sea anemones and their relatives) 
  and phylum Ctenophora (comb jellies) have radial symmetry and are known
  collectively as the radiata.

·        The other major branch, the bilateria, has bilateral symmetry with a dorsal  
  and ventral side, an anterior and posterior end, and a left and right side.

·        Linked with bilateral symmetry is cephalization, an evolutionary trend toward the 
  concentration of sensory equipment on the anterior end.
Cephalization also includes the 
  development of a central nervous system concentrated in the head and extending toward 
  the tail as a longitudinal nerve cord.

      The symmetry of an animal generally fits its lifestyle.

·        Many radial animals are sessile or planktonic and need to meet the environment
   equally well from all sides.

·        Animals that move actively are bilateral, such that the head end is usually first to
   encounter food, danger, and other stimuli.

       The basic organization of germ layers, concentric layers of embryonic tissue that form various
tissues and organs, differs between radiata and bilateria.

·        The radiata are said to be diploblastic because they have two germ layers.

·        The ectoderm, covering the surface of the embryo, gives rise to the outer covering 
  and, in some phyla, the central nervous system.

·        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, 
  such as the liver and lungs of vertebrates.

·        The bilateria are triploblastic.

·        The third germ layer, the mesoderm lies between the endoderm and ectoderm.

·        The mesoderm develops into the muscles and most other organs between the 
  digestive tube and the outer covering of the animal.

 3)    The Bilateria can be 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.  

                                          

       ·        Pseudocoelomates.  In some organisms, there is a body cavity, but it is not 
                  completely lined by mesoderm.

                                           
   
     ·        These pseudocoelomates include the rotifers (phylum Rotifera) and the roundworms
                    (phylum Nematoda).  

·        Coelomates are organisms with a 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.

·        A body cavity has many functions.

·        Its fluid cushions the internal organs, helping to prevent internal injury.

·        The noncompressible fluid of the body cavity can function as a hydrostatic 
  skeleton against which muscles can work.

·        The presence of the cavity enables the internal organs to grow and move
   independently of the outer body wall.

(4) The coelomate phyla are divided into two grades based on differences in their development.

·        The mollusks, annelids, arthropods, and several other
  phyla belong to the protostomes, while echinoderms, 
  chordates, and some other phyla belong to the deuterostomes.

These differences center on cleavage pattern,  coelom formation, and blastopore fate.  

Cleavage:

·        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.

·        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.  

 

Coelom formation:

·        As the archenteron forms in a protostome, solid masses of mesoderm split to form the coelomic cavities, called schizocoelous development.

·        In deuterostomes, mesoderm buds off from the wall of the archenteron and hollows to become the coelomic cavities, called enterocoelous development.  

Blastopore formation (the opening of the archenteron):

·        In many protosomes, 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.