Bio Unit 3 Chapter 28 Part L

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Characteristics of Animals

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• Multicellular and heterotrophic (multicellularity is not unique here but still important)
• Animals ingest and digest internally *
• No cell wall*
• Have organs and organ systems (especially muscle and nervous tissues)*

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Ancestry of Animals

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o Placed in the Supergroup Opisthokonta along with fungi and certain protozoans.
o Domain Eukarya, Supergroup Opisthokonta, Kingdom Animalia
o Two main groups: Invertebrates and Vertebrates. Each group contain several phyla
o The protists today that are considered to be most like the last unicellular ancestor of animals are the choanoflagellates

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Evolution of Body Plans

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all various body plans seen today are found in fossils of the Cambrian period which is relatively early in the fossil record. How come so much diversity in such a “short” time?
Scientists believe that slight shifts in the DNA code and expression of genes called Hox genes are responsible. More on this later. See Science Focus on pages 524-525.

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colonial flagellate hypothesis

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animals are descended from an ancestor that resembled a hollow, spherical colony of flagellated cells.

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Choanoflagellates

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are single celled protozoan that is very small with a flagellum that is surrounded by microvilli in the form of a collar. The movement of the flagellum creates water currents that pull the protist along and also pull debris in the water, (bacteria, organic matter etc.) near it so they can eat it.  If we look at the choanocytes of Sponges (the simplest animals) we see that they look almost identical to the protists called choanoflagellates. A nick name of the choanocyte is “collar cell”.
 Fig 28.2 shows the steps of the colonial flagellate hypothesis. These steps show how an aggregate of single celled flagellates can become a colony and then on to multi-cellularity with eventual cell specialization and tissue development.

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The Phylogenetic Tree of Animals

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The data used to establish the relatedness of animal phyla and such are mostly DNA, rRNA, and morphological characteristics, such as the characteristics of body plans and development. Also the fossil record.

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Symmetry

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refers to a pattern of similarity that is observed in objects.
Three types of symmetry exist in the animal kingdom

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• Asymmetry

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(a lack of symmetry). As seen in sponges

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• Radial symmetry

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bodies are organized circularly, such that any longitudinal cut through the central point produces two identical halves. Seen in medusa and polyp stage cnidarians.

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• Bilateral symmetry

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have a definite left and right half, and only a single longitudinal cut down the centerline produces two equal halves. Seen in the adult form of many animals

• During evolution, cephalization (a head region) also accompanies bilateral symmetry.

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Embryonic Development

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Sponges are the simplest of the animals but they do not have true tissues, just some cell level organization with some specialized cells. However, as we get more complex animals, we see that they have true tissues from at least two of the three embryonic germ layers. These germ layers are the endoderm, mesoderm, and ectoderm. These germ layers give rise to the different specialized tissues and organ systems we are familiar with in most animals.
For example, Sponges do not have these tissues but Cnidarians have embryos that have two germ layers, the ectoderm and endoderm. They are diploblastic.

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Diploblastic

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Tissues from two germ layers (but do not form specialized organs)

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Triploblastic:

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Tissues from all three germ layers .

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All animals except Sponges, Cnidarians and Comb jellies are triploblastic.

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Furthermore, the Triploblastic animals are further divided by animals that are either
Protostomes or Deuterostomes.

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Protostomes

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mouth develops first in embryo (mouth before anus)

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Deuterostomes:

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anus develops first in embryo (anus before mouth)

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Protostomes and Deuterostomes develop differently in three major events:

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Cleavage, Blastula Formation, Coelom Development

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Cleavage:

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cell division without cell growth. The first event after fertilization

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) Blastula Formation:

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a hollow sphere of cells.

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Coelom Development

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a body cavity lined by mesoderm.

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Protostomes

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Cleavage is spiral. The daughter cells sit in grooves formed by the previous cleavages.
The fate of these cells is fixed and they can only contribute to development
in one pathway.
During Blastula formation, the first indentation forms a pore called the blastopore.
The blastopore forms the mouth first in Protostomes.
Coelom development in Protostomes occurs from mesoderm cells near the blastopore and splitting occurs that produces the coelom.

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Deuterostomes

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Cleavage is radial and the daughter cells sit right on top of the previous cells.
The fate of these cells is indeterminate as they can be separated and each one can still become an individual organism.
During blastula formation, the blastopore becomes the anus first and later a second opening forms the mouth.
Coelom development: the coelom arises as a pair of mesodermal pouches from the wall of the primitive gut. Pouches will enlarge and eventually meet and fuse together.

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blastopore

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forms the mouth first in Protostomes

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The Simplest Invertebrates

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Invertebrates are animals that lack a backbone and are far more diverse and numerous than the vertebrates.
All animals are multicellular.

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. Sponges (phylum Porifera

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• Sponges are the ONLY animals to lack true tissues and only have a cellular level of organization.
• These guys do not fit into diploblastic or triploblastic categories. NOT PROTOSTOME
• Only have a few cell types. Examples are amoeboid cells, flagellated choanocyte cells (collar cells)
and epidermal cells.
• Sac-like body plan.
• Asymmetry
• Have many pores and canals. Incurrent canals bring in water. Water exits out large canal called Osculum.
• Sessile for most of life cycle. Filter feeders!!
• Larva can swim to new locations.
• Skeleton made of protein called spongin or made up of “bony or glass like” spicules.
• Spicules are also defensive.
• Can reproduce asexually by fragments budding or sexually with the release of sperm that fertilizes eggs.
• Most are marine but a few are freshwater
• Their choanocytes resemble choanoflagellates!

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Comb Jellies & Cnidarians

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 These two groups of Invertebrates have true tissues.
 Embryos have tissues developed from two germ layers, the endoderm and the ectoderm. Diploblastic.
 Adults show radial symmetry.

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Comb Jellies (phylum Ctenophora)

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are solitary, free swimming marine invertebrates usually found in warm waters.

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Cnidarians (phylum Cnidaria)

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are tubular or bell shaped animals that reside mainly in shallow coastal waters but some are freshwater or brackish water inhabitants. A few are oceanic forms.
• Are named for the specialized stinging cells that most have. These cells are called cnidocytes
• Cnidocytes have a fluid filled capsule called a nematocyst that contains a long coiled hollow thread.
• This thread is pressure sensitive and either is used to just trap prey or to trap and sting prey with a neurotoxin that often paralyzes.
• Body type is a two layered sac. Outer layer: epidermis derived from ectoderm. Inner layer: derived from endoderm, secretes the digestive juices into an internal cavity.
• Internal cavity is called a gastrovascular cavity.
• The two tissue layers are separated by mesoglea, a jelly like substance that is in some ways similar to mesoderm.
• Two main body forms: Polyp and Medusa. Polyp form has mouth up while medusa form has mouth down. Polyp form is usually sessile while medusa form is free swimming. (some exceptions)

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Cnidarian diversity

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Corals, Sea anemones, Hydrozoans (example Hydra), True jellyfish.
(see pictures of sea anemones, corals and jellyfish)
• Corals are like sea anemones but polyps are encased in calcium carbonate (limestone). These are the organisms that build coral reefs and these accumulate over hundreds of years.
• Sea anemones are sessile polyps with upturned oral disks that are surrounded by tentacles.
• Hydrozoans have a dominant polyp stage. Hydras and Portuguese man-of-war are both hydrazoans.

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• Species spotlight! Hydra, the typical Cnidarian. Learn anatomy and life cycle.

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 Small tubular body type
 Cells of the epidermis are termed epitheliomuscular cells because they contain muscle fibers.
 Also have nerve cells that make up a primitive nervous system called a nerve net.

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Diversity among Lophotrochozoans

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 Begins the Protostomes (triploblastic protostomes).
 They are the most diverse of the protostomes.
 Bilateral symmetry during at least one stage of life cycle.
 Embryos have three germ layers in development (triploblastic).
 Some have a true coelom.

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Lophophorans

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• Consists of Byozoans, Brachipods, Phoronids.
• All are aquatic.
• All have a feeding apparatus called a lophophore that is a mouth surrounded by ciliated-tentacle like structures.

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. Trochozoans

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• Have or have had in ancestors, a trochophore, which is a free-swimming marine larva with bands of cilia that control movement and direction.
• Molluscs and annelids have a trochophore stage of development in current modern times (now)
• Flatworms and Rotifers had a trochophore stage of development in ancestor.

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Flatworms (phylum Platyhelminthes) some are free-living (Planarians) while others are parasitic (Flukes and Tapeworms).

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• Have a sac-like body plan with only ONE OPENING, the mouth.
• Gut is said to be incomplete digestive tract because of only one opening.
• Extremely flat body (compressed dorsal-ventral)
• Bilateral symmetry
• Aceolomates (no true body cavity completely lined with mesoderm). Instead of third tissue layer, mesoderm fills the space.

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) Planarians (free living flatworms)

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o Marine, freshwater and some terrestrial but in moist climates.
o Have a muscular pharynx that sucks up food particles]
o Show cephalization with sensory organs for light and touch/”smell”
o Usually reproduce sexually
o Hermaphroditic
o Have a digestive system, excretory system (with Flame cells), nervous system and reproductive system
o NO respiratory system
o NO circulatory system
o Have paired ganglia that serve as brain.
o Have three kinds of muscle layers: an outer circular layer, an inner longitudinal layer, and a diagonal layer. Mucus is secreted and the animal glides on it especially larger forms that have cilia on ventral surface.
o Species spotlight: Dugesia

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Flukes and Tapeworms (parasitic flatworms)

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o Parasitic worms have lost cephalization seen in Planarians.
o Heads and sensory organs have been replaced by suckers and hooks.
o Feed on nutrients provided by the host and some need several hosts to complete their life cycle.
o Reproductive systems are well developed
o Nervous systems are NOT well developed nor or most of the other systems.
o Parasitic worms are covered by teguments, which are protective against the digestive juices of host gut.
o Both Flukes and Tapeworms need primary and secondary hosts. The primary host is the host that the reproductive part of the lifecycle is completed in (the sexually mature adult parasite)

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Flukes (trematodes):

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o Flukes are named for the organs they inhabit.
o Examples are Lung Flukes, Liver Flukes and Blood Flukes
o Body is oval or elongated flattened body
o At anterior end is an oral sucker
o Usually another sucker is at the posterior end for attachment
o Schistosomiasisis a serious disease caused by a genus of Blood Flukes, Schistosoma.
o Learn life cycle on page 532.
o Other diseases caused by flukes and their eggs: dysentery, anemia, bladder inflammation, brain damage, severe liver complications.
o Chinese liver fluke infects humans primarily from eating raw fish.( attention sushi lovers!)

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o Schistosomiasisis

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serious disease caused by a genus of Blood Flukes, Schistosoma

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oral sucker

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anterior end

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teguments

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protective against the digestive juices of host gut.

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Tapeworms (cestodes):

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o Vary in length but can get nearly 20 meters long.
o Modified head region called a scolex that is used to attach to host gut. Has suckers for feeding
o Tegument is called a cuticle
o Are hermaphrodites
o Behind scolex are body segments called proglottids. Proglottids are reproductive structures that contain full sets of female and male reproductive organs (in each segment!!).
o After fertilization, organs dissolve and eggs mature (up to 100,000 eggs).
o Humans can become infected when they eat undercooked meat infected with tapeworms.
o Learn life cycle on page 533.

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. Rotifers (phylum Rotifera)

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. Rotifers (phylum Rotifera)