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Phylum Porifera Essay

Which phyla lack organs? What type of symmetry do they have?

Phylum Porifera which lacks any symmetry (asymmetry) and phylum Cnidaria which has radial symmetry.

List all of the phyla that show cephalization.

Phylum Annelida, Phylum Mollusca, Phylum Arthropoda, Phylum Platyhelminthes, Phylum Nematoda, and Phylum Chordata.

Do all organisms on the table have 3 germ layers (endoderm, ectoderm, and mesoderm)? If not, which Phyla have fewer than three germ layers?

Not all the organisms have three germ layers. The organisms with less than three germ layers belong to Phylum Porifera which has no true germ layers and phylum Cnidaria which has two germ layers (diploblastic).

One phylum has more species than all the others. State the name of this phylum and provide several different examples of species found in this phylum.

Phylum Arthropoda. Examples of species in this phylum include Brazilopelma colloratvillosum, Dermacentor variabilis, Limulus polyphemus, Colossendeis megalonyx, Branchinecta gigas, Pyromaia tuberculata, Asterocheres echinicola, Narceus americanus, Scaphiostreptus parilis, and Musca domestica.

According to chapter 24, fish do not all have the same skeletal structure. Describe the differences among fish from the most primitive to more advanced types.

Fishes greatly differ in their anatomical features especially their skeletal structures. The most primitive group of fishes based on skeletal structure is known as class Agnatha and the fishes in this class are referred to as “the jawless fishes”.  These have a cartilaginous internal skeleton which means that they have no true bones (Layman, 2003). They also lack true teeth, true jaws, and true vertebrae (Layman, 2003).

The other group of fish is referred to as the cartilaginous fishes and they belong to a class of fishes known as Chondrichthyes. Their skeleton is made up of cartilage. Unlike the jawless fishes, fishes in this group have jaws and true teeth which are bony as is seen with sharks (Layman, 2003).

The final group of fish is known as the bony fishes and fishes in this group belong to a class known as Osteichthyes. These fish are very advanced and unlike the other two groups, fishes in this group have a skeletal structure that is made of true bones meaning that their skeletal structure is mineralized (Layman, 2003). They have a bony jaw, bony teeth, and bony vertebrae (Layman, 2003).

Describe the three types of mammals based on how their young develop?

One group of mammals is referred to as placental mammals. The young ones of mammals under this group develop for long inside their mothers. This prolonged development is facilitated by the presence of an allantoic placenta (Go Pets America, 2010). This placenta allows close contact between the vascular systems of both the mother and the growing embryos and this ensures that the embryos are adequately and continually nourished (Go Pets America, 2010). The placenta also allows for removal of waste products from the womb. In addition, the embryos are protected inside their mothers. For example the body temperatures of the mothers are usually constant and this protects the developing young ones from the harsh surroundings. These conditions provide a conducive environment which allows for development of the young ones to a very high degree.

The other group of mammals is known as marsupial. The young ones of mammals under this category develop in the womb just like in placental mammals but their period of development is short. The reason for the short development period is due to presence of a yolk placenta whose ability to nourish the fetus is limited (Vaughan & Ryan, 2010). After birth, the undeveloped young ones continue their development inside their mothers’ pouches where they cling to the mothers’ nipples with their mouths (Vaughan & Ryan, 2010). The young ones get fed on milk through the nipples.

The final group of mammals is referred to as monotremes. Mammals in this group reproduce by laying eggs. With these mammals, two eggs are laid each time and the young ones grow inside the eggs for a short while before they are hatched (Vaughan & Ryan, 2010). Inside the eggs, the young ones are nourished by use of egg yolk. The hatched young ones are always very immature at birth and they continue to develop by feeding on milk from their mothers. But unlike the other groups of mammals, these mammals lack teats and milk oozes from the mothers’ abdomens and the young ones feed by licking the oozing milk (Vaughan & Ryan, 2010).

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