Cnidaria and Porifera are the phyla in which the organisms lack organs. An example of Phylum Cnidaria is Jellyfish, which is radially symmetrical. Examples of organisms in Phylum Porifera include sponges and they lack symmetry on all axes (Ludäscher, 2005).
Phyla that show cephalisation include: Platyhelminthes and Echinodermata.
Not all of them that have three germ layers. Organisms under Phylum Cnidaria have two germ layers, and organisms under Phylum Porifera do not have germ layers (Ludäscher, 2005).
The Phylum Arthropoda has more species as compared to other Phyla. The species under Phylum Arthropoda include insects, crustaceans, and arachnids. For instance, the grasshopper, spider, beetles, butterfly, shrimps, and crabs (Ludäscher, 2005).
There are two types of fish. They include the Cartilaginous or Chondrichthyes fishes and the bony or Osteichthyes fish. Cartilaginous or Chondrichthyes fishes, which are jawed and with paired nares, fins, two-chambered hearts, have scales, and the skeletons, are made of cartilage instead of bone. But they have bony teeth and do not have gill covers. Examples of fish under this group include sharks, skates, and rays. Another group is the bony fish also known as Osteichthyes. This group of fish is the most common. The bony fish have bony skeleton and gill covers (Hairston, 1994).
The monotremes, which are the egg-laying mammals, for instance the platypus. The Marsupials, which are the non-placental mammals, for instance the kangaroos. The marsupials have a pouch in which the young ones develop. The most advanced type of mammals is the placental mammals, for instance, the human beings. The young ones of the placental animals develop inside the placenta to a comparatively advanced stage (Gittleman, 2008).
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