Animal Kingdom: Characteristics of Major Phyla
Phylum Porifera
Phylum Porifera, consisting of 5,500 species, are the simplest animals that exist and are sessile. Their bodies are pierced by pores that communicate with the gastrocele, covered by choanocytes. They have radial symmetry and their skeletons consist of spicules. Examples: Sponges.
Phylum Cnidaria
Phylum Cnidaria, with 10,000 species, are lower animals with more differentiated tissues. They are shaped like a polyp (sessile sack) or jellyfish (free-floating). The surface of their bodies shoots nematocysts, and their classes are diploblastic. Examples:
- Hydrozoans: alternate between the two forms.
- Scyphozoans: jellyfish.
- Anthozoa: marine polyps shaped like flowers.
Phylum Platyhelminthes
Phylum Platyhelminthes, with 20,000 species, are bilaterally symmetrical flatworms and triploblastic without a coelom. Their heads act as information processing centers. They have no skeletal, circulatory, or respiratory systems. Examples: Planarians, tapeworms.
Phylum Nematodes
Phylum Nematodes, with 25,000 species, are found everywhere. They are free-living or parasitic, unsegmented roundworms that present a rigid cuticle covering their bodies. Example: Caenorhabditis elegans.
Phylum Mollusks
Phylum Mollusks, with 93,000 species, have soft, unsegmented bodies split into a head and visceral mass, covered by a limestone shell. Classes:
- Monoplacophorans: Neopilina.
- Polyplacophora: Chiton.
- Scaphopods: Tusk shells.
- Gastropods: Slugs.
- Bivalves: Clams.
- Cephalopods: Squid.
Phylum Annelids
Phylum Annelids, with 16,500 species, are roundworms segmented into somites that are repeated. Their bodies have hairs of chitin. They have bilateral symmetry and are triploblastic with a coelom. Types:
- Oligochaetes: Earthworms.
- Polychaetes.
- Hirudinea: Leeches.
Phylum Arthropods
Phylum Arthropods, with more than 1 million species, have a chitinous exoskeleton and segmented, articulated appendages. Subphyla:
- Chelicerates: Spiders.
- Myriapoda: Centipedes.
- Crustaceans: Crayfish.
- Insects: Grasshoppers.
Phylum Echinoderms
Phylum Echinoderms, with 7,000 species, have no head and exhibit radial symmetry. They move through their ambulacral apparatus of tubes and sacs full of water. Types:
- Asteroidea: Starfish.
- Ophiuroidea: Brittle stars.
- Echinoidea: Sea urchins.
- Crinoidea: Sea lilies.
- Holothuroidea: Sea cucumbers.
Phylum Chordata
Phylum Chordata have a skeletal cord that runs along the body through the notochord. They have gills and a tail. They are triploblastic, coelomate, and bilaterally symmetrical, and they have a heart in a ventral position. Types of chordate vertebrates:
- Chondrichthyes: Sharks.
- Osteichthyes: Common fish.
- Amphibians: Springer frogs, salamanders (urodeles), caecilians (gymnophiona).
- Reptiles: Anapsids (turtles), Lepidosauria (lizards), Archosaurs (crocodiles).
- Aves.
- Mammals: Monotremes (platypus), Marsupials (koala), Eutherians (common mammals).
Hormones in Labor
The placenta secretes progesterone and estrogen, which stimulate the growth of mammary glands, and both inhibit the secretion of FSH. Estrogen stimulates increased muscle mass in the walls of the uterus and makes it sensitive to oxytocin. At week 38, the fetal adrenal cortex is stimulated to produce fetal blood that is transported to the placenta. The placenta responds by secreting less progesterone and more prostaglandins. The mother secretes oxytocin, which increases fetal pressure on the cervix. Oxytocin and prostaglandins stimulate uterine contractions.
Hormones in Milk Production
Mammary glands grow and prepare for milk production. After delivery, milk is secreted into the ducts of the mammary glands. This process is mediated by placental lactogen, which stimulates growth. Prolactin stimulates the mammary glands to produce milk, and oxytocin reaches the muscles around the breasts, causing them to contract.