Mollusca

Mollusks have three main parts:  muscular foot, visceral mass and mantle.  The visceral mass contains most of the internal organs.  The mantle is a tissue draped over the visceral mass secreting a shell.  The mantle cavity houses the gills, anus and excretory pores.  The radula scrapes up food.  Mollusk larvae are ciliated and called trochophrores.

Polyplacophora are chitons which are ovals with 8 dorsal plates composing the shell.

Gastropoda.  Torsion occurs often.  Torsion is when one side of the visceral mass grows faster than the other causing the animal to rotate 180 degrees.  This makes the anus and mantle above the "head."  An example would be snails: their mantle cavity also functions as a lung.  Eyes are at tips of tentacles.

Bivalvia.  In these animals the shell is two halves, dorsal and ventral sides.  An adductor muscle draws them together.  Gills provide the means for feeding and gas exchange.

Cephalopoda.  These are the only mollusks with a closed circulatory system; members of Cephalopoda also have a complex brain and/or nerve system.  They have a beak-like jaw and toxins.  The shell is reduced and internal or missing altogether - instead of external, like the other mollusks; the only exception to this is the Nautilus.  Ammonites were the chelled cephalopods that predated the modern species.

Lophophorates

These are named after the structure, a lophophore.  A lophophore is a circular fold of a body having ciliated tentacles.  They have a U-shaped digest tract.  There are three phyla of lophophorates:  Phoronida, Bryozoa and Brachiopoda.  While an embryo, they closely resemble deuterostomes; the only exception would be phoronids:  the blastopore becomes the mouth.

Phoronids.  These live in tubes made of chitin.

Bryozoans.  These are colonial and most resemble mosses.  Below is the surface structure of most bryozoans.

Brachiopods.  Also called lamp shells.  These look like clams but the difference is easy to miss sometimes:  its shell is dorsal/ventral whereas a clam's shell is lateral.

Annelida

Any annelid is a segmented worm - like the earthworm.  The coelom is partitioned by septae with longitudinal blood vessels (going top to bottom).  The respiratory organ is their skin.  Organs called metanephridia are very important in the removal of waste:  they are a pair of ciliated excretion tubes; there is one pair per segment.  Their primitive nervous systems is composed of ganglia and a ventral nerve cord.  Worms are hermaphroditic.  Cross-fertilization occurs when two annelids exchange sperm.

Oligochaeta.  (earthworm and most others) These eat dirt.

Polychaeta.  These worms have many bristles per segment, called parapodia.  Parapodia function as gills and a mode of propulsion. The fanworm is a member of this class.  It is a tube-dweller, trapping food in tentacles that it extends from inside the tube.

Hirudinea.  These are leeches.

Arthropoda

General traits of arthropods include an exoskeleton, specialized segmentation and jointed appendages, and a division of labor among body parts.  The arthropod exoskeleton is their cuticle, impermeable to water; it is made of layers of chitin and protein and can be as thick/hard as armor or as thin/flexible as paper.  To grow arthropods must remove and replace their exoskeleton.  This process is called molting.  Molting uses a lot of energy and leaves the organism vulnerable at this time.

Arthropods have an open circulatory system: hemolymph fluid is pumped by the heart through arteries to a sinus cavity where the blood bathes the tissues to reach veins that lead back to the heart and gills.  All the sinuses together is called the hemocoel.  The hemocoel is not part of the coelom; the coelom usually gets to be very reduced as the organism ages.  Aquatic arthropods have gills and terrestrial arthropods have specialized internal surfaces - like tracheal systems in the grasshopper.

Trilobitomorpha.  Trilobites were some of the earliest arthropods that died in the Permian extinctions.

Cheliceriformes.  Ticks, spiders and scorpions belong to this classification.  Arachnids will be the "poster child" in this explanation.  All chelicerates have chelicerae, no antennae and simple eyes.  Chelicerae are the fang or pincer appendages by the mouth at the anterior end.  Altogether the spider has 6 pairs of limbs: chelicerae, pedipalps, and 4 pair of walking legs.  The chelicerae inject digestive fluids into the prey and the spider sucks up the liquid that results.  The pedipals mainly function in sense and feeding.  Arachnids breathe through book lungs, stacked plates in an internal chamber that filter out oxygen.

Uniramia.  Insects, millipedes and centipedes all belong to different classes: insecta, diplopoda and chilopoda respectively.  All uniramians have one pair of antennae, unbranched appendages, and a land evolutionary history.  Millipedes eat decaying matter and release stink balls when threatened.  Centipedes are terrestrial carnivores with poisons.

Entomology is the study of insects.  Insects have wings that are extensions of the cuticle and not true wings.  Insects have complex organ systems.  Malgighian tubes are outpockets of the digestive tract that remove metabolic waste from the hemolymph.  They have a trachea system throughout their body and complex nerve system of 2 nerve cords, ganglia and brain.  The brain, though, is just fused ganglia.  Insects choose mates by colors, sounds and pheromones (odors).  They also undergo either incomplete or complete metamorphosis.  Incomplete metamorphosis is when the insect does not have a larval stage; it becomes a nymph and just grows to be an adult size.  Complete maturity includes the larval stage of an insects lifecycle.

Crustacea.  Crustaceans diffuse nitrogeneous wastes through their cuticle and have one or more larval stages.  Decapods are large marine arthropods a calcium carbonate chitinous exoskeleton and a carapace.  Copepods are plankton.  Isopods are small marine crustaceans but also include the terrestrial sow bugs and pill bugs.

Next:  "Deuterostomes."