A snail may not seem to have much in common with an earthworm, but in fact, these two very different looking animals are related.  Mollusks and annelids were probably the first major groups of organisms to develop a true coelom.  Also, the fertilized eggs of both groups develop into a distinct larval form called a trochophore.
 

 This is one of the most successful animal phylas and we may find these organisms in many different environments such as freshwater, marine and terrestrial environments. This is the largest animal phyla next to the Arthropods which contains the insects. All mollusks share the following characteristics.

1. Body Cavity- all mollusks have a true coelom, but in most it is only a small space surrounding the heart.

2. Symmetry- Most mollusks have bilateral  symmetry, and many have one or more shells.

3. Organ systems- They have organ systems for circulation, respiration, digestion, excretion, and reproduction.

4. 3 Part Body Plan- The body of every mollusk has 3 distinct parts; the muscular foot, the head ( may only have sensory tissue or organs) and the visceral mass ( which generally develops into the organs of respiration, circulation and digestion.
 

Respiration- In the mantle cavity( a space between the mantle and the visceral mass) are the organs for respiration called gills. These gills are ciliated and the cilia cause water to pour over the gills. These gills may extract 50 % of the dissolved oxygen from the water that passes over the gill area. Some terrestrial mollusks like snails lack gills but their mantle cavity acts as a modified lung for oxygen exchange.

Circulation- Most mollusks have a 3 chambered heart and an open circulatory system. In open circulation, the blood does not flow entirely in vessels, and at some point pours out over the tissues to bath them in blood. Other mollusks like the octopuses and squids have a closed circulatory system in which the blood travels entirely in vessels.

Excretion- Mollusks use their coelom as a place to dump waste. Structures called Nephridia, which recollect nutrient materials and body fluids and put them back into the tissues of the organism and filter nitrogenous waste to excrete them outside the body.
 

  There are 7 classes of mollusks, but we will discuss the larger 3 phyla. It is important to look at the characteristic differences between each of the 3 groups.

Class Bivalvia- The bivalves get their name from Bi ( meaning two) and valves ( meaning shells). These mollusks are unique among the mollusks because they lack a distinct head, but do have a region of cells called ganglia, which serve as a simple brain. Sensory cells along the edge of the clamís mantle respond to light and touch and transmit these stimuli back to the ganglia located in the visceral mass.
Layers of the Valves or Shells
 Outer Layer- is a tough layer which serve to protect the shell
 Middle layer- is made up of calcium carbonate which strengthens the shell.
 Inner layer- is a smooth layer which serves to protect the soft internal parts.

Adductor Muscles contract to cause the valves or shell halves to close.
The bivalves are sessile( stationary ) filter feeders, they use siphons( one taking water in and one letting water out of the mantle cavity) to move water through the bivalves bodies. The gills have a layer of mucus covering the outer surface of the gills in which food particles  are trapped as the water passes over the gills. Cilia now move the food particles to the bivalves mouth.

Sexual reproduction occurs in the bivalve group with most groups being either male or  females, but some are hermaphrodites( have ability to produce both egg and sperm) . They have a larval stage called a trochophore and a second free-swimming larvae called a veliger.

Pearl Production- occurs as sand or pebbles get into the visceral mass and are covered with a layer of nacre, ( material produced form the smooth portion of the inner shell.)

Class Gastropoda - Commonly called the stomach footed mollusks because they move around on their ventral surface( belly),  the gastropods are snails or slugs. There are over 80,000 species of marine gastropods that have successfully invaded freshwater and terrestrial habitats. The head is distinct with usually 2 antennae. The foot is adapted for locomotion. Terrestrial species develop a slime layer they can glide on. Many gastropods have a single shell. In gastropods the visceral mass rotates 180 degrees called torsion, which causes the rearrangement of the organs and moves the mantle cavity from the back to the front.
 Respiration is carried out a variety of ways. Aquatic snails breathe through gills. Some snails that lost their shells respirate through their skin and terrestrial snails exchange oxygen & carbon dioxide through their mantle cavity inside their shell acting like a modified  or simple lung. Their shell and mantle cavity must remain moist, so they become inactive during hot dry weather.
     Feeding habits are varied among gastropods, most are like the terrestrial snail as a herbivore ( plant eating), and scrape algae off rocks and cut plants, which allow them to be major agricultural pests.  They have a structure called a radula which is a modified jaw. Some gastropods like the Whelks and oyster drills use  their radula to bore holes in other mollusks shells and suck out the soft internal parts. Cone shells use their radula like a harpoon injecting poison into their prey.

Class Cephalopoda- More than 600 species including squids, octopuses, and nautiluses. The name cephalopoda means head - footed. Squids have 10 tentacles and octopuses have 8 and nautiluses have 80 to 90 . All cephalopods are active marine predators.
 Jet Propulsion- like all mollusks that draw water into their mantle cavity through siphons, squids and octopuses have modified this process to help them move faster in water. They also release a dark fluid called ink to help them hide.
 Most cephalopods lack an external shell, except the nautilus.
 The cephalopods are the most intelligent of all invertebrates, they exhibit complex behaviors, and can be trained to distinguish among class of objects. They have an eye that is similar toe that of a vertebrate eye.
 There are both male and female sexes in the cephalopods.
 

 We get the phylum name annelida from the Latin word annelus meaning ring. These rings or segments is an evolutionary advantage for burrowing. All annelids have basic characteristics.

1) Body Cavity- Coelom
2)  Segmentation- the body is segmented, and divided into nearly identical units.
3) Organs Systems- include a highly specialized gut, a closed circulatory system and many nephridia.
4) Bristles- Most annelids have external bristles called setae. Marine annelids also have many fleshy appendages called parapodia.

Closed Circulatory System- Blood moves through a closed system faster than an open circulatory system and blood is under a greater pressure. Enlarged blood vessels act as a modified heart, in fact the earthworm has 5 blood vessels.

Respiration- The earthworm and other terrestrial annelids undergo cutaneous respiration, which is exchanging oxygen through the skin.. The annelids blood contains hemoglobin much as ours which allows for the attachment of oxygen to the blood so it can travel through the circulatory system.

Annelid Diversity

Class Polychaetea- the polychaetes are marine  segmented worms that live in virtually all ocean habitats. This group has a well developed head  unlike most annelids. These organisms have a pair of parapodia which is a distinct characteristic from other annelids, these are used to swim, burrow or crawl.

Class Oligochaete- which includes the earthworms, literally eat their way through the soil. The earthworms do several things to help us as they do this.
a)  aeration of the soil
b) soil fertilization- castings, excremental remains of the worms are good fertilizer.
c) decomposition of vegetative material-
Gizzard- tough muscular organ used to grind up soil to get nutrients from it, as the earthworm will eat a ton of soil during its life.

Reproduction- Earthworms are hermaphrodites. As the worms join head to tail, they swap sperm to fertilize each others eggs. As hermaphrodites each worm has the ability to produce their own egg and sperm, but self fertilization cannot occur, so another worm is needed to fertilize their eggs.

Class Hirudinea- which includes the leeches. A leech lacks segmentation and has suckers at both ends. Leeches are parasites on most mammals and other vertebrates and can be found in many locations, grass, water, and even trees to attach to birds.
Recently leeches have been used in surgical procedures as ancestors did many years ago to insure blood flow. Today we insure blood flow to surgically reattached body parts.
 

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