Segmented worms - metameric organization (one compartment after another — somites — segments organized about the same). Annelids are very complex animals.

First animal with a true coelom which runs the entire length of the body. Triploblastic; bilateral symmetry. These animals have a mesentery holding the gut in place. They have a thin cuticle. For the first time, we have an animal with slime which is a lubricant. These animals also have circular and longitudinal muscles.

Digestive System - Digestive system is complete: mouth, intestine, anus. We get many new structures. Mesentery holds the intestines in place. Associated with that, we have a thick band which holds all the organs in place called a peritoneum — nothing can move. These are animals who’s digestive system is designed for what they eat: meat eaters have simple systems; herbivores have complete systems to digest plant material.

Nervous System - First time we have an animal with a plan. A keynote of the invertebrate group is that they all begin with this animal. They have a ventral nerve cord. Associated with the nerve cord up in the head is a brain. There are different lobes of the brain — no longer a big nerve dot.

Muscles - circular and longitudinal muscle.

Nephridia/Nephridiapores - this refers to kidneys. These are extremely efficient structures. There are two nephridia in each segment (somite).

Circulatory System - new system. Very advanced in the annelids. The circulatory system is complete. It has a heart or hearts, major arteries, major capillaries, major veins. Because of these four structures, it is called a closed system. These animals also have hemoglobin and great powers of regeneration.

No respiratory system - the animal breathes either by skin or respiratory capillaries — these are capillaries close to the skin which allow for CO2 and oxygen exchange. These animals are either dioecious or monoecious. Under monoecious, there are two types: self-fertilized or cross-fertilized.

Habitat - found almost everywhere — terrestrial, aquatic and also ectoparasitic. These are extremely important animals. They turn over soil and put in oxygen. Ectoparasites are used in medicine (leeches).

POLYCHAETA - First Class

Clam worms, sand worms, blood worms

Characteristics of the class:

New structure outside of the body called a parapodium. This comes out of the ventral body and is like a big paddle or oar. Used for movement. The parapodium serves two functions: Movement and Exchange of CO2 and oxygen through respiratory capillaries.

At the front exterior, the animal possesses eyes and tentacles and palps. All of these are sensory. The animal is a carnivore.

The animal has a pharynx which can evert. At the end of the pharynx are two jaws used to capture and grind food.

To move the paddle, there is a third muscle group in the san worm only called oblique muscle.

Intestine is a straight tube — these are meat eaters which is easy to digest. Digestive system consists of mouth, intestine and anus all in a straight line tube.

Nephridia in each somite. Hearts are located in the esophagus.

The animal has no reproductive organs. When these animals go into breeding conditions, there are massive quantities of egg and sperm that just appear. Reproduction occurs in the summer and fall. They are dioecious. When the eggs hatch, there is a zygote which looks nothing like the parent. Have a larval stage — trocophore larvae. They produce massive amount of offspring — they can swim so their chance of survival is better.

It is estimated that one clam worm can turn over a ton of soil on the bottom of the ocean every year. It takes nutrients into the soil and releases CO2. Primarily found on oyster beds or in the substrate sand. They live in all the substrate. These worms are literally more valuable in cash than the best steak you can buy.

OLIGOCHAETA - Second Class

Earthworm

Habitat - found as aquatic and terrestrial — black or tube worm for aquariums are the aquatic form.

Nervous System - a brain with ventral nerve cord.

Circulatory System - contains five pair of hearts (pumping chambers), a ventral and dorsal blood vessel. Dorsal vessel pumps.

Nephridia - take out nitrogenous waste. Two nephridia per segment.

Digestive System - earthworms are herbivores. They have an extra muscular pharynx, a tube esophagus, and two new structures:

1. Crop - a holding bag — thin-walled structures.

2. Gizzard - muscular structure — used to grind up cellulose

The animal then has a long intestine.

Epidermis - has setae which are needle-like structures — four pair for each segment which grab hold of soil and push against it. These are used for movement through soil. Then there is a layer of circular muscle, then longitudinal muscle (looks like fingers).

Then there is the coelom; two nephridia pores — one on either side; ventral nerve cord (center of back); ventral blood vessel. The top is the dorsal blood vessel.

The animal has a new structure called the typhosol which is a fold in the intestine which increases the surface area — used to grind up more plant material.

Reproductive System - the animal is monoecious but cannot self-fertilize. The genital opening of the female is on one segment and the male is on another segment. These animals cross-fertilize. Fertilization occurs when east fertilizes west and west fertilizes east.

The bump on an earthworm is called the clitellum. It secrets mucous which forms a cocoon around the fertilized eggs; as the worm passes through it, it closes at the ends. The zygotes form inside and, when hatched, look like the adult (no larval stage).

HIRUDINEA - Third Class

Leeches - terrestrial and aquatic. Found in all waters. Black leeches are used in medical experiments to help the circulatory system.

Externally, the leeches anatomy is segmented with a huge posterior sucker used as a hold fast. Also has an anterior sucker with three razor teeth which leave a propeller cut. These animals are ectoparasites.

They have two eyes; are segmented; and have a modified digestive system. After hooking on, there are enormous salivary glands to keep a wound open. Once a wound is open, the animal has a long proboscis (1/3 the length of its body) which goes into the wound and starts sucking out blood. Behind this proboscis, the animal possesses a massive crop — a holding bag.