Invertebrate Unit - BIOLOGY JUNCTION

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Unsegmented Worm

Unsegmented Worms

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Phylum Platyhelminthes
Characteristics

Called flatworms because bodies are flattened dorso-ventrally

Acoelomate – solid bodies without a lined body cavity
Have 3 body layers — outer ectoderm, middle mesoderm, & inner endoderm
Bilaterally symmetrical
Show cephalization (concentration of sensory organs at anterior or head end)
Body cells exchange oxygen & carbon dioxide directly with environment by diffusion
Single opening into gastrovascular cavity; two-way digestive tract
Some are parasites & others are free-living
Parasitic worms have thick cell layer called tegument covered with a nonliving cuticle covering their bodies as protection inside hosts
Includes 3 classes — Turbellaria (planarians), Trematoda (parasitic flukes), & Cestoda (parasitic tapeworms

Class Turbellaria

Most are marine but includes freshwater planarian (Dugesia)

Planarians

Spade-shaped at the anterior end & have two, light-sensitive eyespots
Can sense light, touch, taste, & small
Have 2 clusters of nerve cells or ganglia to form a simple brain
Nervous system composed of a nerve net
Capable of simple learning
Move by tiny hairs or cilia over a mucus layer that they secrete
Feed by scavenging or protozoans
Have a single opening or mouth located at the end of a muscular tube called the pharynx which can be extended when feeding
Flame cells help remove wastes to excretory pores

Hermaphrodites that cross-fertilize eggs that are then deposited into a capsule until hatching in 2-3 weeks
Reproduce asexually by fragmentation

Class Trematoda

Includes parasitic flukes
About 1 cm long & oval shaped

Require a host to live
Have both oral & ventral suckers to cling to host & suck blood, cells, & body fluids
Oral sucker around mouth at anterior end sucks blood
May be endoparasites (live inside a host) or ectoparasites (live on the outside of host
Covered in tough, unciliated tegument
Nervous & excretory systems like turbellarians
Hermaphrodites
Have a long, coiled uterus that stores & releases 10,000+ eggs
Eggs released through genital pore & develop into larva
Show complex life cycles
Life cycle of sheep liver fluke:
* Adult liver flukes live in sheep liver & gall bladder where they mate & form eggs
* Eggs enter intestines, pass out with feces, & hatch in water
* Larva enter snails, asexually multiply, then leave snail & form cysts
* Cysts (dormant larva with hard, protective covering) clings to grass
* Sheep ingest cysts when they eat grass
* Cysts hatch in digestive tract & bore through intestines into bloodstream
* Mature & reproduce in the liver

Schistosomiasis (disease caused by parasitic blood flukes) infects people in Asia, Africa, & South America causing intestinal bleeding & tissue decay that can result in death

Class Cestoda

Includes tapeworms
Adapted for parasitic life
Tough outer tegument prevents being digested by host
Anterior end called scolex contains hooks & suckers for attachment to intestine of host

Long, ribbon-like bodies up to 12 m in length
Nervous system extends length of body but lacks sense organs
Lacks mouth & digestive tract but absorbs digested nutrients from host
Grows by making body segments called proglottids
Each proglottid produces eggs & sperm that cross-fertilize with other segments & also self-fertilize (hermaphrodites)
Oldest, mature proglottids containing eggs at posterior end break off & pass out with feces
Life cycle of beef tapeworm:
* Cattle eat grass with proglottids containing fertilized eggs
* Eggs hatch into larva & bore through cow’s intestine into bloodstream
* Larva burrow into cow’s muscle & form cysts
* Humans eat beef (muscle) & cysts travels to intestines
* Cyst breaks open & adult beef tapeworm forms

BEEF TAPEWORM LIFE CYCLE

Phylum Nematoda
Characteristics

Called roundworms
Includes Ascaris, hookworms, Trichinella, & pinworms
Pseudocoelomates have fluid-filled body cavity partially lined with mesoderm
Pseudocoelom contains the body organs & provides hydrostatic skeletal support for muscles
Have long slender bodies that taper at both ends

Covered with flexible cuticle
Digestive tract with anterior mouth & posterior anus; called one-way digestive tract
Separate sexes in most species
Most are free living
Some are parasites on plants & animals
Ascaris is a parasitic roundworm living in the intestines of pigs, horses, & humans
Ascaris life cycle:
* Enter body in contaminated food or water & hatch in intestines
* Larva bore into bloodstream & carried to lungs & throat
* Larva coughed up, swallowed, & return to intestines to mature & mate
* Block the intestine causing death

Hookworm eggs hatch in moist soil & larva bore through bare feet of new host
Trichinella are human parasites caused by eating undercooked pork containing the cysts
* Cause disease called trichinosis
* Cysts cause muscle pain & stiffness

CYSTS IN CONTAMINATED PORK

Phylum Rotifera
Characteristics

Known as rotifers or wheel animals
Transparent, free-swimming microscopic animal
Freshwater & marine
Have a ring of cilia around mouth that rotates like a wheel to bring in food
Feed on unicellular algae, bacteria, & protozoa
Have a muscular organ called the mastax behind the pharynx to chop food
Nervous system composed of anterior ganglia & 2 long nerve cords
Show cephalization (head end)
Have 2 anterior, light-sensitive eyespots

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Starfish Dissection

Starfish Dissection

Introduction:

Echinoderms are radially symmetrical animals that are only found in the sea (there are none on land or in fresh water). Echinoderms mean “spiny skin” in Greek. Many, but not all, echinoderms have spiny skin. There are over 6,000 species. Echinoderms usually have five appendages (arms or rays), but there are some exceptions.

Radial symmetry means that the body is a hub, like a bicycle wheel, and tentacles are spokes coming out of it (think of a starfish). As larvae, echinoderms are bilaterally symmetrical. As they mature, they become radially symmetrical.

Most adult echinoderms live on the bottom of the ocean floor. Many echinoderms have suckers on the ends of their feet that are used to capture and hold prey, and to hold onto rocks in a swift current.

Sea Stars
Sea stars (group name Stelleroidea) are sometimes called starfish, though they are not real fish (they lack both vertebrae and fins). There are two sub-types of sea stars:

Asteroideas are the true sea stars and sun stars.
Ophiuroideas are brittle stars and basket stars.

The differences between the two sub-types lies in how the arms connect to the central disk. Ophiuroids have arms that do not connect with each other. There is a distinct boundary between arm and central disk. Asteroids have arms that are connected to each other. Also, it is harder to tell with asteroids where the central disk ends and the arms begin.

The sea star’s top surface (or skin) looks spiny if you examine it. If you look very closely you will notice that there are different types of growths on the surface. Some bumps are used to absorb oxygen, they are called dermal branchiae. Pedicellaria are pincher-like organs used to clean the surface of the skin. Barnacle larvae could land on a sea star and start growing if it were not for these organs.

How Do Sea Stars Move?
Each sea star had hundreds of tiny feet on the bottom of each ray. These are tube feet, or podia. These tiny feet can be filled with sea water. The vascular system of the sea star is also filled with sea water. By moving water from the vascular system into the tiny feet, the sea star can make a foot move by expanding it. This is how sea stars move around. Muscles within the feet are used to retract them.

Each ray of a sea star has a light sensitive organ called an eyespot. Though it can not see nearly as well as we do, sea stars can detect light and its general direction. They have some idea of where they are going.

Can Sea Stars Grow New Arms?
Given enough time, sea stars can grow back arms that have been damaged or removed. For a few species, the severed arm can grow back into a complete sea star! For most sea stars, however, a severed limb dies.

What Do Sea Stars Eat?
Sea stars eat many things. A sea star’s diet can include: barnacles, snails, sea urchins, clams, and mussels. A few species, such as the spiny star of the North Atlantic, eat other sea stars! Many sea stars eat mussels and clams in an interesting way. They surround the shell and use the suckers on their feet to pull the two shells (or valves) apart. The sea star has enough force in its arms to actually bend the shell! This creates an opening between the two shells that is only .01 inches wide. Using this tiny gap, the sea star puts its stomach into the clam’s shell and eats its insides. When it is done, nothing is left but an empty shell.

Materials:

Preserved starfish, dissecting pan, scissors, scalpel, forceps, T-pins, pencil, lab apron, safety glasses

Procedure: