Category: Invertebrate Unit
Invertebrate Crossword
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Click to download Invertebrate Crossword
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
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:
Dorsal view of starfish showing external anatomy
Ventral view of starfish showing external anatomy
Dorsal view of a dissected starfish showing rectal cecum, anus, madreporite, pyloric stomach, pyloric duct
Dorsal view of a dissected starfish showing madreporite, stone canal, cardiac stomach, and ampullae
Dissection showing where cardiac stomach opens into the mouth
Close up of madreporite and stone canal
Dorsal view of a dissected starfish showing pyloric caecum and pyloric ducts
Dorsal view of a dissected starfish showing gonads and ampullae
Ventral view of starfish showing external anatomy
Starfish Prelab
Starfish Prelab | ![]() |
1. In what phylum are starfish found?
2. What is the habitat for starfish?
3. On what do starfish feed?
4. What system in their body helps them catch & hold their food?
5. What does echinoderm mean in Greek? Why is this a good name for this group?
6. Name 2 classes of echinoderms & a member of each class.
7. Where does water enter a starfish? Where does it leave?