Parasite

Nobody knows how many kinds of parasites there are. They range in size from the smallest virus, inconceivably small at one one-thousandth of a micron, to whale tapeworms a hundred feet long. Some, like ticks, fleas live on the surface of their hosts. Other - trematodes, tapeworms, nematodes - live within, denizens of blood, intestines, liver and heart.

Many are specialists. The so-called pork tapeworm can mature only in humans. One kind of trematodes lives' only on the first gill arch of one species of cod. The green parrot of Mexico harbors at least 15 species of feather mites; each lives on a particular part of a particular feather on the bird's body. Fleas, ticks and leeches are usually more catholic in their choice of hosts, feeding on the blood of a variety of animals. A rat flea deprived of a rat will happily settle on a bat.

Certain parasites are innocuous. Two varieties of mites five harmlessly on the landscape of the human face, tucked into hair follicles of the forehead and cheeks. However, many parasites pester, maim or kill the animals they infect. Several of those: hookworms, little leechlike -vampires of the intestinal tract, suck the blood of a billion people a year and kill perhaps 100,000. Blood-dwelling trematodes infect 200 million people, sometimes causing severe disease of the gastrointestinal tract, bladder and liver.

For humans, the worst offenders are protozoa's, the parasites that cause malaria. Nearly half a billion people are infected with malaria, and each year as many as two million die from the disease, mostly in tropical Africa.

Plant parasites such as tropical mistletoes send their roots deep into the vascular system of their green hosts. Or kleptoparasites, animals that steal or pirate food, and social parasites, which exploit the labors of other species.

What all these things have in common is their way of life. Parasites are not a natural group, like birds or animals or mammals, but a collection of organisms that live at the expense of other organisms, relying on them to survive and reproduce and usually doing harm in the process.

Tests text 1. Fish Anatomy

1. The fin spines and rays are connected by a thin fleshy material that collects….

a) spines c) excess isthmus

b) excess mucus d) silt

2. The size, shape and arrangement of the fish depend on…….. a) surrounding c) mucus

b) predecessors d) the kind of fish

3. ……….have delicate «deciduous» scales.

a) trout and salmon c) salmon and perch

b) trout and perch d) shark and trout

4. A series of small sensor organs used by the fish for detecting turbulence and pressure changes is called …...

a) isthmus c) scales

b) lateral line d) fin

5. …..of fish play the same role as lungs of animals.

a) mucus c) gills

b) rays d) fins

6. Fish secrete ……….to protect themselves from parasites and disease.

a) silt c) shark

b) gills d) mucus

7. ….. have hard, rough scales.

a) perch, rockfish c) shark, trout

b) shark, salmon d) catfish, salmon

8. On the ocean bottom lies……

a) salmon c) flat salmon

b) flat flounder d) perch

9. Fish anatomy is greatly influenced by ……….

a) shape c) water

b) surrounding d) gills

10. Shark’s scales are really modified…...

a) gills c) teeth

b) fins d) rays