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The illustration shows the digestive, nervous, and excretory systems in a flat, worm-like Planaria. The digestive system starts at the ventral mouth opening in the middle of the animal, and then extends to the head and tail with many lateral branches. The nervous system has two cerebral ganglia at the eyes in the head, and two longitudinal nerve cords with transverse connections along the length of the body to the tail. The excretory system is arranged in two long mesh-like structures down each side of the body. An enlargement shows a detailed flame cell, which has a bundle of cilia at one end. The cilia extend down into an excretory tube, which has slits near the cilia to allow waste fluid to enter the excretory tube and exit the animal at the excretory pore.
This planarian is a free-living flatworm that has an incomplete digestive system, an excretory system with a network of tubules throughout the body, and a nervous system made up of nerve cords running the length of the body with a concentration of nerves and photosensory and chemosensory cells at the anterior end.

Since there is no circulatory or respiratory system, gas and nutrient exchange is dependent on diffusion and intercellular junctions. This necessarily limits the thickness of the body in these organisms, constraining them to be “flat” worms. Most flatworm species are monoecious (hermaphroditic, possessing both sets of sex organs), and fertilization is typically internal. Asexual reproduction is common in some groups in which an entire organism can be regenerated from just a part of itself.

Diversity of flatworms

Flatworms are traditionally divided into four classes: Turbellaria, Monogenea, Trematoda, and Cestoda ( [link] ). The turbellarians include mainly free-living marine species, although some species live in freshwater or moist terrestrial environments. The simple planarians found in freshwater ponds and aquaria are examples. The epidermal layer of the underside of turbellarians is ciliated, and this helps them move. Some turbellarians are capable of remarkable feats of regeneration in which they may regrow the body, even from a small fragment.

Four photos of flatworms are depicted. Photo a shows a dark, opaque, wavy-edged, speckled flatworm, about three times as long as it is wide. Photo b shows a transparent, brown flatworm, with a length about eight times the width, and a slightly arrow-shaped head with two eyes. Photo c shows an oval, transparent brown flatworm, with a circular sucker at the front end and one near the middle. Photo d shows a very long, narrow, flat, white tapeworm.
Phylum Platyhelminthes is divided into four classes: (a) Bedford’s Flatworm ( Pseudobiceros bedfordi ) and the (b) planarian belong to class Turbellaria; (c) the Trematoda class includes about 20,000 species, most of which are parasitic; (d) class Cestoda includes tapeworms such as this Taenia saginata ; and the parasitic class Monogenea (not shown). (credit a: modification of work by Jan Derk; credit c: modification of work by “Sahaquiel9102”/Wikimedia Commons; credit d: modification of work by CDC)

The monogeneans are external parasites mostly of fish with life cycles consisting of a free-swimming larva that attaches to a fish to begin transformation to the parasitic adult form. They have only one host during their life, typically of just one species. The worms may produce enzymes that digest the host tissues or graze on surface mucus and skin particles. Most monogeneans are hermaphroditic, but the sperm develop first, and it is typical for them to mate between individuals and not to self-fertilize.

The trematodes, or flukes, are internal parasites of mollusks and many other groups, including humans. Trematodes have complex life cycles that involve a primary host in which sexual reproduction occurs and one or more secondary hosts in which asexual reproduction occurs. The primary host is almost always a mollusk. Trematodes are responsible for serious human diseases including schistosomiasis, caused by a blood fluke ( Schistosoma ). The disease infects an estimated 200 million people in the tropics and leads to organ damage and chronic symptoms including fatigue. Infection occurs when a human enters the water, and a larva, released from the primary snail host, locates and penetrates the skin. The parasite infects various organs in the body and feeds on red blood cells before reproducing. Many of the eggs are released in feces and find their way into a waterway where they are able to reinfect the primary snail host.

Questions & Answers

now that we have an estimate for the diameter of the cell.what estimate can we make about the volume of the cell?
faxhood Reply
why too much insulin result in low blood sugar
Leri Reply
difference between DNA and RNA
Eyitayo Reply
DNA is deoxyribonuclaic acid. Deoxy refers to a lack of oxygen. The Ribose moity is missing an OH group. I think it is missing from the second C of the ring. RNA is ribonucleic acid. DNA has our genetic code in on it. RNA is translated from DNA and carries the blue print for protein synthesis.
Eric
The OH group on RNA prevents it from being reactive. But it is very unstable though. Would you want such a power tool floating around in your body, no. And you have three types of RNA: mRNA, tRNA and rRNA. Please let me know it this helped?😄
Eric
o
Browse
what determines the aeration level in the soil
Shola Reply
what is homeostasis?
Sarita Reply
What is biology
Don Reply
Biology z the study of life
GOLDEN
what's biology
buran
biology is the study of living nd none living organism
Chinaza
Biology is the study of life
Quadri
yes Sir
Said
what's cell biology
Prince
biology is the study of life
Raheal
what is asexual reproduction,?
Awoi Reply
A type of reproduction which does not involve the fusion of gametes or a change in the number of chromosomes
Serena
Reproduction without sex... In which form a single organism or cell makes a copy of itself.
Serena
Please explain the concept of mitosis and meiosis
Serena
I guess you could use it for study buddies and brushing up on what you need to
Serena
what is mitosis
Klp
Asexual reproduction?
Serena
why pepsin and trypsin released in active form?
Yacqub
mitosis is the type cell division in which two daughter cells have same no. of chormosomes
syed
Hi
Don
chromosome number remains the same in mitosis
mc
Ello
Henry
Hii
Nikky
Yrr help me.
Nikky
Physical chemistry..... Koi h jo mujhe physical chem ki notes send kr ske
Nikky
what is asexual reproduction
Targbe
what makes golgi body in plants
Abdulkareem Reply
name the membrane of the plants
Abdulkareem
how can turners syndrome be corrected before birth
Balinda
which animal survive from being preyed just because of being humble, slow, and not aggressive
Balinda
Plants have golgi body's also. Plants are eukaryotic cells. And membrane bound organelles are a characteristic of eukaryotic cells. Moreover golgi body's are creatted from the ER. Also do not forget plants have plastids and animal cells do not.
Eric
During organs transplantation, the organs cannot be taken from just anybody since the graft would be rejected sooner or later due to
Liter Reply
Non-MHC compatibility on the organ and an attack from the patient's immune system.
Eric
what makes golgi body in plants
Abdulkareem
why trypsin and pepsin released in active form
Yacqub
Let us remember MHC'S on our cells. This is how our cells determine self from n o n s e l f. Transplanted tissue has to have a certain amount markers. These have to match to the recipiant's markers. Even with this, immunosuppresant medacine is prescribed to the recipient.
Eric
Even with these measures the body may still reject the transport. This can occur even after the recipient excepting the transport for some time.
Eric
what is integument system
Joy Reply
This system is our skin. This includes the skin lining our alimentary system which includes the tissue from our mouth to our anus. Our skin is our largest organ system. It is mostly made up of epithelial tissue.
Eric
Cellular respiration
Lucy Reply
This is how our cells make energy. They use glucose + oxygen. There are other facors involves also. But these are the main two reactant used, for aerobic respiration. The main product is ATP. ATP is a high energy molecule which is paramount for life.
Eric
what are the characteristics of living things
Ruth Reply
Movement Respiration Nutrition/Feeding Irritability/Sensitivity Growth Excretion Reproduction Deat/Life span
Hashim
What makes children from the same father and mother sometimes don't look alike?
Hashim
One of the main one's is the abilit reproduce. That is why viruses are not considered living. Because they do not have the ability to replicate on they own.
Eric
identification of problems
Nana Reply
what happens in the process of raising the human arms
Nana
what is biology
Brandi Reply
first step in scientific method
Brandi
The study of living things.
Eric
In an investigation the pancreatic duct of a mammal was blocked.It was found that the blood sugar regulation remained normal while food digestion was impaired.Explain
Mac Reply
To begin with, obstruction of pancreatic duct will alter the blood sugar level as the juices responsible for glucose regulation will be rendered inconsequential. This will in turn affect the rate of digestion and absorbtion of digested food substances by the Villus .
Muktar

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Source:  OpenStax, Concepts of biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11487/1.9
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