Flatworm sex

Sex.  It's how babies are made.  Whenever a student asks about sex in class I always begin my answer the same way:  "When two people/lions/jellyfish/etc. love each other very much, the engage in a special hug...(pause)...and then babies come out!"  But the truth is that not only is love not always a part of it, other individuals are not always involved.  Critters can reproduce many different ways, and students are often surprised to find that the world of animal reproduction is incredibly diverse.  

Below is an excerpt from the Zoology/Evolution text that I wrote.  It deals with one of the many methods of reproduction and it involves flatworms, or to use their correct phylum name, Platyhelminthes.  The students have already read the previous section of the chapter dealing with the flatworm's incomplete digestive system and a few other physiological tidbits, but this is the bit that I normally get the biggest response from.
  

 Flatworms can typically reproduce sexually, but they are mostly hermaphrodites which means both types of sex organs are present in each organism.  This is a great adaptation and makes reproduction so much easier.  Let me explain.  Imagine that you are a male worm and in the mood for a little reproduction.  You cruise out into your little part of the puddle in the hopes that you might meet the female worm of your dreams.  You know the kind - all flat and sexy.  Anyway, your little auricles tell you that there is another flatworm a few centimeters to your left and you begin your long crawl over to her.  On the way over you are working on your lines, fixing your pharynx so it is not dirty from your latest dinner/poop, and, in general, getting ready to mate.  When you get over to her you begin the courtship process and are immediately pushed off because the love of your life happens to be rather more male than you had expected.  You might find that you enjoy this other male's company and that you even support the same wormy soccer team, but alas, tonight is about reproduction and another male simply will not help in this matter. 

So you move on to the next worm in the pond who also happens to be male.  The same is true of the next 3 worms you try your little wormy pick-up lines on ("Hey there - I couldn't help but notice the cleanliness of your mouth/anus and I was wondering...").  Hopefully you are starting to see that it can be a bit of a problem to find a mate if the population is split into males and females.  With hermaphrodites you don't have to find a female worm or a male worm, you simply have to find another worm.  All worms are potential mates.  Not that they will all be willing to mate with you, but the possibility is there.  This is especially helpful if you happen to be a parasite and are literally stuck to a space on the intestinal wall. 

It is a little shameless self promotion, but if you like this sort of thing and are looking for something that is entertaining and informative, check out the rest of the text that I wrote for my Zoology/Evolution class at my store on Teachers Pay Teachers.  Available here.