Posted by: Rob Viens | June 27, 2012

Tolerably Contented with Planaria

On June 27th, Darwin was making the most of his last few days in Brazil and getting out to do some collecting.  Even though he loved invertebrates (as a general rule) I think he was a little disappointed at the lack of “higher” animals and/or any real variety in the local geology.  He notes:

“This is my last day on shore, so I was determined it should not be an idle one.— In the bay I found some fine Corallines; the examination of which occupied me during the whole day.— Upon the whole I am tolerably contented with what I have done at Rio in Natural Hist: several important branches have been cut off: Geology is here uninteresting, Botany & Ornithology too well known.— And the sea totally unproductive excepting in one place in Botofogo Bay.— so that I have been reduced to the lower classes, which inhabit the dry land or fresh water.— The number of species of Spiders which I have taken is something enormous.— The time during these eleven weeks has passed so delightfully, that my feelings on leaving Botofogo are full of regret & gratitude.” (June 27)

But, even with his complaints, there was one invertebrate that Darwin did get excited about – the class of flatworms known as the planaria.

From his comments, it is pretty clear that the world of science already knew a bit about aquatic planaria – found in both freshwater and saltwater.  You may remember them from high school biology – they are the little guys that look like this (image from

planaria species
The little dots that look like “eyes” are, in fact, light-sensitive organs.

Flatworms occupy their own phylum on the taxonomic tree of life – the Platyhelminthes. Planaria (class Turbellaria) are one of the main subdivisions of the Playhelminthes. For the most part, they are not parasitic like the other types of flatworms – the flukes and tapeworms.  We’ll stick with the planaria for now – even I am not that into the idea of writing about tapeworms.

Planaria are bilaterally symmetric – one side of the body is a mirror image of the other (we humans are, too). However (unlike humans), planaria are quite simple – having no body cavity, no circulatory system, and no respiratory system.  In fact, one of the reasons why they are “flat” is that they need to breath and absorb nutrients directly through the skin.  If they were “fat” it would be harder to get food and oxygen to the center of their body – flat is just more efficient. To help increase their surface area even further, the “skin” of the planaria is covered with little fine “microvilli”.  These are like the bristles on a brush, creating a lot more surface area for absorbing oxygen and nutrients.

Because diffusion through the skin is so important, flatworms can’t dry out.  Therefore, they live in moist environments (water, tropical soils, inside other animals, etc.). And many also have cilia on the body surface – little moving “hairs” that are usually used for movement (like little oars or propellers) or for moving food particles closer to the body.

I guess it falls in the realm of a thematic element this week, but it is worth noting that some planaria use penis fencing during their “courtship” – that’s right, penis fencing.  Planaria are hermaphrodites, so during reproduction individuals compete over who gets to be the “male” and who gets to be the “female”. The male impregnates the female and, well, you get the rest… I’m not sure which side is considered the “winner” is in this little duel. Probably best not to speculate…

Penis fencing between two planaria – each has 2 penises (the white protrusions) (from Wikipedia Commons)

planaria penis fencing

Lastly – one of the most interesting aspects of flatworms is that if you cut them up, each piece will regenerate into a new individual!  Ever the scientist, Darwin could not let this well know experiment pass him by. He is a description of some of his experiments:

“Some specimens which I obtained at Van Diemen’s Land, I kept alive for nearly two months, feeding them on rotten wood. Having cut one of them transversely into two nearly equal parts, in the course of a fortnight both had the shape of perfect animals. I had, however, so divided the body, that one of the halves contained both the inferior orifices, and the other, in consequence, none. In the course of twenty-five days from the operation, the more perfect half could not have been distinguished from any other specimen. The other had increased much in size; and towards its posterior end, a clear space was formed in the parenchymatous mass, in which a rudimentary cup-shaped mouth could clearly be distinguished; on the under surface, however, no corresponding slit was yet open. If the increased heat of the weather, as we approached the equator, had not destroyed all the individuals, there can be no doubt that this last step would have completed its structure. Although so well-known an experiment, it was interesting to watch the gradual production of every essential organ, out of the simple extremity of another animal.” (Voyage of the Beagle)

Anyway, what really excited Darwin about his discoveries in Brazil was that he found some planaria species were terrestrial.  More on that tomorrow… (RJV)


  1. […] he started to really observe behavior and function. He dissected flatworms and lighting bugs (see Tolerably Contented with Planaria), be looked for cause and effect, he tried to explain why fossil bones looked similar to, but yet […]

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