As noted yesterday, Darwin’s real excitement in finding flatworms came from the fact that he “discovered” two new species that lived on land rather than in the water (i.e., terrestrial planaria). From his reaction, the idea of planarians that did not live in the water was a new one to the world of biology. It was such a radical idea, that Darwin’s friend and mentor, John Henslow, assumed that he had incorrectly identified them. After leaving Rio in July, he writes to Henslow:
“Amongst the lower animals, nothing has so much interested me as finding 2 species of elegantly coloured true Planariæ, inhabiting the dry forest! The false relation they bear to Snails is the most extraordinary thing of the kind I have ever seen.— In the same genus (or more truly family) some of the marine species possess an organization so marvellous.—that I can scarcely credit my eyesight.— Every one has heard of the dislocoured streaks of water in the Equatorial regions.— One I examined was owing to the presence of such minute Oscillaria that in each square inch of surface there must have been at least one hundred thousand present.— After this I had better be silent.— for you will think me a Baron Munchausen amongst Naturalists.” (Correspondence to John Henslow, July/August 1832)
Baron Munchhausen riding a cannonball by August von Wille, 1872:
Baron Munchausen, but the way, was an 18th century German nobleman who liked to tell exaggerated tales of his travels. For example, he told of “riding” a cannonball and visiting an island made of cheese. If you’ve never had the chance, be sure to checkout Terry Gilliam’s movie The Adventures of Baron Munchausen from the 80’s. Granted it has been a while, but I do remember it being entertaining. Anyhow… back to the very real and fanciful world of flatworms…
Geoplana burmeisteri from southern Brazil (from Wikipedia Commons)
The land planaria (family geoplanidae), like all planarians, have to stay moist, so they are commonly found under logs, amidst decaying vegetation or even in the soil. Many are predators (though there are some scavengers) who have been known to actively hunt other invertebrates that live in the soil. They have sense organs that allow the to detect smells, or follow trails left by slugs, snails, insects and even other planarians in the undergrowth. Once they catch their prey they coat them in their mucus, which contains digestive enzymes – thereby tenderizing their food before ingesting it.
Here is what Darwin had to say about his discovery after returning to England:
“The few observations which I was enabled to make were almost exclusively confined to the invertebrate animals. The existence of a division of the genus Planaria, which inhabits the dry land, interested me much. These animals are of so simple a structure, that Cuvier has arranged them with the intestinal worms, though never found within the bodies of other animals. Numerous species inhabit both salt and fresh water; but those to which I allude were found, even in the drier parts of the forest, beneath logs of rotten wood, on which I believe they feed. In general form they resemble little slugs, but are very much narrower in proportion, and several of the species are beautifully coloured with longitudinal stripes. Their structure is very simple: near the middle of the under or crawling surface there are two small transverse slits, from the anterior one of which a funnel-shaped and highly irritable mouth can be protruded. For some time after the rest of the animal was completely dead from the effects of salt water or any other cause, this organ still retained its vitality.” (Voyage of the Beagle)
Pseudogeoplana reticulata from southern Chile (from Wikipedia Commons)
There are several reports about the importance of these planarians to the health of the soil, and some types are considered “indicator species” for soil ecosystem. Indicator species are the “canaries in the coalmine” – they are often the first to be impacted when the environment changes and can signal a change that may be harmful to the whole ecosystem. In other words, when the planaria ain’t happy, nobody’s happy.
There are over 800 known species of land planaria, yet surprisingly there has not been a lot of research done on this family of organisms and a lot still remains to be discovered. Darwin got the ball rolling by discovering them almost 200 years ago – now its time for some new young scientists to take over.
Geoplana ladislavi from southern Brazil (from Wikipedia Commons)
By the end of the day today, June 28th, Darwin was back on board the Beagle – his home away from home. And from the sound of things, there was a certain comfort to returning to his cabin:
“Removed all my things from shore & am now once again in the intricacy of my own corner, writing this journal.— It is something quite cheering to me to hear the old noises.— the men foreward singing; the centinel pacing above my head & the little creeking of the furniture in the Cabin &c.” (June 28)
He was about ready to move on to the next leg of his adventure. (RJV)