Animal Form and Function 1: Cnidarians and Flatworms

I’m a big fan of this sentiment:

Being a geek is all about being honest about what you enjoy and not being afraid to demonstrate that affection.  It means never having to play it cool about how much you like something.  It’s basically a license to proudly emote on a somewhat childish level rather than behave like a supposed adult.  Being a geek is extremely liberating.  –Simon Pegg


I try to keep Pegg’s axiom in mind when I’m teaching.  Every winter, I teach the laboratory portion of a course in animal diversity, much of which consists of dissections.  Every week during that course, my students dissect one or more specimens from selected animal phyla and observe without cutting preserved specimens of several more.  Each week comes with a theme, such as convergent evolution, divergent evolution, or metamerism, and ideas such as the state of the body cavity pervade the entire course as well.

Every session of that class where the time is available, I spend the last 15 minutes or so showing videos, still images, and articles about the weirder and more wonderful aspects of the animals within that session.  For many students, classes like these are their one and only encounter with creatures like ragworms and tarantulas.  Augmenting the dry and often harried perspective offered by detailed dissection guides and midterm exams with the novelty and wonder of their living forms is one way that I try to keep this course interesting even for (gasp) pre-medical students who don’t necessarily enter the course brimming with enthusiasm about crayfish and sea stars.

Infectious enthusiasm is the best teaching aid there ever was, and I bring it to my classes even if I have to add my own material to an established course.  This is that material.

The first session in which I have time for these supplements deals with cnidarians (corals, sea jellies, and sea anemones) and flatworms.

One theme that appears again and again with the cnidarians in particular is symbiosis.  A LOT of cnidarians have algae living inside their tissues, called zooxanthellae, to the point that many species can rely entirely on these symbionts for their sustenance and never capture outside prey.  Many coral reefs are composed of corals that rarely eat and instead rely on local shallow waters to bring them enough light to survive, grow, and build their colonies.  Climate change presents a several-fold insult to such animals.  A deeper ocean means less light reaches the corals; more frequent storms bury reefs in silt; and more carbon dioxide in the atmosphere acidifies the ocean and dissolves shell material.

Yet corals are not even the most striking example of this symbiosis, in this writer’s opinion.  That honor goes to the inland jellies of Palau.

The jellies in these lakes lived on an ordinary reef in recent geologic memory, but became trapped in an inland lagoon as the reef turned into an atoll.  Such a lagoon, cut off from the ocean, tends to lose much of its diversity, as various species get cut off from their food supplies, breeding grounds, and other resources.  The jellies could easily have gone that way after extirpating their prey.  Instead, their symbiotic algae saved them.  These jellies no longer eat, or even sting, and follow the sun across the surface of their lakes to keep their zooxanthellae fed.  Naturally, swimming with them is among the tourist options in sunny, former-US-protectorate Palau.

Another idea I like to get across is the impressive size of many cnidarians.  The animals encountered in a dissection course are, for obvious reasons, typically quite small, and North American students are rarely in a position to see the most impressively sized examples of common animal phyla.  Cnidarians in particular, usually encountered as minuscule polyps in a colony or as manageably-sized anemones for aquaria, often convey the mistaken impression that they are all about the size of a kitten.  The giant sea anemones of the Caribbean, as wide across as serving platters, are still not as good at disabusing students of that idea as the lion’s mane jelly, Cyanea capillata, shown here:

The record-holder for this species had a bell 2.29 meters across and tentacles 37 meters long.  This is longer than the longest blue whale ever recorded and one of the contenders for the longest animal, period, known to science.  Fortunately for non-jelly-enthusiasts, the lion’s mane is confined to polar latitudes.

Another ongoing misconception about cnidarians is that they are mostly immobile.  Only the jellies have obvious locomotor functions, and the other best-known cnidarians–corals–are indeed unable to move around.  Anemones, however, are highly muscular creatures, as this session’s dissection reveals, and can crawl around on their pedal bases.  Surprisingly, however, they can also swim:

They’re rather clumsy, to be sure, but they do swim.

Flatworms are another interesting bag.  Traditionally regarded as one of the most primitive animal phyla, the flatworms now appear to be about as advanced as mollusks and annelids, their “primitive” body plans likely a convergent characteristic.  The majority of known flatworm species are parasites, including tapeworms, flukes with complicated two- and three-host life cycles, and a lesser-known group that commonly attacks fish.  Free-living flatworms are rarely spotted in nature, as they are small, secretive animals found at the floors of ponds and under rocks.

Unless one visits the tropics, that is.
Immediately, one can see that this thing is not the slug it otherwise might resemble, with its horseshoe-shaped head auricles and lack of a distinct muscular foot.  Even this animal, however, cannot compete with the delirious beauty of many marine flatworms.

Unlike the common laboratory species Dugesia, the undulating swimming pattern of the marine flatworm is usually unequivocal, and they combine Dugesia‘s strangely cute charm with the sorts of bold colors that only the tropics can provide.

No visit to the marine flatworms would be complete without a look at their surreally violent mating behavior.

Flatworms are all hermaphroditic, possessing both testes and ovaries.  When flatworms meet during their breeding season, they spar, sharpened penises flared and wielded as weapons in what ethologists call with straight faces “penis fencing.”  The one that manages to wound the other also inseminates the other, which then goes off to nurse its wounds with the flatworm’s nigh-legendary regenerative capacity.This video also displays another aspect of flatworm biology: their acoelomate body plans.  Flatworms do not have a distinct body cavity in which their organs hang the way vertebrates do.  Instead, they are composed of solid muscle and connective tissue all the way through, with their organs embedded therein.  This was long thought to be a primitive characteristic, but modern evidence suggests that flatworms instead developed this trait secondarily, reverting from a more obviously derived body plan.  Regardless of its history, one consequence of an acoelomate plan is that, when a flatworm is wounded, it has no circulatory fluid or opening into its deep interior to contend with thereafter.  Instead, it simply leaks a bit of its internal tissue, as seen in this video.

Future installments in this series will share my instructional aids for other animal phyla, including mollusks, annelids, arthropods (lots of those), echinoderms, and chordates.  Stay tuned 🙂
Animal Form and Function 1: Cnidarians and Flatworms