ACOELOMATE EVOLUTION, 3, FLATWORMS

Flatworms: planarians, flukes, tapeworms

The above sounds less pretentious than Platyhelminthes: Turbellaria, Trematoda, and Cestoda.  For purposes of showing the ancestral links from protozoa through sponges, corals, jellyfish, flatworms and an unknown intermediate (probably a ribbon worm) link to annelids then deuterostomes, the list is more comprehensible.  Side shoots from the ancestral line to chordates leave far more passed than are listed. 

Planaria

Planarians are the turbellarians most like the ancestral link of interest.  Flukes and tapeworms are of interest too, but are all parasites and neither is a link to any other major group.  The common planarian has been an example of flatworms for many students because it is abundant world-wide. 

Marie Jenkins (1963) made an observation of planarians that may be instructive.  She found ones that were cultured in a slippery container could not divide by pulling themselves apart to regenerate two new planarians.  They just kept growing longer until they eventually formed a second head at the distant tail end.  Apparently, the head releases some chemical messenger that inhibits head formation until it is too far away to be in an effective concentration.

Other turbellarians much smaller than planaria have an anterior mouth, are not flattened, and have new mouths and a fission plane develop before separation.  The preparations for division can produce a chain of connected potential individuals up to 16 in number.

Possible descendent groups

Small flatworms may have given rise to Aschelminthes, including a branch becoming the nematodes and another branch, the rotifers.  An intermediate group, the Gnathostomulida, have some flatworm characteristics, and like most aschelminths are adapted to life in sediments of the sea and freshwater. 

The central selective action shaping the aschelminths was their adaptation to the interstitial water (water filling the spaces between sand grains and other small particles of the bottom of aquatic habitats and beaches) where small size enabled their movement while excluding slightly larger predators.  Besides their minute size similar to large protozoans, they often have a forked posterior with each short branch having adhesive glands.  In the nematodes the posterior toes are missing but some have a pair of gland cells and can attach temporarily to the substrate.  Many parasitic nematode species are much larger than their microscopic free-living relatives.

A feature of aschelminths that make it very unlikely they were ancestral to any mainline animals is the fact that they lack the ability to regenerate, probable because they adapted to miniaturization by reduction of chromosomal material as cells of the embryo differentiate into the adult.  A specific number of cells and or nuclei are found in adults of some smaller aschelminths.  Nematodes are unusual in lacking cilia; having only longitudinal muscles in the body wall; and having those muscles enervated by muscle cell processes (the processes lack muscle fibers) that reach either the dorsal or ventral nerve to receive the nerve impulses.  Rotifers are very numerous in lakes and their sediments and beaches.  Pennak (1978) describes in his introductory material the importance of the interstitial habitat as a route for some smaller organisms as they adapt and invade fresh water.

And the likely link

The nemerteans are thought to be descendants of flatworms also because some of them have rhabdites in their ciliated epidermal cells.  But nemerteans, like most aschelminths, have added an anal opening to the digestive system.  Nemerteans also have a blood vascular system so it is possible they were part of an ancient complex derived from flatworms that served as intermediates on the way to annelids at an early stage in the evolution of higher animals.  The living nemerteans do not have clear evidence of an ancestral role, but they are most representative of living animals approximating an intermediate form.  In my unpublished 2010 manuscript, Evolution Insights, I refer to the putative ancestor as a protonemertean.

The hypothetical protonemertean may have used a central branch of the turbellarian gastrovascular system to complete the digestive system with the posterior opening never disappearing after the individuals divided.  Lateral branches of the gastrovascular system may have lost their connection to the gut and become modified into blood vessels.  The benefit of a long body may have been the selective force keeping individuals attached as they evolved coordination as one organism, becoming the annelid worm central to the remaining major phyla evolution.

The soft body of this step in evolution may not have left a fossil record.  The steps along the way may not have anatomically instructive living descendants.  The answer may be in carefully targeted molecular phylogeny studies.  I don’t expect to have another eureka event like the one that made me see the pogonophorans were the missing link between annelids and the deuterostomes.

References

Jenkins, M. M.  1963.  Bipolar planarians in a stock culture.  Science, 142:1187.

Pennak, R. W.  1978.  Fresh-Water Invertebrates of the United States. 2^nd Ed.  Wiley, New York.

Joseph G. Engemann    July 22, 2013      minor editing November 7, 2014; also this note that the features noted in the post about nematodes, almost certainly precludes nematodes from an ancestral role in the Ecdysozoa, as also noted in other posts.