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. 

ACOELOMATE EVOLUTION 2 CNIDARIANS

Cnidaria: corals, anemones, hydroids, jellyfish

The Cnidaria are characterized by production and use of nematocysts.  The nematocysts are complex organelles contained in certain cells of cnidarians.  The previous post suggests how they may have developed from modifications around the spicules inherited from ancestral sponges. 

Two main body types are found in the sexual stage of cnidarians.  The polyp or hydroid body type is the original type if the findings of Kazmierczak (1984) are accepted.  Generations of biologists assumed something like the simple hydra was one of the earliest cnidarians.  The new evidence makes an extinct coral the likeliest candidate for the ancestral origin of cnidarians.  Corals and other in the class Anthozoa have the polyp stage predominating, and no medusa (or jellyfish) stage.  Those in the class Hydrozoa usually have both polyp and medusa stages.  In the class Scyphozoa the jellyfish stage dwarfs other stages.  The medusa stage is the sexual adult stage in cnidarians having a medusa. 

Tentacles, furnished with many nematocysts, and partitions or tubes in the digestive cavity (gastrovascular cavity) in considerable variety are often part of structural diversity of cnidarians.  The medusa stage was recognized by early biologists as having a very similar structure to an inverted polyp stage.  So, thinking of a coral-like polyp stage as the starting point in pre-Cambrian seas, it is easy to imagine an early extinction event making life for the coral so difficult that its polyp, released from the coral skeleton, managed to survive a marginal extinction event to preserve genes enabling such a release.  Ultimately, repeating the process eventually produced forms with medusae dominating the life cycle.

Besides the microscopic features such as the spicule-nematocyst connection, gross features providing a base for determining selection for the sponge-cnidarian transition involved a number of events. (1) The attached bottom dwelling lifestyle was conducive to retaining radial symmetry.  (2)  The upward facing osculum of the sponge provided an opening for gradual evolution of a mouth and transition of the spongocoel to a gastrovascular cavity as adaptations for acquiring larger particulate food developed.  (3) Spicule deposition shifted from the generalized sponge skeletal elements to the external cup-like coral skeleton.  Adaptations for muscles, nerve, and other new and useful soft structure elements optimized for size made vegetative growth of colonies by budding a suitable solution.

Extinct tetracorals were common early fossil corals.  Prior to or along with their radiation into the vast range of anthozoan hard corals, soft corals, and anemones, it is likely that they led to the hydrozoan medusae that were the ancestral hydrozoans.  The four radial gastrovascular canals and related parts may be due to the square cups of tetracoral skeletons affect on selection/development of soft parts.

The hydra is one of the hydrozoan polyps.  Several nematocyst types are found in hydrozoa, about four kinds in hydra.  Hydra is specialized for fresh water existence by loss of the medusa stage.  The freshwater jellyfish retained the medusa stage but the polyp stage does not have tentacles; their polyps bud from a connection in the sediment, some polyps bud off medusae but most have a mouth for feeding.  Similar hydrozoan medusae are found in salt water species.

Tracing vertebrate roots through cnidarians

Just as sponges underwent much diversification after giving rise to cnidarians, cnidarians gave early rise to the forerunner of the flatworms.  The prevailing opinion that phylum Cnidaria begin at a rudimentary stage is incorrect.  The structure of the hydrozoan medusa needed relatively little modification beyond elongation to produce the ancestor of the common planarian.  The centrally located manubrium of the jellyfish is positioned similarly to the proboscis of planaria.  The four branches of the gastrovascular cavity are reduced to the three in the planaria; the fourth was eventually lost due to inability to develop in the compressed space above the proboscis.  The sensory complexes were lost in all locations except where the head developed at the anterior or forward branch of the gastrovascular cavity. 

Hadzi (1963) noted the similarities of flatworm and medusa and proposed the flatworms as intermediate between protozoans and cnidarians, a view that has not been accepted.  Ax (1963) seemed to think the long evolutionary history of existing groups precluded any from being ancestors of any other major group.  But successful adaptations, in my opinion, could very likely persist as their variations give rise to great diversification and other phyla.  The spin-off of new groups was easier before adaptations became well fixed or stable.  Such a series of events in the early history of animal diversification seems more compatible with the Cambrian “explosion” of animal groups.

As the jellyfish body flattened and elongated for bilateral and mobile life as a flatworm, the outer longitudinal muscle fibers and inner circular muscle fibers are now known and positioned as outer circular and inner longitudinal muscle layers.

The nematocysts took on more degenerated and/or restricted function as the rhabdites of the flatworm epidermis.  Other developments added to the complexity of flatworms.

References

Ax, P.  1963.  Relationships and phylogeny of the Turbellaria.  Pp. 191-224 in E. Dougherty.  The Lower Metazoa.  Univ. of California Press, Berkeley.  478 pp.

Hadzi, Jovan.  1963.  The Evolution of the Metazoa.  Macmillan, New York.  499 pp. 

Kazmierczak, Jozef. 1984.  Favositid tabulates: evidence for poriferan affinity.  Science, 225:835-837. 

Joseph G. Engemann    July 19, 2013  

ACOELOMATE EVOLUTION, 1, SPONGES

The evolutionary lineage of our post-protozoan ancestors can be followed through three phyla that left many existing related groups.  The related groups diversified into forms, most of which are not in our ancestral line.

PORIFERA, THE SPONGES

The first post-protozoan phylum is the sponges.  They are so different from other animals that many have thought they were an evolutionary dead end or side-shoot that was not in the mainstream of evolution.  The peculiar amphiblastula larva of some sponges was so different that it seemed to preclude them being in the mainstream.  But Bergquist (1978), in her book on sponges, illustrates a wide variety of sponge larvae, some of which have great resemblance to the planula larvae of some cnidarians.  The mainstream position of sponges was proposed by Tuzet (1963).

If we do not consider the sponges as in the mainstream, it would be necessary to postulate a similar organism as an intermediate form between protozoans and cnidarians.  For more on the intermediate nature of sponges you can find some discussion in the introduction to sponges in the second (1968) and third (1981) editions of Invertebrate Zoology which I edited.  I later found research describing spicules in nematocysts of some cnidarians and it made sense in terms of a spicule, nematocyst, and rhabdite transition found in sponges, cnidarians, and flatworms.

Spicules

Spicules, both calcareous and siliceous ones, found in an early fossil anthozoan (Kazmierczak, 1984) show cnidarians are most likely derived from a sponge most like the Sclerospongea which also have both types of spicules.  The Calcarea, but not the Hexactinellida or Demospongea, may have had an earlier ancestral role, but it is just as likely a primitive member of the Sclerospongia was in that ancestral role.  Hexactinellida have only siliceous spicules, Demospongea have only siliceous spicules if spicules are present (bath sponges in this group lack spicules).

As sessile (attached) animals, sponges were dependent for survival on the selection of things that made them unpalatable to new and mobile predators.  A diversity of toxic substances are found in today’s sponges.  Spicules also may deter predators.  Spicules also served as skeletal elements preventing collapse of the canal systems essential to sponge growth to larger forms.

Protruding spicules increased the sponge’s ability to passively deliver adhering toxins.  Improvements in partially enclosing the spicule in a tube of poison allowed selection for increasingly more effective delivery as the evolution to cnidarians continued.  The nematocysts of most cnidarians may have lost the need for the spicule once the tubular delivery system for poison was effective.

Nutrition

Sponge nutrition involves trapping of small food particles by the collar of the flagellated cells lining some of the chambers of the sponge.  The particles are taken into the cells for digestion.  Water transporting the food particles passes in through small openings (the ostia) on the surface and passes through the chambers with the flagellated cells before exiting through a larger chamber and a large opening, (the osculum).  Sponges may have symbiotic, photosynthetic microorganisms in their tissues.  These were especially important for nutrition during the early evolution into cnidarians before the capacity for feeding on larger living organisms developed. 

Other features

As indicted in a previous post, sponge tissue structure is much like loose connective tissue of vertebrates.  Rudimentary muscular and nervous cellular structure has been observed.  Sexual reproduction is present and eggs produce ciliated or flagellated larvae.  Asexual reproduction occurs in several ways.  Internal buds are produced by some sponges, fragments can regenerate new sponges, and bath sponges can regenerate from a portion left attached to the sea bottom.

The sponges of today are mostly specialized in ways that do not indicate the ancestral position of an unknown ancient member of the Sclerospongea, already specialized in its own way.  Living Sclerospongea were discovered in the last half of the last century; they live hidden away in cavities in coral reefs and are presumably quite different from the ones that evolved into cnidarians.

After listing seven points supporting the ancestral role of sponges, but before the 1984 report by Kazmierczak, I made the following statement in the 1981 invertebrate text:  “In view of the preceding it is reasonable to anticipate further evidence of an important phylogenetic position for ancestral sponges with hopes for clarification or resolution of this issue.”  The discovery of a spicule at the apex of some nematocysts is further new evidence.  The conclusions presented are based in part on hypothetical interpretations of the role of spicules in the absence of any other reasonable hypothesis.

References

Bergquist, P. R.  1978. Sponges.  Univ. of California Press, Berkeley.  268 pp.

Engemann, J. G., and R. W. Hegner.  1981.  Invertebrate Zoology, 3^rd ed.  Macmillan, N. Y.  746 pp.

Kazmierczak, Jozef. 1984.  Favositid tabulates: evidence for poriferan affinity.  Science, 225:835-837.
Tuzet, O.  1963.  The phylogeny of sponges according to embryological, histological, and serological data, and their affinities with the Protozoa and the Cnidaria.  pp. 129-148.  In E. Dougherty, The Lower Metazoa.  Univ. of California Press, Berkeley.  478 pp.

Joseph G. Engemann     July 18, 2013