THE ANNELID THEORY
A major focus of my posts is clarifying why the abandoned annelid theory of chordate origin should be reinstated. People don't abandon theories without reason. But reasons for abandoning a theory may have a validity that doesn't extend to all instances where they are applied.
A theory discredited before publication
An undergraduate research project by a former student, studying the protozoan Stentor, provided new observations for us that got us quite excited. The protozoan is one of the larger protozoans that can be seen, although poorly, with the naked eye. They are usually attached to solid surfaces in their aquatic environment. Like many other ciliates, they have a macronucleus with many (sets of?)chromosomes, and a micronucleus with one set. In the photo below you can see the trumpet like shape they have when attached and the beaded macronucleus inside. They use cilia to circulate water to the large oral end and filter out food particles.
Figure above. Two Stentor protozoans. The macronucleus is elongate and enclosed within a membrane that is constricted to make the macronucleus appear beaded.
The observation that excited us was the presence of many free-swimming
Stentor's taking on the shape typical of the mobile state. There was a concentrated population and many had a large central enclosure, like the picture below shows in the upper
Stentor, that encloses another
Stentor.
Figure above. Free-swimming Stentor with an enclosed Stentor.
Some ciliates are known to reproduce by producing internal buds. But it had never been reported for this protozoan. So he was excited when he told me about it. He had even dissected one to release the internal bud and when released it swam away.
Observing the process for a day or so we finally came to the awareness that it was not a bud. It was another
Stentor that had been swallowed whole. Apparently, cannibalism occurs when food supply decreases in crowded populations.
The annelid theory
The annelid theory was popular a hundred years ago. It may never have been universally accepted. But it was as good as any competing theory of chordate ancestry at the time. The clincher responsible for its rejection was the observation that developmental stages often are a great clue to relationships that are not evident in adult organisms.
The annelid theory had support for the correspondence of relationships of systems of annelids and inverted chordates, or vice versa.
Two Figures above. Annelid/Vertebrate systems as illustrated in early 20th Century texts, the upper in Romer and the lower in Lull, with one inverted so you don't have to turn your monitor upside down.
The drastic change in embryology of annelids versus chordates was used to discredit the theory. But the evidence I present of the probable role of pogonophorans in expediting the shift should be sufficient to reconsider the annelid theory. Gould* presented a good assessment of why it should be reconsidered due to a better understanding of the limitations of the "biogenetic law".
In reviewing volume five of Hyman's
The Invertebrates (1959, McGraw-Hill, New York), I found on page 224 the beginning statement on relationhips of pogonophorans read "It is not open to doubt that the Pogonophora belong to the Deuterostomia." Later she says "the Pogonophora appear most closely related to the Hemichordata." Earlier, on page 201, she said about the hemichordate-chordates, "Such identity is inconceivable except on the basis of a common ancestry. Hence a phylogenetic relationship between hemichordates and chordates is not open to question."
Those comments were made before the evidence of relationship of pogonophorans to earlier polychaete annelids was discovered.
A bit about Libbie Henrietta Hyman
Libbie Henrietta Hyman was perhaps the most outstanding zoologist of the last century. She produced a comparative anatomy manual used by many of the pre-med students of her era. Then, in her position at the American Museum of Natural History in New York, she produced a multi-volume treatise on invertebrates that remains a remarkable resource.
In the final chapter of volume five discussing new developments of topics in earlier volumes she uses colorful language to express definite opinions. For example on page 750 - "The author hoped that the enterocoel theory was dead and buried, as it deserves, but . . .". Then "The author regards the enterocoel theory as fantastic nonsence, for which there does not exist a single scrap of genuine evidence."
She criticizes views putting
Proterospongia in line with the origin of sponges. She provides arguments that the anthozoans cannot be the earliest cnidarians. Both of those views are contradicted in earlier posts of this blog.
She had command of a vast amount of biology of organisms so her opinions have had considerable influence. She proposed putting several phyla in a natural grouping she named Aschelminthes. It had some acceptance until some experts in some of the groups disagreed, perhaps they thought it minimized the importance of the groups they researched. So she later abandoned the view although I think it was correct in the inclusion of most of the groups it included.
A lesson learned
The short life of many ideas and theories, changed because of varying emphasis on major or minor features, gives me pause when I think about my evolutionary ideas. I have learned from the evidence to support the coalescence of the annelid theory as correct in the connection suggested, that gross features can be important. They can also mislead, as the importance of radial versus bilateral symmetry did in forming major clusters of phyla; the transition either way among the two symmetries is eventually selected by the habit of being attached to a substrate versus having active movement over a surface. So it is difficult to state unequivocally that some parameter is the ultimate measure of the direction of relationships.
*Gould, Stephen Jay.
2002.
The Structure of Evolutionary Theory. The Belknap Press of
Harvard University
Press,
Cambridge, Massachusetts. 1433 pp.
Joseph G. Engemann March 2, 2015
Emeritus Professor of Biology, Western Michigan University
