Snails & Fairy Tales


Danish author Hans Christian Andersen, who is most famous for writing beloved fairy tales such as “The Little Mermaid”, “Thumbelina”, “The Snow Queen”, “The Ugly Duckling”, and “The Emperor’s New Clothes”.  Image credit: Thora Hallager via Wikipedia.

The Curious Sengi recently spent a few days in Copenhagen, snurfling about that wonderful old city.  Going to visit the iconic statue of The Little Mermaid (Den lille Havfrue) was nowhere near the top of my list of things to do, but since I was already walking the ramparts of the star-shaped citadel of Kastellet, I dropped down to the harbor below to take a look.  For a brief moment, I saw her:  alone, looking sadly across the murmuring waves.  But even the wet chill of mid-autumn did nothing to deter tour buses from roaring up the drive and disgorging scores of tourists.  They gawk.  They pose with big smiles and snap selfies.  For a few, maybe this is the fulfillment of a dream.  For others, it is just another check mark closer on the itinerary to lunch.  But standing back and watching the rowdy proceedings, you cannot help but feel a bit pained.  She’s such a poor little slip of a thing, caught in the moment of her greatest despair.


These daily disturbances are the least of the insults The Little Mermaid statue has endured. She has been variously decapitated, dismembered, blown up, and drenched in paint. As an easily accessible and visible tourist attraction, the statue has become the focus of protest statements and simple vandalism (Wikipedia 2016).  Image credit: Curious Sengi.

Though there is a growing awareness that the fairy tales put on the big screen by Disney are intensely sugar-coated variants of the original stories that inspired them, I had to wonder how many of those tourists really knew “The Little Mermaid”, written by Danish author Hans Christian Andersen (1805 – 1875).  I certainly had to refresh my memory.  The story is bleak.  Let’s just say that after being abandoned by the prince for whom she had sacrificed so much, the only happiness the Little Mermaid will ever experience is the release of death and the promise of an immortal soul. . . . . or disintegration into sea foam.  I assume there is no noteworthy distinction there.  Andersen’s story is complex, haunting, and even ambiguous.  Then again, so was Andersen himself.

A vignette of Andersen’s life can be found in a small collection of shells at the Natural History Museum of Denmark in Copenhagen.  He began collecting local land snail shells because of Jonas Collin, the son of his close friend, Edvard.  Jonas had taken up zoology and enthusiastically collected snails, an endeavor that filled their lodgings with the stink of the boiled creatures and preservative spirits when Andersen and Jonas traveled together in 1861.


Shells of land snails collected by Andersen during his travels throughout Denmark.  The Natural History Museum acquired this collection in 1905.  Image credit: Natural History Museum of Denmark / Curious Sengi.

During this journey, the much younger Jonas proved to be “harsh, insulting and assertive”; the constant arguments and ungrateful attitude on Collin’s part was shattering to Andersen, who confided his tearful feelings to his diary.  But within this tumultuous relationship between the imperious young naturalist and the older doting author, there were moments of happiness.  At the end of their travels together, the pair parted amicably and Andersen would continue poking around gardens with newly acquired interest to find more specimens to send to his friend.  Andersen included this note in a letter to Jonas’ mother:

Yesterday I found a beautifully coloured snail.  I thought immediately of Jonas and took it up to my room, then went to lunch; but when I went back upstairs, the snail was nowhere to be found.  I searched and searched, then found the beast had crept up the back of the table.  I put it in my soap dish and put a lid over it.  Last night, I wanted to make my specimen, boil the snail, remove the body and keep the shell for my scientific friend, but when I came to retrieve my victim, it had disappeared again, and this time it was lost for all time.  The maid had cleaned up and thrown the snail out the window, but Jonas will see that I was thinking of him, and that will appeal to him (Andersen to Henriette Collin 5 June 1862; quoted from Strager 2014).

Upon hearing the news, Jonas responded rather like a pedantic brat:

That you collect snails for me is very touching, but the fact that you let them run away again, in my opinion, balances out the praiseworthy.  I would very much appreciate snails from Basnaes.  You can keep them alive in a dry box with small holes in the lid.  They survive for years without food by sleeping (quoted from Strager 2014).


Image credit: Natural History Museum of Denmark / Curious Sengi.

Though Andersen’s snail collecting eventually slowed to a halt (Strager 2014), he was mindful of pleasing Jonas, almost to an obsequious degree.  Sometimes the young man expressed gratitude, but it can be puzzling why Andersen was so invested in this unbalanced relationship.

One possible way of understanding Andersen’s relationship with Jonas is to acknowledge the intense, and ultimately unreciprocated, feelings that Andersen had for Jonas’ father, Edvard.  There can be a long discussion about whether or not Edvard Collin was once seen as a lover (Bom & Aarenstrup 2015), but it is clear enough that Andersen put both men and women at the center of his romantic and erotic attentions.  In his private writings, Andersen expresses his deep yearnings, all of which seemed to end in rejection and celibacy (Bech 1998; Bom & Aarenstrup 2015).


Andersen with Jonas Collin, around the time of their snail collecting venture.  Image credit: Bordeux Barberon via Wikimedia Commons.

I wonder if during his rambles with Jonas, Andersen learned that snails are hermaphrodites, capable of reproducing as both males and females, sometimes simultaneously.  Andersen seems to have seen himself as androgynous (Bom & Aarenstrup 2015).  Natural history would hint at a sympathetic connection between Andersen and snails — another unassuming, ugly duckling-like character that Andersen identified with.  And yet he cast the snail as an egocentric curmudgeon in the story “The Snail and the Rosebush” as a jab against certain philosophers in his acquaintance (Strager 2014).  No soft spot for snails there and, thus, our natural history fairy tale falls apart.

Like “The Little Mermaid”, the story of Andersen’s life was filled with much more pain and complexity than is perhaps understood.  His collection of snail shells in the Natural History Museum of Denmark is but a small glimpse of that life.


Statue in Bratislava, Slovakia commemorating Andersen and some of his famous fairy tale characters, including the grumpy snail.  Image credit: weepingredorger.

Read Andersen’s short story:  “The Snail and the Rosebush


Bech, Henning.  1998.  “A Dung Beetle in Distress.”  Journal of Homosexuality 35 (3-4):  139 – 161.

Bom, Anne Klara & Anya Aarenstrup.  “Homosexuality.”  H.C. Andersen Centret:  FAQs.  Syddansk University, 11 August 2015.  Accessed 27 November 2016.

Strager, Hanne.  2014.  Precious Things:  The greatest treasures of the museum.  Tam McTurk, translator.  Gylling:  Narayna Press.

Wikipedia contributors.  “The Little Mermaid (statue).”  Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 24 November 2016.  Accessed 27 November 2016.


Mantles Most Splendid: Tridacna spp.


Giant clams (Tridacna spp.) on display at the California Academy of Sciences in San Francisco.  There are more or less seven species of giant clams inhabiting the shallow tropical reefs of the Indo-Pacific (Klumpp, Bayne, & Hawkins 1991).  Image credit: California Academy of Sciences / Curious Sengi.

The elegant shapes and colorful patterns of shells often define the identity of the molluscs we encounter and gleefully collect.  But what we hold in our hands is a largely inert structure of calcium carbonate, devoid of its occupant, its essence incomplete.  What is missing are the squishy bits — the animal itself.  The soft tissues of clams and snails can be breathtaking in their colors and patterns.  One of the better known examples of this are the giant clams, Tridacna spp.


Discarded shells of the largest clam in the world, Tridacna gigas.  Image credit: David Hall via Arkive.

Residing the shallow sunlit seas of the tropical Indo-Pacific, tridacnids are the largest clams in the world, with one individual measuring 1.4 m (4.6 ft) in length and 263 kg (863 lbs) in weight (Beckvar 1981).  While the shells have been collected as curios for their child-bathtub-sized novelty and undulating, fluted edges, there is little else that stands out about these shells.  What is mind-blowing about tridacnids are the vivid colors and patterns of iridescent blue, green, black, yellow, brown and violet of the exposed living tissue, the mantle.


Tridacnid spotted off Thailand.  Image credit: Walkabout Alex.


Get close to a world of color and fascination!  Detail of siphon and feathery gills.  Image credit: Fatherree via

What makes these fleshy frills so psychedelic is a complex interplay between the tridacnid clam and its unicellular dinoflagellate symbionts called zooxanthellae.  The zooxanthellae primarily reside in the upper layers of tissue of the enlarged mantle and siphon, where they are exposed to sunlight and photosynthesize.  The simple sugars that are the result of photosynthesis — glucose and glycerol — are released into the bloodstream, thus providing a source of energy for the tridacnid (Goreau, Goreau, & Yonge 1973; Morton 1978; Fitt & Trench 1981; Wilkins 1986; Buck, Rosenthal, & Saint-Paul 2002).  Generally, clams are filter feeders, drawing in water and sorting out all the plankton and organic particulates for digestion.  While giant clams retain a fully functional and efficient filter feeding apparatus, the clear, nutrient-sparse waters of the tropics has likely driven the evolution of symbiotic relationships that provide alternate energy sources.  Adult tridacnids can probably sustain themselves on photosynthates alone and it is hypothesized that this influx is behind the spectacular growth rate of approximately 100 mm (4 in) per year observed in the species Tridacna gigas (Bonham 1965; Klumpp, Bayne, & Hawkins 1991).


Zooxanthellae live as free cells into a branched tubular system running under the surface of the mantle and siphon tissue (Buck, Rosenthal, & Saint-Paul 2002).  Giant clams acquire symbiotic zooxanthellae through the environment; at around 20 to 25 days old, after the floating larvae settles into a more benthic post-metamorphic state, the clam ingests motile zooxanthellae.  How the symbionts are transferred from the stomach into the visceral and mantle tissues is unknown.  Growth accelerates after acquisition of symbionts (Jameson 1976; Fitt & Trench 1981).  Interestingly, free-living zooxanthellae will keep all products of photosynthesis inside their cell, but the presence of host tissues causes photosynthates like glucose and glycerol to be secreted, suggesting some kind of chemical regulation by the host (Muscatine 1967).  The purple spots are reflective iridophores.  Image credit: Science Photo Library via Getty Images.


What does the nuclear tests off Bikini Atoll have to do with giant clams?   Bonham (1965) was able to calculate growth rates of T. gigas from Bikini Atoll by correlating dates of atomic tests and the deposition of radioactive strontium-90 in the growing shell.  Measuring from one radioactive growth line to the next, one specimen measured 10 cm long during the 1956 atomic tests and then 24 cm during the 1958 tests, indicating a growth rate greater than any other bivalve.  Image credit: Getty Images via The Independent.

So here is our first color component:  zooxanthellae use green chlorophyll and a red carotenoid called peridinin as photosynthetic pigments, rendering the single-celled organisms a brown or reddish-brown color (Fatherree 2007).


T. squamosa.  Image credit: Wikimedia Commons.

The host clam itself generates accessory pigments — greens, browns, and yellows — that extend the range of usable wavelengths of light available for the symbiont’s photosynthesis (Fatherree 2007).  In addition, specialized cells (iridocytes) form clusters in the mantle called iridophores that act both as reflectors to redirect light into deeper tissue and as a sunscreen to prevent the zooxanthellae from getting fried from intense sun exposure.  Each iridocyte contains a stack of crystalline reflective platelets that scatter light, which makes pigments appear more vivid, creates those eye-popping electric blue-green colors, and makes the mantle shimmer with iridescence (Goreau, Goreau, & Yonge 1973; Griffths, Winsor, & Luong-Van 1992; Fatherree 2007; Todd, Lee, & Chou 2009).


Iridescent sheen on the blue lip of this tridacnid.  Possible hybrid between T. noae and T. maxima.  Image credit: Jake Adams via Reef Builders.


Blue-rimmed T. derasa.  Image credit: Ora Farm.

Photosynthetic pigments of zooxanthellae, accessory pigments and iridophores of the host tridacnid clam completes the palette of colors.  But how are these colors arranged into striped, spotted, and mottled patterns?  While variation in color is caused by localization of zooxanthellae (Todd, Lee, & Chou 2009), the pattern-generating mechanism remains unknown.  Despite some broad parameters of patterning seen between species, the role of genes remain ambiguous.  Offspring are observed to have a tremendous variety of colors and patterns different from their parents, suggesting a weak genetic link (Fatherree 2007).


Bed of giant clams at low tide on Orpheus Island.  Image credit: goestaf via Panoramio.

Much of the tridacnid’s anatomy and behavior is focused on fostering the intimate relationship with their zooxanthellae symbionts.  The super-sized mantle and siphon blossom from the shell during the day and orient themselves to the light like flowers tracking the sun (Morton 1978; Wilkins 1986). This makes the exposed fleshy parts vulnerable to predation, especially from roaming fish looking for a quick bite.  Tridacnids do have several hundred well-developed pinhole eyes along the mantle and siphon that can detect shadows and moving objects, triggering immediate retraction and closing of the shell (Wilkins 1986; Land 2003; Todd, Lee, & Chou 2009).


Giant clam researcher, Dr. Rick Braley, demonstrates a defensive response in T. gigas — squirting powerful jets of water upon tactile stimulus (Wilkins 986).  Image credit: Aquasearch.

However, responding to each threat by clamming up is hardly beneficial to the zooxanthellae that need to be exposed for hours a day for photosynthesis.  Todd, Lee, and Chou (2009) hypothesized that the colors and patterns of the mantle are actually cryptic, breaking up the outline of the animal and camouflaging it against the cluttered background of the reef.  High levels of morphological variation, i.e., polymorphism, is common in organisms inhabiting complex and unpredictable environments.  Analysis of RGB color values of mantles and background reef substrate indicate some color matching camouflage against fish predators with trichromatic color vision.  That a given set of parents spawns wildly polymorphic offspring might be a bet-hedging strategy that potentially some of the dispersing larvae will match their given substrate.  This also allows for colonization of a large range instead of being limited to one type of microhabitat for which they are a perfect background match.


A study of color and pattern polymorphism in T. crocea by Todd, Lee, and Chou (2009) identified eight general color morphs in this species.  Larger individuals tended more towards greens and browns, while smaller individuals were vivid blue.  The researchers hypothesized a tradeoff:  that the blue structural color caused by iridophores act as a sunscreen that might be more vital to smaller clams than larger ones.  Image credit:  Todd, Lee, & Chou 2009.


T. gigas and diver.  Image credit: Rick Hankinson via Wikimedia Commons.

Though the flamboyant mantles of tridacnids may serve to hide the clams from predators, they have captured the attention of human collectors.  Giant clams have long been an easily-accessible and prized food source for native Pacific Islanders.  With the opening up of global markets, tridacnid meat and shells have come into high demand, quickly exhausting local stocks (Jameson 1976; Benzie & Williams 1995; Foyle et al. 1997).  Overfishing has stimulated successful efforts at industrial-scale aqua- and mariculture (Klumpp, Bayne, & Hawkins 1991).  However, the heavy investment necessary to build large nursery facilities and the seven year growing time to bring species like Tridacna gigas to meat harvesting size was not within reach of many communities throughout the Indo-Pacific.  Instead, villagers have turned their attention to smaller species like T. crocea, T. maxima, and T. squamosa for the saltwater aquarium trade.  It only takes five to seven months for those species to reach a size suitable for shipping, making this an economical feasible option (Foyle et al. 1997).  The ability to breed tridacnids in captivity also creates opportunities to reseed depleted reefs (Beckvar 1981).


T. maxima farmed for the aquarium trade.  More intense blue and green colors seem characteristic of smaller clams (Todd, Lee, & Chou 2009).  Image credit: PacificEastAquaculture via ReefEdition.


Solomon Island local, Erik Koti, cleaning T. maxima and T. derasa in a family-run enterprise to raise giant clams for the aquarium trade.  Koti was trained by WorldFish Center and WWF in sustainable aquaculture methods.  Image credit: Jürgen Freund.

The success of raising tridacnids in captivity may have saved them from overfishing, but there is a new threat, one that impacts all marine creatures with photosynthetic zooxanthellae.  The symbiont in giant clams is a dinoflagellate named Symbiodinium microadriaticum, and this same species is supposedly found in all reef-building corals, many sea anemones, gorgonians (sea fans), and the Upside-Down Jellyfish, Cassiopeia (Fitt & Trench 1981).  With warming of ocean surface waters, symbiotic zooxanthellae are expelled, leaving their hosts bleached white.  One experiment recreated the conditions for an observed bleaching event on the Great Barrier Reef of Australia and determined that increased light intensity and water temperature were the main factors resulting in loss of zooxanthellae.  It is unclear whether the zooxanthellae are leaving or if they are being ejected by their hosts (Buck, Rosenthal, & Saint-Paul 2002).  In some cases, bleaching is a temporary loss of color that can last well over a year.  But in more severe cases, the damage is permanent — the spaces that once held zooxanthellae degenerate and the giant clam is left to struggle on its own, undoubtedly with poor outcomes (Norton et al. 1995).


Localized bleaching in an animal for sale in the aquarium trade.  Stress from transport can cause loss of zooxanthellae.  Image credit: Fatherree via


Bleached and dead giant clam on Australia’s Great Barrier Reef.  Tridacnids derive the majority of their energy from their symbionts.  Rising water temperatures cause zooxanthellae to leave their hosts.  Image credit: Jake Adams via Reef Builders.

Incidence of coral reef bleaching is becoming more frequent and widespread.  This is not necessarily always fatal, but it does require immediate response to global climate change.

A clam is more than just its empty shell, to be discarded or set upon a shelf.  By delving into something as superficial as color and pattern, we have uncovered something about the essence of the tridacnid giant clam — the symbiotic relationship with zooxanthellae that is a vital focus of its form and function.


Baby giant clams. Go, babies, go!  Image credit: Neo Mei Lin via Psychedelic Nature.


Beckvar, N.  1981. “Cultivation, Spawning, & Growth of the Giant Clams Tridacna gigas, T. derasa, & T. squamosa in Palau, Caroline Islands.”  Aquaculture 24:  21 – 30.

Bonham, Kelshaw.  1965.  “Growth Rate of Giant Clam Tridacna gigas at Bikini Atoll as Revealed by Radioautography.”  Science 149 (3681):  300 – 302.

Buck, Bela Hieronymous, Harald Rosenthal, & Ulrich Saint-Pail.  2002. “Effect of increased irradiance and thermal stress on the symbiosis of Symbiodinium microadriactum and Tridacna gigas.”  Aquatic Living Resources 15:  107 – 117.

Fatherree, James W.  2007.  “Why Do Tridacnids Look the Way They Look?”  Tropical Fish Magazine April 2007.  Accessed 17 September 2016.

Fitt, William K. & Robert K. Trench.  1981.  “Spawning, Development, & Acquisition of Zooxanthellae by Tridacna squamosa (Mollusca, Bivalvia).”  Biological Bulletin 161:  213 – 235.

Foyle, Timothy P., J.D. Bell, M. Gervis, & I. Lane.  1997.  “Survial and growth of juvenile fluted giant clams, Tridacna squamosa, in large-scale grow-out trials in the Solomon Islands.”  Aquaculture 148:  85 – 104.

Goreau, T.F., N.I. Goreau, & C.M. Yonge.  1973.  “On the utilization of photosynthetic products from zooxanthellae and of a dissolved amino acid in Tridacna maxima f. elongate (Mollusca:  Bivalvia).”  Journal of Zoology 169:  417 – 454.

Griffiths, D.J., H. Winsor, & T. Luong-Van.  1992.  “Iridophores in the Mantle of Giant Clams.”  Australian Journal of Zoology 40 (3):  319 – 326.

Jameson, Stephen C.  1976.  “Early Life History of the Giant Clams Tridacna crocea Lamarck, Tridacna maxima (Röding), and Hippopus hippopus (Linnaeus).”  Pacific Science 30 (3):  219 – 233.

Klumpp, D.W., B.L. Bayne, & A.J.S. Hawkins.  1991.  “Nutrition of the giant clam Tridacna gigas (L.). I.  Contribution of filter feeding and photosynthates to respiration and growth.”  Journal of Experimental Marine Biology and Ecology 155 (1):  105 – 122.

Land, Michael F.  2003.  “The spatial resolution of the pinhole eyes of giant clams (Tridacna maxima).”  Proceedings of the Royal Society B 270:  185 – 188.

Morton, Brian.  1978.  “The diurnal rhythm and the processes of feeding and digestion in Tridacna crocea (Bivalvia:  Tridacnidae).”  Journal of Zoology 185:  371 – 387.

Muscatine, Leonard.  1967.  “Glycerol Excretion by Symbiotic Algae from Corals and Tridacna and Its Control by the Host.”  Science 156 (3774):  516 – 519.

Norton, J.H., H.C. Prior, V. Baillie, & D. Yellowless.  1995.  “Atrophy of the Zooxanthellal Tubular System in Bleached Giant Clams Tridacna gigasJournal of Invertebrate Pathology 66:  307 – 310.

Todd, P.A., J.H. Lee, & L.M. Chou.  2009.  “Polymorphism & crypsis in the boring giant clam (Tridacna crocea):  potential strategies against visual predators.”  Hydrobiologia 635:  37 – 43.

Wilkens, Lon A.  1986.  “Visual system of the giant clam Tridacna:  Behavioral adaptations.”  Biological Bulletin 170 (3):  393 – 408.

Notes from the Field No. 1: Darwin v. Octopus

Charles Darwin had all the anxieties typical of a recent college graduate:  uncertain of what was to come, depressed by the prospects available in the Real World.  He had already disappointed his physician father once by dropping out of medical school, so he studied theology at Cambridge in order to take up the gentlemanly profession of being a parson.  But young Charles was clearly procrastinating and desperately planning one last hurrah of the naturalist’s life.  Then one late summer’s day, he opened a letter offering him an opportunity to sail around the world aboard the HMS Beagle. . . . 


The first port-of-call in what would become a five year voyage around the world.  The Beagle landed at St. Jago, Cape Verde Islands off the coast of West Africa.  It was Darwin’s first real intoxicating taste of the tropics.  Image credit: Barrow, John. 1806. “A Voyage to Cochin China, in the years 1792, and 1793.” via The British Library (Flickr)

28 January 1832, St. Jago

Found amongst the rocks West of Quail Island at low water an Octopus.— When first discovered he was in a hole & it was difficult to perceive what it was.— As soon as I drove him from his den he shot with great rapidity across the pool of water.— leaving in his train a large quantity of the ink.— even then when in shallow place it was difficult to catch him, for he twisted his body with great ease between the stones & by his suckers stuck very fast to them.— When in the water the animal was of a brownish purple, but immediately when on the beach the colour changed to a yellowish green.— When I had the animal in a basin of salt water on board this fact was explained by its having the Chamælion like power of changing the colour of its body.— The general colour of animal was French grey with numerous spots of bright yellow. . . . Over the whole body there were continually passing clouds, varying in colour from a “hyacinth red” to a “Chesnut brown”.— As seen under a lens these clouds consisted of minute points apparently injected with a coloured fluid. The whole animal presented a most extraordinary mottled appearance, & much surprised very body who saw it. . . . The animal seemed susceptible to small shocks of galvanism: contracting itself & the parts between the point of contact of wires, became almost black.— this in a lesser degree followed from scratching the animal with a needle.— The cups were in double rows on the arms & coloured reddish.— The eye could be entirely closed by a circular eyelid.— the pupil was of a dark blue.— The animal was slightly phosphorescent at night.

The common octopus, Octopus vulgaris. Image credit: Jatta, Giuseppe.  1896.  “Cefalopodi viventi nel Golfo di Napoli (sistematica).” Fauna und Flora des Golfes von Neapel und der angrenzenden Meers 23 via Biodiversity Heritage Library (Flickr).

30 January 1832, St. Jago

Found another. changed its colour in the same manner when first taken. Caught another: I first discovered him by his spouting water into my face when I certainly was 2 feet above him. When seen in water was of dark colour with rings: being with difficulty removed from a deep hole & placed in a puddle of water swam well & emitted a dark Chesnut brown ink.— he continued likewise to spout water, evidently being able to direct his siphon.— When on land did not walk well having difficulty in carrying its head which it continued filling with air as before with water.— From same cause the animal often made a noise when squirting out water. They are so strong & slippery that one hand is insufficient to hold them.— Whilst swimming generally changed colour & seemed to imitate colour of the rocks.—


Image credit: Cloney & Florey 1968.


Darwin was obviously mesmerized by the way the octopus changed its colors like “passing clouds” and he noted the “minute points apparently injected with a coloured fluid.”  What he saw were chromatophores, specialized cephalopod pigment cells.  Each chromatophore is individually innervated (labeled “Axon” in lower left of the diagram) and equipped with radial muscle fibers that pull the chromatophore from a dense ball of dark pigment into a flat, splayed out pigment field that blossoms with color.  


Cloney, Richard A. & E. Florey.  1968.  “Ultrastructure of Cephalopod Chromatophore Organs.”  Zeitschrift für Zellforschung 98:  250 – 280.

The Complete Work of Charles Darwin Online.  John van Wyhe, ed.  2002.

Desmond, Adrian & J. Moore.  1991.  Darwin:  The Life of a Tormented Evolutionist.  W.W. Norton & Company.