Exoskeleton & Endoskeleton

Exoskeleton (pronounced ek-soh-skel-i-tn)

(1) In zoology, an external covering or integument, any hardened external structure, as the shells of crustaceans or the scales and plates of fishes and reptiles, especially when it is of the nature of bone.

(2) All hard parts, such as hair, teeth, and nails which develop from the ectoderm or mesoderm in vertebrates (generally used only in technical literature).

1841: The construct was exo- + skeleton.  The prefix endo- was used as a word-forming element meaning "inside, within, internal.  It was from the Ancient Greek ἔνδον (éndon) (within; inner; internal) from the primitive Indo-European en-do- (an extended form of the root en (in)).

Skeleton was from the New Latin skeleton (bones, bony framework of the body), from the Ancient Greek skeleton soma (dried-up body, mummy, skeleton), from the neuter of σκελετός (skeletós) (dried up, withered, dried body (and as a noun: parched; mummy), from σκέλλω (skéllō & skellein) (dry, dry up, make dry, parch), from the primitive Indo-European (s)kelha- (to parch, wither); related was the Ancient Greek σκληρός (sklirós) (hard).

Skelton was an early variant form.  The noun use of Greek skeletos passed into Late Latin as (sceletus), hence the French squelette and the rare English skelet (1560s), the Spanish esqueleto & the Italian scheletro.  The meaning "bare outline" is first recorded circa 1600; hence the term skeleton crew from 1778 used to describe minimal staffing, the skeleton key a similar allusion to some of a structure being removed.  The phrase skeleton in the closet (source of secret shame to a person, family or institution) is from 1812 and thought an adoption from the imagery in Bluebeard fable (1697) by Charles Perrault (1628-1703). Exoskeleton was a creation in 1841 by English paleontologist Sir Richard Owen (1804–1892).

Endoskeleton (pronounced en-doh-skel-i-tn)

(1) In zoology, the internal skeleton or framework of the body of an animal (generally the bony or cartilaginous skeleton of vertebrates).  Certain invertebrates, such as sponges and echinoderms, also have endoskeletons.

(2) For most (non-technical) purposes, a synonym for skeleton.

1838: The construct was endo- + skeleton  The prefix exo- was used as a word forming element in words of Greek origin meaning "outer, outside, outer part" and was used from the mid-nineteenth century.  It’s from the Ancient Greek ἔξω (éxō) (outer; external).and was related to ex (out of).

Endoskeleton is used almost exclusively in the biological sciences.  For most general purposes, it’s synonymous with skeleton which is the default assumption of use because it’s familiar from humans and most familiar animals.  Exoskeleton has become more widely used in recent years because of the interest in fields such as engineering, robotics and medicine in using external structures, often to augment or replace human functions.

Trilobites

Trilobite (pronounced try-low-byte) translates literally as “three lobes".  Often casually referred to as bugs or sea-bugs, in taxonomy, all trilobites actually belong in the class of trilobite in the phylum arthropod and within the class are ten orders.  It’s not known how many species of trilobites existed but almost 21,000 have thus far been identified in the fossil record, their numbers and variety leading them to be regarded as one of history’s more successful animals.  They inhabited all the seas and oceans and endured some three-hundred million years, surviving several mass-extinction events.  Their long duration, their structure and living habits meant they became a common and frequently discovered fossil, noted since antiquity although the first attempt scientifically to classify one seems to have been by Wan Shizen of China who, in 1689 described trylobite pygidia (tails) as batstones.  The first known scientific drawing was by Welsh botanist, the Reverend Edward Lhuyd (1660-1709) whose sketch of a trilobite was published in "The Philosophical Transactions of the Royal Society.  The drawing, now classified as being a Ogygiocarella debuchii, was then (not unreasonably), called the "flatfish".

Truly ancient, trilobites pre-date the Cambrian explosion and went extinct only towards the end of the Permian extinction event which ended the Paleozoic age.  However, the earlier events took their toll, a few orders vanishing after the Ordovician extinction event while the Devonian event removed all but one order, that last survivor dying out in the Great Permian Extinction.  Why such a successful and prolific creature could not endure these extinctions remains a debate.

All share the same basic structure, having three lobes: a left pleural, a middle axial and a right pleural lobe, their bodies divided into a cephalon (dead), thorax (middle), and pygydium (tail).  Trilobites had a thick, protective exoskeleton which formed a hard calcite shell, something like that of the modern crab and is the reason for their frequency in the fossil record, the exoskeletons usually the only part to survive although, in the rare cases where certain surrounding conditions exist, traces of soft tissue such as antennae can survive fossilization.  As a trilobite grew, it molted its exoskeleton, and many of the fossils which exist are molted frames rather than dead creatures.

Endoskeleton cars

The Birdcage: The Maserati Tipo 60/61 (chassis #2549).

Endoskeleton cars are far from uncommon but some make the concept more obvious than others.  The Maserati Tipo 60/61 (1959-1961) gained the nickname “Birdcage” (by which it’s almost always known) because observers were much taken with the delicacy of the construction.  By the late 1950s, space-frames had become familiar to race-car builders but they were usually robust-looking arrangements whereas Maserati had rendered an intricate latticework of some 200 chromoly steel tubes welded often in triangulated form in the points of highest stress, the designing providing both lightness and rigidity.

Mercedes-Benz 300 SLR (W196S, upper) & 300 SL (W198, lower).

One of the reasons the Maserati’s skeleton looked so delicate was that the space-frame had become associated with Teutonic-flavored construction like that used by Mercedes-Benz for its 300 SL & 300 SLR.  Both shared the same method of construction but despite the names and the the visual similarity between the two, there were few common components beyond the nuts, bolts & screws.  The 300 SL was a road car while the SLR was a lengthened version of the W196R Formula One Grand Prix car with a sexy body and an enlarged (though somewhat detuned) straight-eight engine.

Exoskeleton cars

MVE Exocet (left) & Exomotive’s Exocet Sport V8 (right).

Exoskeleton vehicles are numerous on farms, mine-sites and such but rarely seen on public roads.  They do though have a niche for those who want something which sacrifices just about everything (aerodynamics, weather protection, doors etc) for the nimbleness only extreme light-weight can deliver.  An example is the MVE Exocet, released for public sale in 2010.  It’s an inventive approach to the kit-car concept and takes the classic front-engined, rear-wheel drive approach, based on Mazda’s Miata (the MX-5, introduced in 1989 and a kind of clone of the Lotus Elan of the 1960s but without the problems), the advantage with the Japanese platform being its unusual sub-frame which permits the removal of the body, leaving the engine, drive-train and suspension as a rolling assembly to be transplanted to the Exocet chassis.

The nicely defined shoulder blade and ribcage definition of Lindsay Lohan's endoskeleton.

Because of the light weight, even when using sensible four-cylinder engines the Exocet delivers high-performance but the Americans in particular can’t resist the idea that just about any car can be improved by the installation of a V8 and quite outlandish power to weight ratios are possible.  An indicative example of Exomotive’s Exocet Sport used a 525hp (LS3) version of one of the later evolutions of the small-block Chevrolet V8 which, fully fueled, weighed in at 1690 lb (767 kg), somewhat less than a 2023 Formula One car.  Because it possible to buy, off the shelf (as a “crate” engine), V8 engines with about the same power as a F1 power-plant generates, although there was be something a weight penalty, the potential does exist to build a two-seater roadster with a similar power-to-weight ratio and there are jurisdictions which even allow such a thing to be registered for use on public roads.  Opinions would differ on whether such a build is a good idea but the little machines, if the V8 was tuned more for low and mid-range torque rather than ultimate power, would seem to have great potential in competitions such as short-course events and hill-climbs although the dubious aerodynamics would render it less suited to high-speed tracks.