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Consider centaurs, mermaids/mermen/merfolk, driders/arachne, lamia/naga, and other half-human creatures with similar bodyplans, which I will henceforth refer to as "centauromorphs". They all have a legless human body (which may or may not include the pelvic region) attached to an animal body where its head would normally be; this applies even to the snake-based lamia/naga, because anatomically speaking, a snake's body is mostly an extremely lengthened thorax, with the actual tail only forming a small portion at the end.

So, this begs the question: How would the anatomical terminology work for a centauromorph, considering that there would be duplicate body regions and parts between the humanlike and nonhumanlike halves of the whole body? This is further complicated by the fact that those regions/parts aren't always truly homologous (e.g. the thorax in humans and horses contains the respiratory system and the heart); they can be merely analogous (e.g. the abdomen in humans and insects; whereas it's mostly the digestive and urogenital systems in the former, it contains practically every major internal body system in the latter except the brain, including the lungs and heart that would be found in the thorax in vertebrate animals)?

Furthermore, in the case of arachnid- or crustacean-based centauromorphs, the animal body part that the human portion would be attached to is the cephalothorax (i.e. it's a head and a thorax in one in the animal that the hybrid creature is based on); I don't know about anyone else, but describing a drider as having a legless humanoid body attached at the lower end to the front of a spider's cephalothorax sounds really awkward to me unless it's meant literally, i.e. if you actually amputate a humanoid's legs (or just use a humanoid who lost their legs beforehand) and surgically/magically attach them by the pelvis to an already-existing spider's cephalothorax.

For an example of what seems IMO to be improper anatomical terminology for such duplicate body parts, take Monster Musume's Arachne race (WARNING: it's a fanservice-heavy manga/anime, thus expect frequent NSFW-ness of images on the site). In the official diagram for Arachne anatomy, the thorax and abdomen are qualified with the adjectives "first" (for the human ones) and "second" (for the spider ones). This seems quite lackluster, because anatomical terminology typically proceeds in a medial/proximal to lateral/distal direction with respect to the standard anatomical position, the only exception I know being the numerical order of the digits (it starts from the thumb, which in SAP is actually the most laterally positioned of the digits). Furthermore, it gives preconceptions about the roles of each body region (especially their internal makeup) that do not seem to make any sense biologically; why would a centauromorph have two thoraxes that are separated by an abdomen, with yet another abdomen coming after them all?

PS: This question is particularly relevant in the case of one particular fictional arthropod-based centauromorph species that I'm working on, since its brain is actually distributed between the human head and what would be an arachnid/crustacean cephalothorax (i.e. effectively two brains), but that's a topic for another time I'm afraid, for the simple reason that I'm planning to make a dedicated question for said species considering how much issues I would like to ask for help about.

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  • $\begingroup$ This question might help you with some aspects of your question. Welcome to WorldBuilding! If you have questions about the site please take the tour and visit the help center. Have fun! $\endgroup$ – Sec SE - clear Monica's name Mar 16 '17 at 14:26
  • $\begingroup$ @Secespitus: Yeah, I actually came across that thread when I was making sure I wasn't making a duplicate thread. It's useful, but not that much IMO. $\endgroup$ – MarqFJA87 Mar 16 '17 at 16:03
  • $\begingroup$ perhaps a nitpick, but I don't think mermaids match the criteria of "where its head would normally be". In artwork I have seen, mermfolks' fish part seem to typically lack the pectoral fins and gills present in thoracic region of fish, meaning there isn't any duplication of body parts. $\endgroup$ – TheBlackCat Mar 16 '17 at 17:48
  • $\begingroup$ @TheBlackCat: Just because they lack the pectoral fins and gills that are integral features of a fish's trunk region (fish have trunks, not thoraxes) doesn't negate my point; rather it's a sign of laziness on part of the designer. A fish's tail is rather small, forming about 1/4 to 1/3 its total length barring extremely elongated bodyplans, and doesn't contain little more than muscles for moving the attached fins. $\endgroup$ – MarqFJA87 Mar 16 '17 at 17:57
  • $\begingroup$ @TheBlackCat: That said, the mermaids from the Monster Musume manga/anime look more or less the same as your typical mermaid, only they do have what would be pectoral fins on actual fish, and they have gills on their humanoid torsos instead of on their fish part. $\endgroup$ – MarqFJA87 Mar 16 '17 at 17:58
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Personally, I would just prepend the family of the specific animal type whose body part(s) you're referring to.

ie: hominid-thorax or equid-thorax for a centaur. Optionally omit the family for parts that are not duplicated, like the head.

You mentioned that your particular species has two brains, one in each of the expected areas. However, if you had another species that did NOT duplicate organs - for example, perhaps a centaur only has a heart in the equid-thorax and does not duplicate another in the hominid-thorax because the first is capable of supplying enough blood flow throughout. In that case you might want to refer to the "fake" thorax (the one that doesn't actually contain the expected organs) as pseudothorax or something along those lines. And then "thorax" implicitly refers to the "real one."

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    $\begingroup$ Prepending the family of the specific animal type whose body part(s) are being referred to doesn't seem very scientific to me, unless the creature is an artificially engineered organism that is explicitly made to hybridize two different creatures (e.g. human and horse for centaurs). You don't see biologists describing the platypus as having a duck's bill and an otter's tail attached to a beaver's body except in informal contexts, after all. Your proposal for cases not involving duplication, however, does have precedent in real-life anatomical terms for some animals. $\endgroup$ – MarqFJA87 Mar 16 '17 at 15:57
  • $\begingroup$ Does a platypus have two of any particular body part? Two tails, for example? Yes, it appears to have body parts from a mish-mash of other animals, but without duplication such as the OP describes, it's not really the same thing. Anyway, this is how I would solve the problem, if you have an idea that seems more likely to be used by real scientists for an analogous situation, I'd be interested to read your answer, too =) $\endgroup$ – Steve-O Mar 18 '17 at 3:51
  • $\begingroup$ Well, I've been provisionally using the term "foretrunk" to distinguish the humanlike body-side's trunk from the nonhuman one, since it's indeed located at the front of the whole body, anatomically speaking. "Hindtrunk" would then be the natural choice for the nonhuman trunk; the same principle could be extended to subdivisions such as the thorax (assuming that it's not a "pseudothorax", that is). This, however, runs into another problem when it comes to insect-based centauromorphs: Insect anatomy already uses "prothorax", "mesothorax", and "metathorax" for subdivisions of the insect thorax. $\endgroup$ – MarqFJA87 Mar 18 '17 at 11:03
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    $\begingroup$ Also, even if we do use "metathorax" for the nonhuman body region in question, it would actually be located in the mesosoma rather than the metasoma of the centauromorph's body, which naturally would lead to confusion. At least the prothorax-mesothorax-metathorax division scheme takes place within the same tagma (mesosoma), rather than across two or more tagmata. $\endgroup$ – MarqFJA87 Mar 18 '17 at 11:06
  • $\begingroup$ @MarqFJA87 Actually, when the platypus was discovered, it was in fact described as just that. platypus.org.uk/facts-history.htm scroll down to 'Discovering' to see what I mean. $\endgroup$ – Fayth85 Mar 23 '17 at 11:06
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Well the same way it already is with humans. Are the lower and upper head called differently? Yes and no.

Start talking about the head as one and then differentiate. Jaw, brain, etc.

For the thorax you would have the same as with human legs. It's all thorax, but there are maybe 30 ribs, upper left and lower left kidney, etc. Just like in upper and lower legs that are both part of the leg, but inside there are femur, tibia, patella etc. If we had two identical knees per leg, they probably be either upper and lower or medial and cranial knees. If they were different they might have separate names, but in that case you have to invent those.

So do it just as we already do. What's there twice gets named by relative position (left eye right eye) and everything that's new gets new names. Everything has a name for the whole thing and names for the parts. Medical terminology is very systematic and that system can normally be extended.

For the animal parts take the animal terminology and extend that.

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  • $\begingroup$ I think I'm getting confused. Let's take Monster Musume-style arachne for example; how would your approach apply to the external morphology (i.e. major body divisions) of such a creature? $\endgroup$ – MarqFJA87 Mar 23 '17 at 14:54
  • $\begingroup$ I'm not sure how a spiders thorax is named, so either thorac + whatever the name for that is or upper thorax and lower thorax if its sufficiently close. The connection would need a new name because that's something we don't have. For the legs, take whatever the legs of spiders are named by or name them caudal middle and cranial or front middle and back left/right leg. - That's how I'd do it. $\endgroup$ – DonQuiKong Mar 23 '17 at 15:35
  • $\begingroup$ A spider's body is divided into a cephalothorax (because that's what it is: it's a head and "chest" in one) and abdomen; more formal terms in use would be "prosoma" and opisthosoma", literally "fore-body" and "hind-body". Insects, incidentally, have a head, thorax, and abdomen (prosoma, mesosoma, and metasoma are nearly synonymous with them; it's kinda complicated, because e.g. the mesosoma actually includes 2-3 abdominal segments that are fused with the actual thorax). $\endgroup$ – MarqFJA87 Mar 23 '17 at 20:03
  • $\begingroup$ And yeah, the connection would definitely need a new name; it's the main reason why I even asked this question, in fact. Now that I'm thinking about it, though, maybe the answer is simpler than I was assuming it was. How about using diminutives? For example, the humanlike thorax would be the "thoraculum", and the human abdomen-like connection "abdominculum", possibly prefixed with "pseud(o)-" on account that it's not really a true abdomen (thanks to Steve-O for that suggestion above). It sort of has some precedent in real-life animal anatomy. $\endgroup$ – MarqFJA87 Mar 23 '17 at 20:08
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Centaur anatomy is out of order compared to humans, but it should be possible to label them by number and type. For example: first segment (human/only head), second segment (first/human thorax), third segment (first/human abdomen), fourth segment (second/animal cephalo/thorax), fifth segment (second/animal abdomen), etc.

Although, I find it difficult to believe a hexapedal organism would have multiple torsos as opposed to one as with insects like the praying mantis. It is the closest animal in real life with a centaur body plan.

I think you're the first person to ever use "centauromorph." The terminology varies depending on whoever you are talking to, but in my experience these are variations of [animal]+(cen)taur+ic/oid.

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  • $\begingroup$ Labelling by order of segment sounds even less sensical than by type of segment. And as for the anatomy being out of order, you seem to be either unaware of or forgetting the concept of standard anatomical position. We can just rearrange the "posture" of a centaur's body so that the humanoid portion is aligned with the axis of the equine portion, allowing us to apply terms like "anterior"/"posterior" consistently. $\endgroup$ – MarqFJA87 Mar 23 '17 at 15:08
  • $\begingroup$ And it may not have been obvious, but I am trying to be as holistic as possible/necessary (varies on a case by case basis) when it comes to the biology of the species whose designs I'm working on. Finally, I'm not partial to "hippocentaur" because not only is it uncommonly used, but "centaur" is used by itself to refer to a horse-based hybrid to the point that we have such a thing as ichthyocentaurs rather than "ichthyohippocentaurs". $\endgroup$ – MarqFJA87 Mar 23 '17 at 15:11
  • $\begingroup$ @MarqFJA87: I was referring to the fact that their first abdomen precedes their second thorax. I shall edit my answer later. $\endgroup$ – Anonymous Mar 23 '17 at 16:43
  • $\begingroup$ To be honest, I've only ever seen the numbering approach ever applied to the names of actual segments of an arthropod's major body divisions (e.g. 4th abdominal segment), not to the divisions themselves. And yeah, the praying mantis indeed is the closest real-life example of a centaur-like body plan, though in its case it's the thorax that is uniquely structured so that its prothorax is elongated and flexibly articulated with the subsequent mesothorax. $\endgroup$ – MarqFJA87 Mar 23 '17 at 20:14
  • $\begingroup$ Contrast it to, say, an arachne, whose humanoid body portion is so different from its spider-based one, not only because it lacks an exoskeleton but because it does not plausibly look like it belongs to a segmented organism (as with all arthropods). Of course, some designers I've seen do design their arachne and other arthropod-based centauromorphs to be much more arthropod-like, in which case it's probably much easier to deal with the terminology issues. $\endgroup$ – MarqFJA87 Mar 23 '17 at 20:17
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You answer your own question. Centauromorphs are merely analogous. Descriptions (and depictions) of them are based from 'what we know' rather than from 'what they are'.

Let's split them up first. There are those which are artificial - they are made, not born, and there are those which are natural - they are born, not made.

The former may not be anatomically and structurally coherent, in that they are constructed and may need support systems to survive (as found in China Mieville's Bas-Lag series). They suffer from traumatic stress and find it difficult to identify who they are against who they were.

Whereas those being who are born, not made, are anatomically and structurally coherent, otherwise they could not survive or breed. Their depictions may suggest multiple respiratory organs, or what-have-you, but this is the fault of the artist, not some odd structure of the being itself.

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