Dean, and All.
Your mention of the guide in "Etidorhpa" commenting upon specialization of body parts, heads, organs, limbs, etc. at the expense of other parts and organs, reminded me of one of my favourite books of all time: H.G.Wells' "The First Men on The Moon".
The main protagonist of the story, Cavor, tells his colleague, Bedford - via a radio message from the Moon, after the latter's return alone to Earth - all about the specialized development of the inner Moon's inhabitants, the Selenites, to suit their particular roles or functions in the basic "Lunar Hive" - arms, hands, legs, brains, etc. The Grand Lunar - the ruler of the "Hive" - for example, is a creature comprised mainly of a gigantic but fragile, thin-skulled head (through which the massive brain can be seen pulsing within), with an atrophied body dangling beneath it like a shrunken, useless appendage. He has no other purpose but to think for all of his subjects, and, since they carry him around everywhere, he has no need of limbs or even a proper body.
I don't know if Wells ever read or even heard of "Etidorhpa", which John Uri Lloyd published in the US in 1896. Wells' "First Men In The Moon" was published in 1901.
Another British writer, Olaf Stapeldon, in his staggering future history of mankind, "Last and First Men", also speaks of a similar atrophy of useless body-parts with massive augmentation of the brain, who live in circular "observatories" and act as computerlike repositories of knowledge and wisdom, for the benfit of the general population of the Earth, some thousands of years hence. (BTW. I most heartily recommend all three of the above books to any who haven't yet read them!)
Oddly, this same sort of adaptation seems to be echoed today in the generally-described appearance of ETs such as the Greys and their ilk. One has to wonder what any likely inhabitants of the HE might look like, in view of all this! And one also wonders if these Greys might just possibly BE the actual Inner Earthers themselves!
Gerry ( aka Marsflash)