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The Common Eel, which haunts our streams and rivers in the middle of its 
life-span, retains its fore- (pectoral) fins and has small scales embedded in the 
skin, as contrasted with the Moray and the Conger, which are scale-less. Pipe 
fishes and Sea Horses have lost pelvic fins, and most of them have developed 
an eel-like form. In many other fish, however, this shape is accompanied only 
by reduction of pelvic fins and a migration of them forward, to lie almost below 
the jaws. Such are the Sand-eels, the Ribbon Fish, and many others. 
What, however, is characteristic of "Eels" in general is the double bend. The 
characteristic mode of swimming of active fish, with two pairs of fins, is a single 
S-bend. Eels, the serpentine fishes, have elongated bodies and a double S-bend 
as their swimming movement. Sea-horses are an interesting exception to this rule. 
They have lost the pelvic fins, but swim with the body vertical, by a sculling 
action of the dorsal fin. 
One genus of Crossopterygian fish, Calamoichthys, a living fossil allied to the 
recently discovered Latimeria, has lost its pelvic fins. This fish, and its ally 
Polypterus, are sluggish lung-fish. I remember seeing one in the London Zoo 
Aquarium, and although it had an eel-like shape, I can remember nothing about 
its mode of swimming. When I saw it, it hardly moved at all. Like Latimeria, 
the famous Coelacanth, the fins of Crossopterygian fish are carried on short, 
fleshy limbs, and it is most likely that from this group there developed, in the 
late Devonian period, the earliest Amphibia, and through them all the groups 
which we (being ourselves members of one such group) like to call the Higher 
Vertebrates. 
Although some Amphibia, such as Frogs and Toads, have well-developed 
limbs adapted for leaping, many others have limbs either reduced to, or never 
developed beyond what amount to points of purchase on the ground. They move 
by swinging the spine in an action similar to that of a swimming fish. However, 
one group of Amphibia, occurring in South-east Asia, have entirely lost their 
limbs. These are the Caecilia, or Apoda, worm-like creatures who compensate 
for their loss of legs by leading a burrowing existence. 
As a help in surviving thus, they have retained small scales in the skin and a 
much solider skull than other living Amphibia. Both these characters were 
present in the primitive Stegocephalia, the earliest land vertebrates, as they were 
also' in those fish from which we believe these Amphibia arose. Frogs, Newts> and 
most other modern Amphibia have all lost scales and have very sketchy skulls. 
Their retention of limbs has enabled them to survive such losses. Caecilians have 
not been handicapped by the fact that their eyes have been almost or entirely 
lost. Eyes are anyway of little use to a burrowing creature. They have replaced 
sight by information provided by a sort of short tentacle they have developed on 
either side of the front of the head. 
Now we turn to the Reptiles and the group of "Serpents" par excellence, 
the Snakes. 
The sub-Order Ophidia, the snakes, appear to have developed from lizards 
which lost their legs, probably taking refuge at first by burrowing, and then were 
saved and enabled to diversify by a number of compensating mutations. 
First I would mention a lizard we all know, the Slow-Worm, or Blind-Worm, 
neither slow, nor entirely blind, nor a worm. This lizard has lost its limbs and 
survived by developing no more than a dusky colouration, a slender form, and 
capacity for keeping among the grass-roots. They have kept alive but are 
members of a small and not very successful group. 
True snakes have almost all entirely lost all four limbs. But we must not 
suppose that the lizardish ancestor of the snakes lost two pairs of good limbs 
suddenly in one generation. We are forced to believe that the limbs were gradually 
reduced by a series of unfavourable mutations during a long period while some 
strains so afflicted developed other favourable changes, allowing the pre-snakes 
to survive and propagate. Members of the group Boidae (the Boa-Constrictors 
and Anacondas) and a few others retain some internal structures of the hind-limbs 
and a visible external claw at each side of the cloaca. These claws, rudiments of 
limbs, are better developed in the males than in females, and are not used for 
locomotion but only in mating. 
It seems that when the lizard-like ancestors of snakes lost their limbs and 
took to a burrowing life, they were able thus to avoid extinction when they also 
lost eyelids and the nictitating membrane. This last structure is the "third eyelid" 
