Book - Vertebrate Zoology (1928) 22
Vertebrate Zoology G. R. De Beer (1928)
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Chapter XXII Fins And Limbs
The most primitive chordates relied for their locomotion on the myotomes of the body-wall, which, by contraction on one side and relaxation on the opposite side of the body, can produce the sinuous bendings which pass like waves down the length of the body and propel the organism along. Amphioxus is in this condition.
Improvement of methods of locomotion is connected with the formation of extensions of the body in the shape of fins. The earliest of these to arise were apparently those which lie in the middle line of the dorsal and ventral surfaces : the so-called median fins. In Amphioxus they are foreshadowed, but in Petromyzon well-developed median fins are present, supported by cartilaginous radials provided with radial muscles at their base on each side. Fish likewise have median fins, and these show an advance over the conditions in Petromyzon in that the web of the fin is supported by dermal fin-rays in addition to the cartilaginous radials. These fin-rays are horny (ceratotrichia) in the Selachians ; bony and jointed (lepido- trichia) in the Teleostomes, and in the Dipnoi they are fibrous and jointed (camptotrichia). The median fins of the amphibia have neither cartilaginous radials nor dermal fin-rays at all, and in some of them the fins develop and regress according to the season and the breeding period.
In the fish, in addition to the median fins there appear the two pairs of " paired " fins : a pectoral pair and a pelvic pair.
The method of origin of median fins and paired fins is very similar. In each case a longitudinal fold of skin appears, and into it little " muscle-buds " make their way, having been formed from the myotomes and separated off from them. Cartilaginous radials then appear, and on each side of these, the dermal fin-rays. The fins contain structures derived from several segments of the body, and this is reflected in the number of radial muscles, cartilaginous radials, dermal fin-rays and nerves which the fin contains.
In the most primitive forms, and in early stages of development of other forms, it is common to find that the median fins are continuous and form one fold which extends down the dorsal side, round the tail and forwards again on the ventral side. The presence of a number of separate and discontinuous median fins in many fish is therefore probably due to the sub- division of an originally continuous fin.
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Fig. 157. — The pectoral fin of Cladoselache, showing the radials (r) project- ing parallel to one another and perpendicular to the side of the body (b). (Drawn from a cast.)
If the median fin was primitively continuous, it is possible that the paired fins also were originally continuous folds on each side of the body, and that they became subsequently divided into pectoral and pelvic sections. The fact that in some fish such as Scyllium, there is in early stages of develop- ment a continuous series of muscle-buds given off from all the segments of the trunk makes this possibility fairly probable.
Later on during development the muscle-buds between the positions of the pectoral and pelvic fins come to nothing.
The most primitive paired fins known are probably those of the fossil Cladoselache, in which they are triangular flaps with the apex pointing outwards and the broad base attached to the side wall of the body. The radials are more or less parallel to one another and stick out at right angles to the side of the body. It is important to notice that the radials are scarcely concentrated at all at their base ; in fact the base is the broadest part of the fin. In the body- wall there are some basal cartilages with which the bases of the radials articulate.
The next step in the evolution of the fins was probably the concentration of the radials at the base of the fin. The result of this was that the stalk attaching the fin to the side of the body became narrow, and the fin became free to move in a greater variety of manners. The arrangement of the radials was now in the shape of a fan as in the Osteolepidoti, and the fin itself was in the form of a blunt paddle. The centremost radials formed what may be called the axis of the fin, but this is not well marked in primitive forms in which the fin is short.
By a lengthening of the axis a pointed laurel-leaf-shaped fin is arrived at, like that of Ceratodus (the so-called archi- pterygium). This type of fin is also present in the fossil Pleuracanthus, where it would seem to have evolved indepen- dently from that of Ceratodus. The skeleton of the paired fins of the primitive fossil Dipnoi resembles that of the Osteolepidoti.
On the other hand, by a shortening of the axis and reduction of the radials, the web of the fin comes to be supported mostly by the dermal fin-rays, and this is the condition of the higher bony fish.
The pectoral and pelvic girdles must have arisen in accord- ance with the need for a firm point of attachment of the fins in the wall of the body. The radials at the base of the fin have fused together and grown inwards, and in so doing they may enclose in foramina the nerves supplying the fin. In the pectoral girdle it is usual to find a dorsal scapular and a ventral coracoid element ; the pelvic girdle is not so well developed. These girdles lie in the body-wall and are not primitively connected with any other part of skeleton.
In the bony fish, the scapula, coracoid and pelvis ossify as cartilage-bones, and in addition, a number of dermal bones arise in connexion with the pectoral girdle. In a primitive bony fish like Polypterus, these dermal bones are the clavicle, cleithrum, supra-clei thrum, and the post-temporal which is attached to the hind part of the skull. This girdle, which is composed of dermal bones, may be called the clavicular girdle,
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Fig. 158. — Comparison|between the fin of Sauripterus, A, and a pentadactyl limb, B. (A after Gregory.) c, carpals ; co, coracoid ; h, humerus ; mc, metacarpals ; p, phalanges ; r, radius ; ra, radials ; s, scapula ; u, ulna ; zo, web of the fin, supported by lepidotrichia. The fin of Sauripterus is an example of the blunt paddle- shaped type of fin.
to distinguish it from the other girdle, formed of cartilage or cartilage-bones, to which the term scapular girdle may be applied. The clavicle is present in the sturgeon (Acipenser), but in all higher fish it is lost. It may seem curious that the pelvic girdle never has any additions of dermal bones to the cartilage- bones of which it is composed. The explanation is that the dermal pectoral girdle originally had no connexion with the pectoral fins. It provides a firm attachment for the muscles of the body-wall just behind the gill-slits in those forms (the bony fish) in which the gill-slits are highly developed. The gill-slits occupy a region of perforation and weakness, and they prevent the main mass of the lateral body-wall muscles from becoming attached to the skull. The dermal pectoral girdle, which itself is attached to the skull, gives these muscles some- thing solid to work from. The joining of the scapular and clavicular pectoral girdles is due to the fact that both are situated close behind the gill-slits. Since there are no gill-slits or other source of weakness near the pelvic girdle, the latter has no dermal elements added to it.
From the nature of the water in which they live, the fins of fish are necessarily more or less like paddles. But it is from such paddles (or ichthyopterygia) that the five- digi ted or penta- dactyl limb (cheiropterygium) of the Tetrapods or land- vertebrates was evolved. It is interesting to inquire into the question as to which type of fin most probably gave rise to the limb. The most convenient starting-point is the blunt lobate fin of the Osteolepidoti (and primitive Dipnoi) with a single large radial at its base, and an increasing number of radials arranged fanwise running to the outer border of the fin. In such a form as Sauripterus (one of the Osteolepidoti) the single basal radial of the pectoral fin may perhaps be held to represent the humerus, and the next two correspond to the radius and ulna of the terrestrial fore limb. In a general way the next radials represent the carpals and metacarpals.
The earliest limbs probably had more than five fingers, and the number of rows of radials in the distal part of the fins of Sauripterus is greater than five. But if the pectoral fin of Sauripterus be compared with the arm of a primitive amphibian like Eryops, it is easy to see how the structure of the latter could be derived from that of the former. The evolution of the five-digited, or pentadactyl limb is an adaptation to locomotion on land. During this transformation, the limb- girdles must have become better developed, for an animal in air is relatively heavier than in the water, and the limbs are subjected to greater strains and stresses. At the same time, the girdles of the earliest land-vertebrates closely resemble those of their aquatic ancestors. So in Eogyrinus (fossil Amphibian of the Carboniferous period), the clavicular pectoral girdle is represented by the clavicle, cleithrum, supra-cleithrum and post- temporal, which latter is attached to the hind end of the skull, just as in bony fish. To these is added a median ventral interclavicle. As in all Tetrapods, the scapula (cartilage- bone of scapular girdle) rests on, but is not attached to, the ribs. The pelvic girdle of Eogyrinus is interesting in that the ilium rests on the ribs without fusing with them to form a sacrum. In this respect, the pectoral and pelvic girdles are similar, but in higher forms the ilium becomes firmly attached to one or more sacral vertebrae. In addition to the ilium, the pelvic girdle contains the pubis and ischium.
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Fig. 159. — The forelimb of Sphenodon : an example of a typical penta- dactyl limb with a primitive carpus.
The figures indicate the ordinal numbers of the digits, c, carpals of the distal row, which are five in number ; ce, centralia ; h, humerus ; i, inter- medium ; mc, metacarpal ; p, phalanges ; ps, pisiform ; r, radius ; ra, radiale ; w, ulna : id, ulnare.
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Fig. 160. — Ventral view of the abdominal ribs or gastralia, and pectoral girdle of Sphenodon. cl, clavicle ; co, coracoid ; g, gastralia ; gc, glenoid cavity ; ic, inter- clavicle ; r, ribs (true) ; s, scapula.
Fig. 161. — Ventral view of the pectoral girdle of Ornithorhynchus. ec, epicoracoid (or precoracoid). Other letters as Fig. 160.
In the earliest land- vertebrates, the function of the limbs was not to support the body of the animal but to row it along while its ventral surface rested on the ground. Such move- ment must have been slow, and improvement came in the reptiles, in which the limbs lift the body off the ground. In them, there was no friction to be overcome between the body and the ground, and the higher the body was lifted, the longer the limbs, the longer the stride and the faster was the pace. In the reptiles the clavicular pectoral girdle is reduced to the clavicle and interclavicle (the cleithrum persists only in some primitive forms), while the scapular girdle usually consists of a dorsal scapula and a ventral coracoid. In the Theromorph reptiles the scapular girdle may have two ventral elements, the coracoid and precoracoid. In the pelvic girdle the ilium becomes attached to the sacral vertebrae, and the ischio-pubic foramen appears between the pubis and ischium. In some Dinosaurs a post-pubis is present, extending back beneath the ischium. In Chelonia, the pectoral and pelvic girdles have a peculiar position in that they lie inside the ribs, instead of outside them as in other forms. In birds, the pubis rotates backwards and comes to lie parallel to and beneath the ischium, with which it may to a certain extent fuse.
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Fig. 162. — Diagrams illustrating the evolution of the limbs of Tetrapods.
A and B, views of the early stage when the limbs stick out laterally and the ventral surface of the body rests on the ground. C and D, later stage, when the body is lifted off the ground, the forearm and shank being vertical, and the limbs projecting to the side. E and F, late stage, when the hind limb is rotated forwards from the acetabulum, and the fore limb rotated backwards from the glenoid cavity ; but the hand points forwards and the radius and ulna are crossed.
In mammals, the coracoid, precoracoid, and interclavicle are retained only in the Monotremes. The pelvic girdle of Monotremes and of Marsupials is characterised by the presence of a pair of epipubic bones, which support the pouch, or marsu- pium. The clavicle is often missing in the higher mammals, and especially those which use their limbs for fast running. So the clavicle is absent in the horse, and it is much reduced in the dog. In the more primitive forms, and those which are specialised for tree-climbing and digging, the clavicle is usually present.
The limbs themselves show interesting modifications. In the earliest Tetrapods, the limbs stuck straight out at right angles to the side of the body. When the ventral surface of the body became lifted off the ground, the upper arm and thigh stuck straight out laterally and horizontally ; at the elbow and knee there was a right-angle bend, so that the forearm and shank descended vertically to the ground. At the wrist and ankle, there was another right-angle bend, so that the hand and foot extended horizontally away from the body.
In the mammals, starting from the condition just described, the limbs have undergone a rotation. The hind limbs have been rotated forwards, so that the thigh runs forwards from the hip-girdle, and parallel with the side of the body, the shank runs downwards, and the foot points forwards again. In the fore limb, however, the upper arm has been rotated back- wards parallel with the side of the body, and the forearm runs downwards. But the hand would point backwards if the fore limb had undergone a simple rotation similar to that of the hind limb (though in the opposite direction). As a matter of fact, the hand points forwards, and this is brought about by a rotation of the wrist through 180 about a vertical axis which coincides with the forearm. So it happens that the forearm is twisted, and the radius runs from the outer side of the elbow to the inner side of the wrist, passing in front of the ulna, which runs from the inner side of the elbow to the outer side of the wrist. This is the typical position (of pronation) in mammals ; most Primates, including man, however, are able to uncross the radius and ulna and so turn the palm of the hand upwards (supination).
It is impossible to go into all the types of limb-structure, but it is interesting to consider the adaptations of limbs to the three great media, viz. to locomotion on land, in the air, and in water.
The fingers and toes of land-living vertebrates above the amphibia end in horny claws which may be modified into nails or hoofs. When the whole surface of the hand or foot is applied to the ground, as in the human foot, the animal is said to be plantigrade. Other animals, like the dog, rest only the under surface of the fingers and toes on the ground, while the palm of the hand and sole of the foot take no share in bearing the animal's weight. This is the digitigrade condition. Others again, such as the horse and cow, which rest only on the end joints of the fingers and toes, are unguligrade. [The latter form part of the order Ungulata.]
The limbs of the horse are specialised for rapid movement on hard ground. Only the 3rd digit is retained, and its extremity is expanded and surrounded by the nail which gives rise to the hoof. The other digits have disappeared, leaving only small vestiges of the metacarpals and metatarsals (of the 2nd and 4th digits) in the form of " splint-bones.". The fossil ancestors of the horse show different stages in this process of reduction of the number of digits, and lead back to normal pentadactyl animals. These odd- toed Ungulates are called Perissodactyls. Curiously enough, a parallel process of reduction in number of digits, and of formation of hoofs consisting of a single digit, went on in a group of animals (all now extinct) quite independently of the horses : the Thoatheria. This is a very remarkable case of convergence in evolution.
Fig. 163. — Convergence in the adaptation of limbs for flight, A, in birds ; B, Pterodactyls ; C, bats. cm, carpo-metacarpus ; h, humerus ; mc, metacarpal ; p, phalanx ; r, radius ; w, ulna.
FlG. 164. — Convergence in the adaptation of limbs for swimming, in, A, Ichthyosaurs ; B, Plesiosaurs ; C, birds (penguin) ; D, mammals (dolphin) .
In the " cloven-hoofed " Ungulates or Artiodactyls, the hoof is formed from the end joints of digits 3 and 4, as in cattle, where the metacarpals and metatarsals of the two digits fuse.
Among mammals, limbs with a primitive type of structure are those of the Primates, which preserve all the five digits. In most Primates, the first digit (thumb or big toe) is capable of touching any or all the remaining digits, i.e., is opposable. This structure enables the limb to grasp objects firmly. Apes have this power in feet as well as hands, while man only preserves the capacity to oppose the first digit in his hands.
Three separate and independent groups of vertebrates have become adapted to life in the air, by the modification of the fore limbs into wings. These are the extinct Pterosauria (" flying reptiles "), the birds and the bats. In the Pterosauria, the fourth digit of the hand was enormously elongated, and a web of skin was stretched between it and the side of the body, extending back to the hind limbs and tail. The bird's wing is built on an altogether different principle, for the wing-surface is made up of a number of feathers inserted on the hand and forearm. The skeleton of the forelimb of the bird shows a reduction in number of digits to three, and the claws at the end of the digits have disappeared except in the young of some birds, such as the ostrich and of the Hoatzin. The primitive fossil bird Archaeopteryx had well-developed claws.
The wing of the bat is different, again, for in it the 2nd, 3rd, 4th, and 5th digits of the hand are much elongated, and support a web of skin which stretches out from the side of the body.
The three types of wings just described form another interesting example of convergent evolution on the part of unrelated animals, but the most striking example is that furnished by the limbs of those land-vertebrates which have subsequently returned to an aquatic mode of life and become adapted to it. The adaptation takes the form of a modification of the limbs into flippers or paddles, which superficially may come to resemble the fins of fish, but which betray their descent from the pentadactyl structure of the land-vertebrate's limb in their internal structure. This adaptation has taken place at least nine separate times, in independent groups. Three of these are mammals : the whales, the Sirenia, and the seals. Among the birds, the penguins have modified the wing into a paddle. In the reptiles, the turtles (Chelonia), Ichthyosaurs, Plesiosaurs, Mosasaurs, Thalattosuchia, and Thalattosaurs all show the same modification of the limbs into paddles, and in several fossils it is possible to trace the evolution from normal pentadactyl limbs.
In the more highly modified of these paddle-like limbs (as in the whales, for example), it is common to find that the number of phalanges is increased (a condition known as hyperphalangy). In the broad paddles of Ichthyosaurs, the number of rows of phalanges exceeds five, producing the condition called hyperdactyly.
The 5th metatarsal bone is an object of interest. Normally this bone is straight, as in the amphibia, the most primitive reptiles (Cotylosauria), the Theromorph and allied reptiles and the mammals. In other groups of reptiles, however, it is peculiar in being hook-shaped, and the possessors of this modified type of 5th metatarsal are : Sphenodon, lizards, tortoises, crocodiles, Dinosaurs, and Pterosaurs. It is worthy of note that these groups all have characters in common in the structure of the heart or of the skull, and are regarded as belonging to the great Sauropsidan branch of the reptiles which culminates in the birds. It is probable that the hook-shaped metatarsal is characteristic of this group, and distinguishes it from the other main stem of reptiles (Theropsida) which evolved in the direction of mammals. The evidence from the 5th metatarsal fits in with that obtained from other sources.
Mention must be made of those animals which have lost their limbs. They have totally disappeared in some of the eels. Among the amphibia, the pelvic limbs and girdle have been lost in the Sirenidae, and the worm-like Gymnophiona have lost all the limbs and girdles. Coming to the reptiles, the snakes have lost the girdles and the pectoral limbs altogether, while only very small vestiges of the pelvic limbs remain. Several families of lizards have independently assumed the snake-like form by loss of the limbs, such as the slow- worm (Anguis), some of the Scincidae and the Amphisbaenidae. These forms furnish an interesting example of convergent evolution. Among mammals the pelvic girdle and limbs vanish almost completely in the whales (Cetacea) and Sirenia.
Literature
Goodrich, E. S. Vertebrata Craniata, Cyclostomes and Fishes. Black, London, 1909.
Goodrich, E. S. On the Classification of the Reptilia. Proceedings of the Royal Society, Ser. B, vol. 89, 1916.
Gregory, W. K. Present Status of the Problem of the Origin of the Tetrapoda. Annals of the New York Academy of Sciences, vol. 26 1915.
Watson, DM. S. The Evolution of the Tetrapod Shoulder Girdle and rorehmb. Journal of Anatomy, vol. 52, 1917.
. The Evolution and Origin of the Amphibia. Philosophical Trans- actions of The Royal Society, Ser. B, vol. 214, 1926.
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- Vertebrate Zoology 1928: PART I 1. The Vertebrate Type as contrasted with the Invertebrate | 2. Amphioxus, a primitive Chordate | 3. Petromyzon, a Chordate with a skull, heart, and kidney | 4. Scyllium, a Chordate with jaws, stomach, and fins | 5. Gadus, a Chordate with bone | 6. Ceratodus, a Chordate with a lung | 7. Triton, a Chordate with 5-toed limbs | 8. Lacerta, a Chordate living entirely on land | 9. Columba, a Chordate with wings | 10. Lepus, a warm-blooded, viviparous Chordate PART II 11. The development of Amphioxus | 12. The development of Rana (the Frog) | 13. The development of Gallus (the Chick) | 14. The development of Lepus (the Rabbit) PART III 15. The Blastopore | 16. The Embryonic Membranes | 17. The Skin and its derivatives | 18. The Teeth | 19. The Coelom and Mesoderm | 20. The Skull | 21. The Vertebral Column, Ribs, and Sternum | 22. Fins and Limbs | 23. The Tail | 24. The Vascular System | 25. The Respiratory system | 26. The Alimentary system | 27. The Excretory and Reproductive systems | 28. The Head and Neck | 29. The functional divisions of the Nervous system | 30. The Brain and comparative Behaviour | 31. The Autonomic Nervous system | 32. The Sense-organs | 33. The Ductless glands | 34. Regulatory mechanisms | 35. Blood-relationships among the Chordates PART IV 36. The bearing of Physical and Climatic factors on Chordates | 37. The origin of Chordates, and their radiation as aquatic animals | 38. The evolution of the Amphibia : the first land-Chordates | 39. The evolution of the Reptiles | 40. The evolution of the Birds | 41. The evolution of the Mammalia | 42. The evolution of the Primates and Man | 43. Conclusions | Figures | Historic Embryology
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