Intestinal Loops and Fold Characteristics in Bird Groups

The Digestive System of Birds

The Bird Digestive System

Intestine and Villi

Relative Length of the Intestinal Canal

Intestinal Fold or Loop Types

Intestinal Loops and Fold Characteristics in Bird Groups

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Secondary shortening and widening of the gut (owing to the assumption of frugivorous habits) may reduce the number of loops, and may render the original arrangement quite untraceable, as in Carpophaga, Rhamphastus, and Manucodia. When a bird has acquired strictly piscivorous habits, the gut is considerably lengthened and narrowed and may, as in Pandion and in Haliaetus, render the old formation quite unrecognizable. These are, however, exceptions, which are not numerous;  as a rule the lengthening of the pre-existing loops and the additional intercalation of new ones does not disturb the typical formation, but rather throws interesting lights upon the lines of new departure along which certain birds have become developed, e.g. the Alcedinidae from a Coraciine stock, now modified through the acquisition of carnivorous and piscivorous habits.

All the Ratitae agree in having the second loop right-handed, and the third left-handed; this is a feature which occurs again only in the Crypturi, Gallinae, Opisthocomus, and in the Cuculidae. Moreover, as with the exception of the duodenum none of the loops are closed and well defined, the Ratitae represent in this respect the lowest avine type.

The Gallinae form a well-defined group.  Lowest among them stand the Neotropical Cracidae, through which they lead towards the Crypturi.  The Gallinae have also an unmistakable resemblance to Opisthocomus and thence to the Cuculidae.

The Turnices are traceable to a Ralline or low Gralline stock, with assumed plaglocoelous characters of the second loop.

The pericoelous assemblage is large. It is typically represented by the Wader, of which the Limicolae and the Rallidae form the principal groups.

The Rallidae with Otis and Grus are connected with the Turnices, more distantly with the Crypturi, and still more so with Apteryx. Dicholophus is in all points a Gruine form, like Psophia, and cannot be separated from them. Rhinochetus contains Ralline, Limicoline, and cannot be and Ibis-like features; the only bird which it resembles somewhat closely in its very peculiar intestinal convolutions is Podica.

The Limicolae agree with the Laridae, and also with the Columbae in all essential points. Each of these three groups contains a number of forms which lead in an unbroken series from the typically pericoelous birds with four alternating loops to the typically mesogyrous birds.  Most Columbae and Laridae are mesogyrous, but Sterna and its allies represent pericoelous or lower forms. Neither granivorous, nor insectivorous, nor piscivorous habits have exerted any appreciable influence upon their intestinal convolutions, although of course the stomach and caeca are affected, The presence of the crop of the Columbae is repeated in the granivorous Limicoline genera Attagis and Thinocorys.

Numenius approaches in various ways the Ibises, whence a continuous line be traced into Platalea and Phoenicopterus on the one hand and into the Pelargi proper on the other.

Rather different from the Limicolae are the Pteroclidae. They have four hoops, which are all closed, which are all closed, left-handed, i.e. isocoelous, and straight; the second and fourth loops have their apices turned back, and especially the terminal end of the second resembles somewhat a plagiocoelous formation. The Pteroclidae have consequently various points in common with the Rallidae, Limicolae, and Columbae.

The Alcidae are pericoelous and strictly orthocoelous; they agree with the Laro-Limicolae in the configuration of their first three loops, but they differ from them in the number of loops, which is at least six, the last three of which are are left-handed.  They approach in this respect the Pygopodes. These (Colymbidae and Podicipedidae) shew unmistakable affinities with what may be called generalized or low Gralline forms; their four or five loops are closed, orthocoelous, and alternating. The Pygopodes connect the large assemblage of the Waders with the following congregation, of which the Herodii, Steganopodes, Tubinares, and Spheniscidae are all divergent types. A very close connection exists between the Herodii and the Steganopodes, and this is supported by numerous other characters. The Tubinares are in more than one respect the most specialized outcome of this great collective Order, and reach in the typically mesogyrous Procellariinae their highest development.

The Spheniscidae are very specialized. They possess undeniable characters in common with the Pygopodes, Steganopodes, and Tubinares; they are on the whole orthocoelous, but the extreme length of their gut thrown into numerous straight and oblique, or quite irregular, convolutions renders comparison very difficult.

The Anseres, to which belongs Palamedea as a probably very old member, are all orthocoelous and combine pericoelous and plagiocoelous characters in their second loop. The five or six principal loops are alternating; the last four are closed and straight. As typically orthocoelous, aquatic birds, and as Praecoces they agree with the Pygopodes, and the root of the stock of the Anseres has to be looked for in this direction alone.

The Pelargi, containing the Hemiglottides (Ibis and Platalea), Phoenicopterus, and the Ciconiae, are rather diverging forms, which can be characterized as possessing four very long and mostly closed loops (with occasional secondary loops intercalated), of which the first three have a tendency to coil their apical ends into more or less irregular spirals: this leads sometimes to an almost mesogyrous formation.

The Hemiglottides approach nearest to the Limicolae, although their points of resemblance with Numenius may possibly be cases of convergence only. Very closely allied to, in fact inseparable from the Hemiglottides, and connecting them with Tantalus, and thus with the Ciconiae proper, is Phoenicopterus; there is not one single feature in the whole of the Digestive System in which this bird differs from the Pelargi or resembles the Anseres except in the presence of small but functional caeca, which are nearly lost in the Pelargi. But these caeca stand in direct relation to the food of the Flamingoes, which consists of the confervae in the mud of the lagoons. The zoophagous Pelargi have lost them, the phytophagous Flamingoes have preserved them.

The Ciconiinae proper, represented by Ciconia, and connected with the former genera by Tantalus, are essentially telogyrous ; their second loop is right-handed, and accompanies the duodenum; this is a rare feature, and is of taxonomic value for the diagnosis of the subfamilies of the Pelargi.

The Pelargi are often classed with the Herodii, but these two Families differ from each  other in almost every point of primary importance.

There are also certain resemblances between the Pelargi and the Accipitres, the chief connection is formed by the telogyrous character, the mode in which additional loops of the lengthened gut are stowed away, and the tendency to convert some or one of the principal loops into regular spirals. Among the Accipitres, the Old-World Vultures especially exhibit striking Ciconiine similarities

The Psittaci are distinctly telogyrous; all their five principal loops are closed and alternating; this, with the presence of a crop, and the absence of functional caeca, are features which occur again together only in the Accipitres. The absolutely vegetable food of the Parrots would sufficiently account for the differences which exist between them and the entirely zoophagous Accipitres. However, this indication of a possible relationship between the Birds-of-Prey and Parrots is as little binding or satisfactory as other suggestions based upon other organic systems.

Of the Coccyges the Cuculidae possess four intestinal loops, of which the first and second are right-handed. The loops are on the wholr orthocoelous, but the apices of the two middle ones are often turned up, or the second loop is plagiocoelous.  Moreover, they possess fully-developed caeca. In all these respects they resemble to a great extent the Gallinae; and this hint is considerably strengthened by Opisthocomus, which is, barring special features, exactly intermediate between the Cuculidae and the Gallinae. The Musophagidae seem to possess but three loops, the original second loop havng been suppressed in connexion with the frugivorous habits of these birds. The isocoelous feature of the Musophagidae is therefore reduced to a secondarily acquired one, and to a case of convergence towards the typically isocoelous birds.

This Pici (Picidae, Capitonidae, and Rhamphastidae)differ, like the Epopes (Bucerotidae and  Upupidae), from all the remaining birds in the alternating position of their four loops, which in the frugivorous Rhamphastidae, as well as in the extremely short-gutted genus Upupa, are reduced to three by the suppression of the original second loop.  Xantholaema, one of the Capitonidae, has this second loop still indicated. The total absence of caeca in all these birds is a coincidence, while there are no obvious characters, besides the anticoelous convolutions, which point to a close relationship between the Pici and the Epopes.

The remaining are all isocoelous. Of them the Coraciidae standnearest to the hyhypothetical ancestral or central stock, because they are the most generalized group, from which all others can be derived.  The Alcedinidae, which have reached a truly mesogyrous formation, started in one direction from or out of the Coraciidae.

The lengthened gut of the Kingfishers in conformity with their generally piscivorous habits, forms a left-handed spiral by its sccond loop, while the fourth loop is long, and in the more piscivorous members widely open and irregularly placed. The affinity between the Coraciidae and the Alcedinidae in opposition to other groups may be expressed by the term Halcyones.

The Striges verge towards the plagiocoelous type, but all their affinities rest with the Coraciidae and Caprimulgidae combined. These three Families possess long caeca; the Alcedinidae, Cypselidae and Trochilidae, have lost them, the first of these because of their piscivorous and cancrivorous habits.

The Cypselormorphae (Caprimulgidae, Cypselidae, and Trochilidae)agree very much with each other. They all have only three intestinal loops, which are short, in agreement with their principally insectivorous habits. The Trochilidae differ in the possession of a crop. The Cypselidae and Caprimulgidae are somewhat more closely related to each other, and the latter (including Podargus) turn towards the Owls. The Cypselidae are sometimes supposed to be somewhat nearly allied to the Passeres. Their alimentary system does not altogether favour such a view; but perhaps the ancestors of Colius once filled this gap, leaving their existing descendants now in a solitary position..
The Trogonidae stand on a lower level than the Cypselidae, Trochilidae, and Coliidae, on the same level as the Caprimulgidae and Coraciidae, and connect them all with each other. The Trogons still possess well-developed Caeca like the Coraciidae, Caprimulgidae, and Striges, while all the other isocoelous birds have lost them, or have only functionless remnants of them.

The Passeres are a very uniform group. They all possess only three loops, without indications of more; the second and third are left-handed; the second becomes a left-handed spiral, the turns of which depend upon the length of the gut; the third loop is always open, and invariably encloses the duodenum between its descending and ascending branches, the latter branch being situated on the ventral and left side of the descending branch of the duodenum. This arrangement is invariably the same, even in the Mesomyodians, and in such otherwise aberrant forms as Rupicola and Pitta. There is a special line which leads from the Laniine forms through the Austrocoraces (Gymnorhina, Graucaulus, Strepera, and Paradiseidae) into the Coraces proper, which latter have produced some special modifications of the intestinal convolutions, and may be looked upon as the last and highest blossom of the avine tree.
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