OCR Text |
Show 1889.] C O N V O L U T I O N S IN BIRDS. 307 enumeration and description of the intestinal convolutions as they occur in the numerous orders and families of birds, because this will be done elsewhere. The Table (pp. 308, 309) contains, in a condensed form, an account of the principal modifications of the intestinal folds, and the diagram (Plate XXXII.) shows the affinities, or, to speak more cautiously, the convergent similarities, of all the principal families, as they are suggested merely by a study of their intestinal arrangements. The birds will be discussed only from this point of view in order to test, and to draw attention to, the taxonomie value of those characters which are exhibited by the modes in which the mid-gut is stowed away in the abdominal cavity. Many of these similarities are perhaps merely coincidences, and in this case can have no taxonomie significance; but if these similarities coincide with those of several other organic characters, they are entitled to a higher rank as indicating not convergence but common descent of those birds in which they persistently occur. There seems to be a sort of belief prevailing that the intestinal convolutions are very variable and unreliable in the same species, that they are a matter of accident ; hut, on the contrary, I have found them constant to an astonishing extent, not only in the same species but in many large families. Of course secondary shortening and widening of the gut (owing to the assumption of irugivorous habits) may reduce the number of loops, and may render the original arrangement quite untraceable, as in, e. g., Carpophaga, Bhamphastus, Manucodia. When a bird has acquired strictly piscivorous habits, the gut is considerably lengthened and narrowed, and may, just 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. In the following Table the order adopted is one of mere convenience, without necessarily indicating near relationship. The second column contains the number of principal loops ; this can best be ascertained by spreading the intestine out on the table without tearing the mesenteric connections. The next three columns refer to the 2nd, 3rd, and 4th principal loops : r means that the loop in question is a right-handed one, like the duodenum ; I that it is retrograde, or left-handed ; o signifies that the loop is open ; cl that it is closed. The last column indicates in a few words the type of formation. , The diagram (Plate XXXII.) requires some explanation. All the birds of which the names are written inside the inner of the two concentric circles are on the whole orthoccelous, whilst those ii laced between the two concentric circles are cycloccelous ; of the latter, the underlined families are telogyrous, the others mesogyrous. |