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Show 328 DR. GADOW ON THE ANATOMY OF PTEROCLES. [Mar. 21, exist in our common Pigeon, and thus they are in contrast with those birds in which ca?ca are altogether wanting, like Woodpeckers, Parrots, and others. Garrod likewise included the Passerine birds amongst the Menotyphla (to use a Haeckelian term for animals possessing ca?ca). Now I think this is not correct; and we must, consider this matter a little further. Garrod himself came to the conclusion that the ancestral bird-stock did possess ca?ca; as this is undoubtedly true, it follows that all those birds which are now found without caeca must have lost them, either phvlogenetically or even during their ontogenetic development. In fact we see, in embryos of such birds as have when adult only very small quite rudimentary ca?ca, that these organs are, in the embryo, just as well developed as in birds with long ca?ca; but these ca?ca, in a Pigeon for instance, do not grow any further. They are in early life stopped in their development, and thus remain in a rudimentary state. Again, in all those birds which are completely devoid of caeca the tendency to suppress these organs is simply carried out to the extreme. We cannot, therefore, group the birds into birds with ca?ca and birds without caeca; and this is especially wrong, as there exist many birds which, although apparently allied to each other, differ greatly in the presence or absence of ca?ca. If we want to take the caeca into consideration at all, we must take another point of view : that is, are the caeca of any use to the birds in question or are they not ? Now, apparently, in all birds which have well-developed caeca they are useful, although we must confess that we do not know in what way. Again, in birds with very small caeca, where these organs are simply vermiform-like processes, and which never contain any chyle in their extremely small lumen, they cannot have any physiological function, else they would not have been suppressed. No doubt in some cases, in which they are not quite aborted, as for instance in the Crows and in our common Pigeon, the glands in their walls may still produce some secretion, which then may be made the best of. But this is one of the cases in which rudimentary organs are not completely stopped in their functions although they are useless, simply because the animal hitherto has not been able to get rid of them entirely: thus, for instance, the appendix vermiformis of man, or another example still more striking, our thymus gland, which, although a gland, is now without a duct, and thus rather a paradox. But to return to our question. It is clear that birds with rudimentary caeca have to be grouped together with lipotyphlous birds, i. e. birds which have lost these organs. The great development of the caeca therefore constitutes a considerable difference between the Pteroclida? and the Columbida?, as the former and the Gallinacei are decidedly menotyphlous and the Columba? lipotyphlous. In the Gallinacei the whole digestive tract always forms four very distinct loops: the duodenal one is the first; the next two loops are formed by the ileum ; in birds which, like Perdix, have a compara- |