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Show 1892.] BODY-CAVITY IN SNAKES. 495 in Crocodiles [cf. (5) figs. 42 and 43, in which a small unpaired space is seen in the region of the stomach l], (iv.) The omental space is, as described above [§ IV. (iv.) note], well represented in all the other groups of Sauropsida. (v.) As to the liver-sacs, the task of comparison is not so simple. The ventral liver-sacs of Crocodiles are no more comparable with those of Snakes than with those of Birds. W e also soon see that there is a considerable difference between the body-cavity in the pulmohepatic region of Snakes and that in the corresponding region of the Bird. Thus, if we start from the median ventral ligament of the liver and proceed outwards, we find in the ease of the Fowl [(5) fig. 45] that the spaces 1, 1 are bounded laterally by the body-wall, or rather by the " oblique septum " ; but in Snakes (fig. 3B) we pass all round the liver and reach the median dorsal attachment of that organ. This difference is clearly correlated with the fact that while in Snakes the " pulmohepatic recesses " are totally or almost totally absent [see above, § VI. (i.)], in Birds they are remarkably well developed [(5) fig. 45, 2, 2']. In short, the "avian diaphragm" is developed out of fibrous tissue on the ventro-mesial face of the lungs, which tissue bounds these spaces (2, 2') externally. But while the fibrous tissue, which forms a sort of diaphragm ventral to the lung in Snakes, and bounds the right liver-sac dorsally, appears to have a somewhat similar relation to the lung, its other relations are different. In fact, but for its adherence to the lung, one might, from its topographical relations, rather compare the diaphragmatic tissue in Snakes with that in Mammals. It would seem hardly more possible to closely homologize the " diaphragm " and liver-sacs of Snakes with those of Birds than the diaphragm of Mammals with that of Birds. In the case of animals which have descended from common ancestors along more or less divergent lines, we ought not of course to expect to be always able to compare directly corresponding parts, but rather to have to content ourselves sometimes with tracing the condition of things in each back to their common origin. It may be remarked that the lungs of Varanus (Monitor) are excluded from the peritoneal cavity by a sort of " membranous diaphragm"2, and this is less markedly the case with the lungs of certain Chelonia. It may well be, therefore, that the relations of the lung in Snakes can be explained by a reference to one or both of these groups; but this embryology alone can decide. 1 It is marked in these figures with a number 1, like the paired ventral liver-sacs, because I was led to regard both the ventral part of the posthepatic septum (/3) which closes the liver-sacs posteriorly, and the fibrous tissue (j3) which closes this gastric sac posteriorly, as but lateral expansions of the median ventral attachment of the stomach and liver, from which point of view the sac in the region of the stomach and the paired ventral liver-sacs might be grouped together. The embryology of Snakes seems to be in favour of the essential similarity between the septa /3f3 (see § VI.); but as in Snakes this gastric space is so distinctly posterior to the two liver-sacs, it may with advantage be considered apart from them in both those animals and Crocodiles. 2 Martin, P. Z. S. 1831, p. 138. See also P. Z. S. 1889, p. 608. 34* |