OCR Text |
Show 496 MR. G. W. BUTLER ON THE [June 14, (vi.) Besides the definite subdivision of the body-cavity described above, there are found in Snakes a number of modifications which tend to obliterate the pleuroperitoneal cavity. Consideration, however, shows us that these modifications of the simple primitive condition (like those which lead to the definite subdivision of the body-cavity) take place along lines which are followed to a greater or less extent in other groups of Sauropsida. And Snakes differ in this respect, to some extent, among themselves. Thus, the obliteration of the pleural cavity m a y be compared with what we find in Birds (and possibly also in Monitor Lizards, if not in Chelonia). Again, the complete exclusion of the kidneys from the body-cavity, although the rule with the Snakes, is, we see, not confined to this order. A partial exclusion is rather usual [ante § IV. (i.) and notes.] Thirdly, the remarkable development of extra-peritoneal lymph-cavities in Snakes, which still further reduces the space available for the body-cavity proper-is but an exaggeration of what we find in other Reptiles (e. g. Monitor and Amphisbaenidae) [ante §IV.(0)]. Lastly, the relation of the alimentary canal to the peritoneum, so often described, certainly does appear to be a peculiarity of Snakes; concerning this, Snakes differ among themselves, some departing less than others from the common type (in which the intestine hangs by a mesentery into the peritoneal cavity). The firm attachment of the stomach in Crocodiles seems essentially similiar to that in Snakes. § VIII. Conclusions. (1) In Snakes, as in Crocodiles, the pleuroperitoneal cavity is, for great part of their embryonic period, very similar to that of adult Lizards; and the same is true of the earlier stages of Birds and Mammals. (2) In Snakes the simple, primitive, lacertilian condition becomes during the later stages of development modified, in the following ways:- (a) The pleural part of the body-cavity becomes obliterated. (b) The peritoneal part of the body-cavity becomes subdivided into a definite number of closed spaces, viz.: (i.) A posterior, or intestino-genital space. (ii.) A gastric space on the left side. (iii.) A n " omental" space. (iv.) T w o paired liver-sacs. (3) An examination of representatives of nearly all types of Snake (see list) shows that however much they may differ, in actual size, in the proportion of their parts, in various anatomical characters, and in their habit of life, they all agree in the plan on which their peritoneal cavity is subdivided. (4) The subdivision of the body-cavity of Snakes proceeds in |