496 MR. G. W. BUTLER ON THE [JuDC 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 pleuroperitoueal cavity. 



Consideration, however, shows us that these modifications of the 

 simple primitive condition (like those which lead to the definite sub- 

 division 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 may 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 Amphisbsenidee) \_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 Ciivity). The firm attachment of 

 the stomach in Crocodiles seems essentially similiar to that in Snakes. 



§ VIIL Conclusions. 



(1) In Snakes, as in Crocodiles, the pleuroperitoueal 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. 

 (6) 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.) An " omental" space. 



(iv.) Two 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 



