THE YPSILOID APPARATUS OF URODELES. 285 



from the linea alba, at least lends a certain steadiness to the 

 apparatus while at the same time it cooperates with the ypsi- 

 loideus posterior and the ypsiloid portion of the transversalis to 

 pull the arms strongly inward. 



Corroboration of the above explanation is furnished by those 

 occasional specimens which happen to have been preserved with 

 the ypsiloid muscles contracted. These cases show that the 

 effect of the concerted contraction of the muscles associated with 

 the ypsiloid cartilage is not only to bend the stem of the cartilage 

 upward (dorsally) at its articulation with the pelvic girdle but to 

 curve the flexible arms upward and inward (medially). Evidently 

 the result of the contraction of the three pairs of muscles con- 

 nected with the ypsiloid cartilage is a decided constriction of the 

 posterior region of the abdomen and a consequent compression 

 of the organs contained within^ it. 



To understand in what way this action of the ypsiloid appa- 

 ratus controls the relative buoyancy of the anterior and posterior 

 regions of the body, the shape and position of the lungs must 

 be considered. The lungs of Dicinyctylns are exceedingly simple 

 structures, mere sacs with no trace of the usual amphibian con- 

 dition in which the cavity is subdivided by partial partitions. It 

 seems impossible, in fact, that such very simple structures with 

 so small a supply of blood can justify their existence merely as 

 respiratory organs. In shape, also, the lungs of Dicinyctylns are 

 peculiar. Narrow anteriorly, they widen gradually and round off 

 quite abruptly at the posterior end. The whole form is most 

 adequately described, perhaps, as club-shaped. The statement 

 often given as to the size of the lungs (viz., one third to one half 

 of the length of the body cavity) I find quite incorrect when the 

 observations are made upon freshly killed specimens. If the 

 lungs of such a specimen be inflated through the glottis not even 

 sufficiently to float the body in water (and therefore not unduly), 

 subsequent dissection shows that the lungs extend the entire 

 length of the body cavity so that their rounded, bulging, free 

 ends lie on either side in the angle between the ilium and the 

 vertebral column. 



It is thus easy to see the cause of the bulging of the lateral 

 and ventral walls of the posterior part of the body cavity. 



