THE YPSILO1D APPARATUS OF URODELES. 275 



very general sense that both are differentiations from the rectus 

 abdominis ; they are differentiated from different layers and in 

 connection with structures which are not themselves homologous. 

 They are, in other words, independent differentiations occurring 

 in widely separated forms .and in response to absolutely different 

 physiological needs. 



PART II. THE FUNCTION OF THE YPSILOID APPARATUS. 

 A. Respiratory Habits of Lunged Salamanders. 



Beyond the statement of the very evident fact that the 

 ypsiloid cartilage furnishes the attachment for certain of the 

 abdominal muscles (Wiedersheim, '75), I have been unable to 

 find, in the literature upon the subject, any explanation of its 

 function. There is, however, as has already been said, so appar- 

 ent a correlation in the Salamandrida between the presence of 

 the apparatus and that of the lungs, that the explanation of its 

 function will involve, first of all, a discussion of the respiratory 

 habits of lunged salamanders. 



The more obvious respiratory movements of lunged salaman- 

 ders when breathing air have been very clearly described by 

 Bruner ('96). In brief, two forms of aerial respiration occur, one 

 merely a bucco-pharyngeal, the other a pulmonary respiration. 

 Both of these may be readily observed in the case of any lunged 

 salamander. The first takes place almost constantly and with 

 great rapidity. It begins with an enlargement of the bucco- 

 pharyngeal cavity by lowering the hyobranchial apparatus ; this 

 results in air being drawn in through the nares. Following this 

 inhalation is an exhalation in which the floor of the mouth rises 

 again. These movements follow each other so quickly that the 

 visible effect is a rapid fluctuation of the throat. The mouth 

 remains tightly closed during the entire process, and the respira- 

 tory currents make use of the nasal passages alone. 



At frequent, though irregular intervals, during bucco-pharyn- 

 geal respiration, acts of pulmonary respiration occur. These are 

 easily distinguished externally from the bucco-pharyngeal form 

 by the fact that the depression of the floor of the mouth is a 

 prolonged and exaggerated one, during the latter part of which 

 a contraction of the M. constrictor naris occurs. According to 



