228 



COMPARATIVE ANATOMY 



leads up to that of Crocodilia. Thus there is a continuous and an unbroken, 

 series from the lower forms to the higher. 



The structure of the auditory organ of Hatteria shows many striking, 

 peculiarities : it thus, like that of Chainreleo, occupies an isolated position. 



While the cochlea gradually becomes more independent of the 

 sacculus, the latter shows the greatest variety both as to form and 

 size in the different types. Thus, for instance, it is usually very 

 small in Birds, while in Lizards (e,g., Lacerta) it is of considerable size 



A 



.c/r 



FIG. 182. RIGHT MEMBRANOUS LABYRINTH OF A, Lacerta viridis, AND B, 

 Alligator mississipiensis, from the outer side. (After G. Retzius.) 



ade, aperture of the ductus endolynrphaticus ; esc, sacculo-cochlear canal ; frt, 

 foramen recessus seal tympani ; tv, tegmentum vasculosum ; -sr, septum 

 cruciatum ; mb, membrana basilaris. (Other letters as in Figs. 180 and 181.) 



The aperture of communication between the utriculus and sac- 

 culus persists, though it gradually becomes narrowed, as does also 

 that between the sacculus and cochlea. The connection between 

 these latter may be drawn out to form a canal (canalis reuniens), and 

 this is particularly the case in Birds ; in Crocodiles an inter- 

 mediate condition between Birds and Lizards is seen. The mem- 

 branous labyrinth of Birds, however, is always specially characterised 

 by the peculiar arrangement of the anterior and posterior 

 canals, which are greatly arched, and the position of their openings 

 into the sinus superior (canal commissure) is reversed. 



In lower types (Swimming Birds) this peculiarity is less marked than in 

 the higher forms, and it would be interesting to ascertain the condition 

 of these parts in the Struthionidse, as one would expect to find in them 

 important points of connection with Reptiles. 



