212 



MARINE BIOLOGICAL LABORATORY. 



living vertebrate. And it is quite possible that the 

 auditory saucer may contain the rudiments of the two 

 primary sense-organs of the Cyclostome ear. The 

 parent sense-organ soon divides transversely into two 

 nearly equal parts, which are the anterior and pos- 

 terior sense-organs respectively. The auditory vesicle 

 is at the same time partly separated into two cham- 

 bers (incomplete canals) to accommodate them. These 

 chambers are the utriculus and sacculus. 



c.r 



r.p. 



viyzon. 



Fig. 15. — The Cyclostome type ear 

 shown here is a very simple structure 

 as compared with the complicated 

 organ found among the Gnatho- 

 stomata, but compared with its an- 

 cestral condition, i.e. the auditory 

 vesicle, it is seen to have gone 

 through many changes, and stands 

 to-day midway between its earliest 

 condition and the highest differen- 

 tiation known. This diagram is con- 

 structed on the basis of the anatomy 

 of the only two known forms of 

 this group, Myxhie and Petro- 

 The canals and their organs retain a very primitive condition 



of structure. 



caa Anterior canal. 

 cap Posterior canal. 

 cr. a Crista acustica anterior. 

 cr. p Crista acustica posterior. 

 Endolymphatic duct. 

 The utriculo-sacculus. 

 and 2 Branchlets of the utricular 

 branch of the auditory 

 nerve, 

 and 2 The saccular branches of the 

 same. 



d 

 n.s 

 w I 



J I 



ode 



mac. 71 

 mac. s 

 I and 2 



Terminal portion of the endo- 

 lymphatic duct, which in 

 neither of these Cyclostomes 

 opens on the surface of the 

 body in adult life. 



Macula acustica utriculi. 



Macula acustica sacculi. 



The portions (i) of the sense- 

 organs that remain in the 

 parent cavity and the portion 

 (2) which migrate into the re- 

 cessus utriculi and lagena 

 respectively. 



Each of the two sense-organs of the second genera- 

 tion after a while divides into two unequal parts in such 



