214 



MARINE BIOLOGICAL LABORATORY. 



The sense-organ numbered six in the Cyclostome 

 stage (the crista acustica anterior, or anterior canal 



o.d.e. 



9ia. 



t.a.a 



cr.a.post. 



Fig. 1 6. — The Gnathostome type 

 ear shown here illustrates the 

 method and the extent of the 

 modifications which the Cyclo- 

 stome ear has suffered m its 

 descent to this group. The an- 

 terior and posterior canal organs 

 have each budded off an organ 

 which, in the case of the anterior 

 canal, produces a well-formed 

 canal, the external, present in 

 all the members of this group, 

 w^hile the posterior bud never 

 produces a well-formed canal. 

 The utricular recess, which con- 

 tains a number of sense-organs 

 derived from the division of the 

 macula utriculi, is represented as 

 forming a tubular prolongation of 

 the utricular cavity, in order to 

 more clearly show the parallelism 

 of this group of organs with the 

 lagenar group. In no known 

 form does the production of the canal proceed to the extent shown 

 in the diagram. The sense-organs of the lagenar group are repre- 

 sented as separated from each other by an interval, whereas the 

 actual relation in nature is much more intimate. 



# N.vni. 



ca Anterior canal. 

 cp Posterior canal. 

 ch External canal. 

 c. ab Abortive canal. 

 u Utriculus. 



rec. utric Recessus utriculi. 



rec. sac Recessus sacculi. 



d Ductus endolymphaticus. 



s Sacculus. 



o. de Orifice of the same. 



organ, of Myxine and Petromyzon) has divided to pro- 

 duce sense-organs ten and twelve of the Gnathostome 

 stage, i.e. the cristae acusticas anterior et horizontalis, of 

 the human ear. 



