196 MARINE BIOLOGICAL LABORATORY. 



medium. This duct communicates with some of the 

 sense-organ canals lying in the skin, and is innervated 

 by one of their nerves. 



The sense-organs, six in number, with perhaps a trace 

 of a seventh, are placed, three in the ampullae of the 

 semicircular canals, as already described, these being 

 desis^nated the cristae, while the other three are known 

 as the maculae and papillae acusticae ; the one in the 

 recessus utriculi, 6V being the utricular macula vui, 

 while the one in the saccular recess SC is called the 

 macula acustica sacculi. It has budded off the lagenar 

 spot, or papilla acustica lagenae, /. 



These organs are derived from the division of a single 

 parent organ, as we shall see later, and hence their dif- 

 ferences of form are secondary. The trunk of the 

 auditory nerve (Fig. i, ac) is composed of two branches, 

 each of which gives off three branchlets to the anterior 

 and posterior divisions of the ear respectively. We 

 may thus speak of an utricular and a saccular branch, 

 each supplying the structures belonging to {i.e. de- 

 veloped from) its part of the ear. In Torpedo the 

 anterior branch or the ramus utriculi gives off one 

 branchlet to the crista of the anterior ampulla, another 

 branchlet to the crista of the external ampulla, and a 

 third to the macula utriculi. This latter branchlet is 

 in reality composed of several branchlets ; for the 

 macula utriculi is a compound sense-organ, as the 

 sequel will show. 



The posterior or saccular branch gives off one 

 branchlet to the crista of the posterior ampulla, a 

 second branchlet to the macula sacculi, and a third 

 to the papilla lagenae, while there is possibly a fourth 



