130 THE VOYAGE OF H.M.S. CHALLENGER. 



several of the Synaptidae have such olfactory organs either attached to the nerve-ring 

 itself or lying immediately in its proximity. Lately Danielssen and Koren as well as 

 myself have exhibited in our respective reports that Elpidia glacialis and Kolga hyalina 

 are provided with such organs not only at the nerve-ring but also all along the two 

 lateral ambulacra! nerves of the trivium. 



In Elpidia glacialis the main nerve-cords, excepting the odd one of the ventral 

 surface, communicate at their junction with the pharyngeal ring with a small auditory 

 sac. On making a closer examination of the ambulacra! nerves one will find that 

 each of the ventral lateral cords carries these sacs, usually five or slightly more, 

 scattered all over their length, while the remaining nerve-cords seem to be without 

 any such ; one or two may usually be observed near the points where the pedal 

 nerves join the main cords. The auditory vesicles are provided interiorly with an 

 epithelial lining, are spherical, about 0"2 mm. in diameter, and seem in general to be 

 attached close to the nerve-cords ; they contain five to twenty otoliths of a characteristic 

 ovate shape, with one of the ends rounded, while the other is slightly tapered and 

 truncated. The otoliths (PL XXXVI. fig. 25) measure in length about 0*036 mm. 

 and in breadth - 02 mm., and are made up of from three to four distinct layers, the 

 innermost of which has a granular appearance. When regarding the layers posteriorly, 

 viz., from the rounded end, they are marked out by concentric circles. 



Danielssen and Koren mentioned in their aheady cited memoirs that Kolga 

 hyalina possesses no less than fifty -six auditory sacs, two of which are situated close to 

 each of the chief dorsal nerves not far from the ring and twenty-six along each lateral 

 ventral nerve. It is most probably a fact that the odd main cord is regularly without 

 any auditory vesicles, and that the dorsal cords only possess such vesicles anteriorly in 

 the neighbourhood of the pharyngeal ring ; at least I never found any exceptions to 

 this rule. According to Danielssen and Koren, each auditory vesicle in the above- 

 mentioned species contains from 20 to 130 otoliths, the form of which is either 

 oblong and composed of different layers, or round and presenting concentric circles. 

 I suppose that these latter, the rounded ones, are only the former observed from the 

 round end, and thus the layers must of course present the appearance of concentric 

 circles. My intention is to enumerate below all the cases in which auditory vesicles 

 have been found, but I draw a special attention to the fact that I have not had the 

 opportunity of examining as many forms as I should have wished, owing to the scant)^ 

 material at my command. 



In Kolga nana about thirteen auditory vesicles are present along each of its ventral, 

 lateral nerve-stems, each vesicle containing about twenty otoliths. 



In Parelpidia elongata auditory vesicles are found at the nerve-ring enclosing up to 

 thirty or thirty-five otoliths, the length of which varies from 0*021 mm. to 0'04 mm. 



Elpidia incerta possesses a great number of auditory vesicles disposed along each side 



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