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



Furthermore the attempt has been made to render the descriptions as easy of reference 

 as possible, by arranging each in a series of uniform paragraphs and taking up the organs 

 in a definite order. 



Dimensions have been given in the case of new species, although, except as 

 indicating general proportions, I do not attach much value to them, owing to the great 

 contractility of the animals when alive, and to the irregular manner in which they 

 sometimes seem to be affected by reagents. 



It may be a matter of astonishment to some that no stress has been laid upon the 

 radula in the comparison of different species. When the examination of the present 

 collection was first commenced, drawings were made of this organ, and it was intended 

 to publish and use them for diagnostic purposes, but it became evident in a short time 

 that without a much more extended investigation than was possible under the circum- 

 stances no results of value could be hoped for. It appears that in almost every radula 

 each row of teeth differs a little from the one preceding it, and very frequently five, six, 

 or even more rows must be examined before a given form repeats itself ; two rows of 

 teeth from the same specimen will often differ as much as two from different species. 

 From this it was evident that the majority of the figures hitherto published were valueless 

 for comparative purposes, inasmuch as they show only one row of teeth, and it appeared 

 wiser to defer the consideration of this particular organ, in the hope of making a thorough 

 examination of the whole matter with larger material at some future date. 



