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



of the different species in a clear way, so that it might be possible, at a glance, to survey 

 and compare the different forms. With this view I have not only given figures of the 

 entire Sponges, of special fragments, and of the skeletal elements, but also numerous 

 ideal (less highly magnified) sections through the body-wall. In these diagrammatic 

 figures, which are composed from a number of microscopic sections, the skeletal elements 

 are indicated in blue. If I had attempted to copy the individual sections exactly as 

 they appeared, the essential and typical could not, as a rule, have been distinguished 

 from the unessential and accidental, except, of course, by giving a larger number of 

 illustrations than seemed justifiable for such a slight possible advantage. 



Since I had repeated occasion to restudy the rich material, even after the plates were 

 printed off (i.e., at a time when it was no longer possible to introduce corrections), I have 

 been forced sevei'al times to indicate a change in my original opinion, by a correction in 

 the text, or by a change in the specific designation of the plates. 



