262 THE VOYAGE OF H.M.S. CHALLENGETl. 



four species just enumerated, and comparing them with their shallow-water allies, we 

 cannot fail to be struck with one very remarkable fact, and this is that while the shallow 

 water species are characteristically more or less amorphous in external form, or, at the 

 most, simply digitate or ramose, those from below the thousand fathom line have, almost 

 without exception, beautifully symmetrical and definite shapes. This interesting circum- 

 stance is prolialjly to be accounted for by the fact that the deep-sea forms are exposed to 

 precisely similar external conditions for very long periods of time, and variations in any 

 one particular direction (which prove to be advantageous to the species under its particular 

 external conditions) have time to develop into fixed and definite characters ; while in the 

 shallow-water forms the ever changing external conditions necessitate corresponding 

 changes in the sponge, and no external forms can become fixed and permanent, excepting 

 such as, from their very general and unpronounced character, are suited to the ever 

 varying conditions under which they are placed. 



We have already had occasion to show, in discussing the value of external form as 

 a guide to classification, that it is of all characters perhaps the most variable and, 

 consequently, the least trustworthy. The particular external form which a deep-sea 

 sponge assumes may not infrequently be explained by reference to the conditions under 

 which it lives, and in this fact lies strong confirmation of the views to w'hich we have just 

 given expression. 



Thus in Tedania actiniiformis, a sponge whose shallow-water congeners are charac- 

 teristically shapeless, the peculiar external appearance (PL XL fig. 2), and chiefly the 

 arrangement of the pores and oscula, are explained Ijy the fact that the sponge lives on a 

 bottom of mud in which it is nearly buried. 



Many of the most remarkable external forms which we know are due to the 

 necessity of obtaining some means of support to prevent the sponge from sinking bodily 

 into the soft mud or ooze which is so prevalent at great depths. Thus we can easUy 

 account for the " Crinorhiza " forms already discussed,^ and for the analogous " Tricho- 

 stemma " forms.^ Nor need we be surprised that species of distinct (though allied) genera 

 {e.g., Cladorhiza, Axoniderma, and Chondrocladia), all living under precisely similar 

 conditions, arrive at a precisely similar solution of this difficult problem of support, viz., 

 the " Crinorhiza" form. 



Other perfectly definite and characteristic external forms found in deep-sea sponges 

 are, however, as yet unaccounted for. Amongst these we may mention those of Stylo- 

 coi'dyla and Tentorium; but by far the most extraordinary and beautiful is that exhibited 

 by Esperiopsis challengeri (PL XVIIL) which has been fully described elsewhere," and 

 which comes from a depth of 825 fathoms (hence it is not included in the foregoing list, 

 which contains only species from a depth of over 1000 fathoms, though of course a 

 deep-sea form). 



' Vide p. 87. ^ yide p. 216 £< seq. ' 3 Vide p. 80. 



