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



being 6 (Station 147, IGOO fathoms) ; but this question will be more fully dealt with in 

 discussing the bathymetrical distribution. 



The geographical range of individual species^ appears, with few exceptions, to be 

 somewhat restricted. As exceptions we may mention Halichondria panicea, Reniera 

 cinerea, Chalina i^crgamentacea, Tedania digitata, lophon jxittersoni, Axinella jJrofiaida, 

 Suberites carnosus, Tentorium semisuhei'ites and Latruncidia apicalis. Five of these 

 (Nos. 1, 2, 4, 5, 7) are very generalised types, with few definite specific characters to go 

 by, and might consequently be expected to have a very wide range. One, Axinella 

 profunda, comes from very deep water (2300 and 2385 fathoms) at widely remote 

 localities in the North and South Pacific respectively (cf. p. 181), but apparently under 

 exactly similar conditions in the two cases ; while the distribution of the remaining three 

 is very hard to understand. The genera and families, on the other hand, have mostly a 

 very wide range, although some appear to be characteristic of special localities and to 

 be comparatively rare elsewhere. Thus the Chalininse and. Eetyoninse are highly 

 characteristic of the Indo-Australian area, and the Tedaniinse more or less so of the 

 Patagonian area. 



DISCUSSION OF THE BATHYMETRICAL DISTRIBUTION. 



We have already had occasion to point out that the Monaxonida are not a predominant 

 group in very deep water. If we analyse our table of bathymetrical distribution as we 

 did that of geographical distribution we shall find this conclusion fully confirmed, 

 especially if we bear in mind the large proportion of Challenger dredgings which were 

 taken in deep water as compared with those taken in shallow water. Our four bathy- 

 metrical areas may then be arranged as follows, according to the number of species^ 

 obtained in each : — 0-.50 fathoms (85 species); 50-200 fathoms (55 species); 200-1000 

 fathoms (46 species); 1000-3000 fathoms (24 species). Here then we have the 

 very definite result that the number of species varies in inverse proportion to the 

 depth, or in other words that the number of species gradually diminishes as we go 

 downwards. 



According to Vosmaer •' (and we have no reason to doubt the correctness of his 

 statement) the greatest depth at which a Monaxonid sponge has hitherto been obtained 

 is 860 fathoms (recorded for Cladorhiza). This depth is now extended to 3000 fathoms 

 (for Cladorhiza longipinna, nobis) by the results of the Challenger investigations, which 



1 For details as to the geographical distribution of individual species the reader is referred to the Table and to the 

 Description of Genera and Species. 



2 For the purpose of simplifying the discussion, varieties are here again treated as distinct species ; which these are 

 will be seen by reference to the Table of Geographical and Bathymetrical Range. 



3 Bronn's Klass. u. Ordnung. d. Thierreichs, Porifera, p. 455. 



