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



structure, — they appear to be of a more trustworthy morphological character. The function 

 of the skeleton is to support the soft parts. The heterogeneous and homogeneous skeletal 

 fibres are equally fit for this function ; and when once a sponge has adopted the 

 heterogeneous fibres, it would but conserve them. One might logically compare the 

 heteroeeneous and homogeneous skeletal fibres with calcareous and siliceous skeletal 

 spicules, these latter propping the sponge-sarcode equally well in both cases ; and since 

 it is necessary to separate systematically the Calcarea and Silicea, it must be equally 

 necessary to separate the Ceratina and the Psammonemata. 



To sum up, the procedure of the above-mentioned systematists may be regarded as 

 very logical. Yet each question admits of numerous answers equally logical and 

 undeniable ; and it is not to be forgotten that what we think to-day to be thoroughly 

 logical, we may perhaps regard to-morrow as quite impossible, that the really logical is 

 that which alone corresponds with the reality. The ascribing of such systematic 

 importance to the properties of the skeleton in question cannot be reconciled with the 

 reality, and in no case can these properties serve as the basis for the primary subdivision 

 of the group Keratosa. Such a subdivision would express that the Keratosa with 

 homogeneous skeletal fibres form one phylogenetic branch of the group, the Keratosa 

 with heterogeneous fibres another, phylogenetictilly equivalent to the first, the 

 intermediate connecting links having died out. This, however, is not the case, the 

 differences in the structure of horny fibres, as already pointed out by Vosmaer, 

 being but of a quantitative nature. There are amongst the Keratosa, forms, the horny 

 envelope of whose heterogeneous skeletal fibres is very thin {Apliisina, Aijlysilla, 

 Darwinella), so that the pith-substance forms the main part of the fibre ; there are 

 again other forms ( Verongia, lAiffaria), the horny walls of whose fibres are far thicker, 

 and, at least in some representatives of the genus Luffaria, there are to be found 

 amongst fine fibres, fibres quite similar to those of Euspongia, or Spongelia, viz., fibres 

 apparently entirely devoid of any central diff"erentiation (PL IX. fig. 6). I say 

 apparently, for, thanks to F. E. Schulze, we know that each normal horny fibre does 

 possess what he calls an " Achsenstrang." He has been able to discern it in the fibres of 

 Euspongia ^ and Spongeliaj'^ and that certain homogeneous fibres show no differentiation, 

 even under high microscopic power, seems to be due to the fact that, hand in hand 

 with a more voluminous development of the pith-substance, there is a variation 

 in its chemical and optical properties. In the fibre of an Aplysinu or Aplysilla the core 

 and the surrounding horny laminae can be readily distinguished under very low 

 magnifying power, and the designation " hollow-fibred," which has been adopted by 

 many spongiologists with respect to forms like Aplysilla or Verongia, owes its 

 existence to the fact that even when simply dried out, the skeletal fibres of the above 

 forms show no more trace of the pith-substance. 



1 Zeitschr. f. loiss. Zool, Bd. xxxii. p. 633. ^ Ibid., pi. \i. figs. 6 and 7. 



