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



be in many points quite artificial ; but we must console ourselves with the impossibility 

 of altering the matter, and although wdth respect to this group we have to deal with 

 anatomical cho,racters almost exclusively, we must ground upon them our systematic 

 arrangements, proceeding, however, with all possible prudence, and bearing in mind the 

 necessity of a critical attitude towards our own conclusions. 



These general remarks will now be followed by more special observations as to the 

 systematic value of various characters of the sponges in question. I begin with the 

 properties of the skeleton. Its high systematic significance has always been recognised ; 

 what is more, it has been exaggerated. The systems of Duchassaing de Fonbressin and 

 Michelotti,^ of Gray,^ Hyatt,^ Carter,* are founded simply on its properties. In the diag- 

 noses of the last-named naturalist, indeed, the " sarcode " is also very often spoken of ; but 

 these particulars might in almost all cases be omitted ; and it is precisely owing to the 

 circumstance that the former spongiologists were inclined to pay to the properties of the 

 skeleton an exclusive attention, that its modifications for a long time past have been 

 submitted to a careful study, and considerable difi'erences in its structure discovered. 

 It has been stated that while in some of the Keratose Sponges the horny fibres show no 

 differentiation in their central and peripheral parts, the fibres of the skeleton of many 

 others admit of a distinction into a central pith-substance (" Marksubstanz " of German 

 authors) and of a horny laminar envelope ; and while the homogeneous fibres are almost 

 always more or less cored with foreign bodies, so that the horny substance shows in 

 many instances a tendency to disappear entirely, the heterogeneous skeletal fibres, on the 

 contrary, are in most cases quite free from any foreign enclosures. Finally, it has been 

 stated that in certain sponges [lanthella) the horny envelope of their skeletal fibres is 

 charged with true ceUs (Flemming,® Carter ''). To the first of these characters the greatest 

 systematic significance has been repeatedly ascribed, and the two spongiologists to whom 

 we owe the most elaborate systems of Keratosa (Carter and Hyatt) have made use of it 

 in order to subdivide the grouji into two main divisions (Ceratina and Psammonemata, 

 Carter ; Aplysinte and Sponginse, Hyatt), which proceeding deserves a more detailed 

 critical study, since we have recently learned from the spongiological writings of F. E. 

 Schulze that the above-mentioned differences in the properties of the skeleton present a 

 certain antagonism with regard to the internal structure of the soft parts, that both in 

 Ceratina or Aplysinse and Psammonemata or Sponginse two types, or at least modifica- 

 tions, of the canal system, are to be seen. Schulze ascertained, in fact, that while an 

 Aplysina, and on the other hand a Euspoiigia or Cacospongia, are characterised, in 

 the organisation of their canal system, by comparatively small, round, or pear-shaped 



1 Spongiaires de la mer Caraibe, Harlem 1864. ^ Ptoc. Zool. Soc. Lond., 1867, pp. 503, 508. 



• Revisiou of the North American Porifers, Boston Soc. Nat. Hist., 1875 and 1877. 



* Ann. and Mag. Nat. Hist., ser. 4, vol. xvi. pp. 132, 134-140, 1875. 



6 WUrzhurger Verhandl., N. F., Bd. ii. • Ann. and Mag. Nat. Hist., ser. 5, voL viii. p. 112, 1881. 



