REPORT ON THE MONAXONIDA. liii 



CHAPTER III.— THE CLASSIFICATION OF THE MONAXONIDA. 

 I. The Data of Classification. 



That the detailed classification of any group of sponges would be a difficult task, one 

 might be led to suppose on d priori grounds alone, from the consideration of the low- 

 position which sponges occupy in the animal kingdom, or, in other words, from the con- 

 sideration of their low degree of specialisation and the consequent scarcity of definite 

 distinguishing characters. No two sponges are exactly alike, and yet all the so-called 

 species in a genus resemble one another so nearly, and are so often still connected by 

 intermediate forms, that the distinction of species often becomes purely an arbitrary 

 question and a matter of individual judgment. The characters of the species do not 

 appear to be nearly so firmly fixed as in higher groups, they seem to be in a very plastic 

 condition, and capable of almost infinite modification according to their surroundings. 



This being so it is obvious that we must, for purposes of classification, endeavour to 

 discover those structures in a sponge which are least subject to modification. That 

 external form is useful for this purpose only to a very limited extent, and when the 

 utmost caution is exercised, is now agreed on all hands. If further proof be needed of 

 this fact the Challenger collection supplies it in abundance, and it is perhaps worth our 

 while to give one remarkable illustration. 



We shall describe in the systematic portion of the present work a number of species 

 shown by their spicules to belong to three distinct genera, viz., Cladorhiza, Chondro- 

 cladia, and Axoniderma, all characterised by the same external form, and that a very 

 remarkable one. In brief, each sponge consists of a small, subglobular or conical body 

 provided with an equatorial zone of long, stiff, supporting processes. To this peculiar 

 external form {vide PL XX. figs. 2, 4, 5, 7, 8) we have given the name " Crinorhiza 

 form " ; all sponges as yet known to possess it come from very great depths and 

 live on a bottom of soft mud or ooze, and there can be no doubt that the long, radiating 

 processes, usually associated with a single vertically descending, central, root-like process, 

 are for the purpose of supporting the sponge and preventing it from sinking into the 

 soft mud on which it lies. We only find the Crinorhiza form in species which live in very 

 deep water ; species of the same genera which inhabit shallower water do not possess it. 



It would be an easy matter to adduce further instances in support of the same law, 

 namely, that similar external conditions beget similar external forms even in species of 

 distinct genera, but it is needless to multiply proof. Moreover, the external form of 

 one and the same species (as shown by its spiculation) varies greatly, sometimes with no 

 apparent reason at all, as, for example, in Petroda similis, nobis (PI. III. figs. 3, 4). 



We must then look elsewhere than to external form for a guide to classification. 



