CXLV 
THE VOYAGE OF H.M.S. CHALLENGER. 
organisms are eadi to be regarded as a nexus of more or less dependent variables ; all 
change together, but at different rates ; but amongst the less dependent are the micro- 
scleres, which thus maintain a relative constancy amidst the shifting changes of the rest. 
To the same conclusion Schulze appears to have been led, since in his already published 
system of the Hexactinellida, the systematic importance of the microsclere stands next in 
order to the Lyssacine or Dictyonine character of the main skeleton ; just as in our classi- 
fication it follows on the Lithistid or Choristid character. The same result will I expect 
be attained in the case of Monaxonida, when this large group has been more exhaustively 
studied. That this expectation is founded on something more than analogy will appear 
from some remarks {vide Appendix II.) on those Monaxonid Sponges which my colleagues, 
Messrs. Eidley and Dendy, transferred to me for examination on the supposition that 
they belonged to the Tetractinellida, a supposition which we shared together, and which 
was strengthened by the apparent absence of any close affinities between these Sponges 
and the rest of the Monaxonida with which my colleagues were familiar. 
It will probably conduce to clearness if I give here an abstract of the classification 
of the Choristida so far arrived at, omitting the definitions, which will be given later : — 
Order I. CHORISTIDA. 
Suborder I. SIGMATOPHOEA. 
Family I. Tetillidse. Family II. Samidse. 
Suborder II. ASTEOPHOEA. 
Demus A. Streptasteosa. 
Family I. Theneidse. Family II. Pachastrellidse. 
Demus B. Euastrosa. 
Family I. Stellettidse. Appendix — Epipolasidse ? 
Demus C. Stereastrosa. 
Family I. Oeodiidse. Family II. Placospongidae. 
Suborder III. MICEOSCLEEOPHOEA. 
Family I. Placinidse. Family II. Corticidse. Family III. Thrombidse. 
In order to arrive at more general results first, we shall postpone a consideration of 
the families of the Choristida till we have passed in review the Lithistida. 
