clx 
THE VOYAGE OF H.M.S. CHALLENGER. 
6. NOTE ON PHYLOGENY. 
Some remarks on the relationship of the Monaxonida to the Tetractinellida will be 
found on p. 419. 
Of relationships among the Tetractinellida so highly probable that they may almost 
be regarded as definitely ascertained, a representation is given in the following scheme, 
in which direction of descent, when this is supposed to be known, is indicated by an 
arrow, when not by a straight line : — 
Sigmatophora Sterrastrosa 
1 
I 
Lithistida Streptastrosa Euastrosa 
If we attempt to extend our investigations to the whole of the Tetractinellida and 
the associated Monaxonida we encounter contending hypotheses, upon the relative merits 
of which it is at present impossible to decide. Thus if we accept the certainly very 
tempting hypothesis, that the trisene is descended from a calthrops or a microcalthrops, 
and the sigmaspire (as analogy in the case of the Lithistida would suggest) from a 
euaster, then the two most probable schemes of affiliation are the following, first : — 
Tbteactinellida. Monaxonida. 
Sigmatophora Meniscopliora 
Astrophora Lithistida Spintharophora 
I 
Microsclerophora 
And next : — 
Meniscophora 
Sigmatophora 
Astrophora Spintharophora 
I 
Microsclerophora — 
These two schemes are not mutually exclusive, for it is possible that the Menisco- 
phora are not a monophyletic group ; part may have been derived from the Sigmatophora 
and part from the Spintharophora ; and the possibility of a Sigmatophorous origin for a 
part at least of the Meniscophora is suggested by the frequency with which the trisene 
