Ixvii 
REPOET ON THE TETRACTINELLIDA. 
regarded as sigmaspires reduced to arcs, a view which is supported by the occasional 
occurrence of S-shaped forms as apparently accidental variations of the normal C-shaped 
sigma. I have myself no doubt as to the derivation of the Desmacidine chela from the 
sigma, but as my views on this point are at variance with those of my colleague, Mr. 
Dendy, and as this form does not occur in the Tetractinellida, I shall not now discuss the 
question. 
The spiraster, which we shall next consider, is the crux of sclerological studies. It 
may be conceived as originating in two different ways, which, however, are not mutually 
exclusive. It may arise as a spiral extension of the centrum of a euaster, or by the 
development of spines about a sigmaspire ; the former view appears to have been 
generally accepted, I fancy rather as a tacit assumption than as a result of investigation. 
In the Theneidse there is good reason to believe, at least so it seems to me, that 
the spiraster originates from a sigmaspire or polyspire, for in its smallest forms, which are 
probably the least removed from the ancestral, it always presents a well-marked spire of 
about two turns, with comparatively small spines, which are produced at intervals in a 
radiate direction from the outer side of the spire. The spire is in this case the most 
conspicuous part of the spicule, and since we have already found the sigmaspire becoming 
spined in the. Tetillidse, which are amongst the simplest sponges of the Tetractinellida, 
we are fairly led to conclude, in the absence of evidence to the contrary, that the sigma- 
spire is the parent of the spiraster. 
Further, in the Theneidse the euaster would appear to proceed from the spiraster 
and not the spiraster from the euaster ; thus as the spiraster increases in size the spire 
becomes reduced, and the spines increased in length, till the various forms of metaster 
result ; the increase in size continuing, the spire sinks into insignificance, and the spines 
acquire comparatively colossal proportions ; the plesiaster, which can thus be traced through 
a continuous series of transitions from the spiraster, is scarcely distinguishable from the 
euaster. In the lower forms of the genus Thenea the course of development proceeds no 
further, but in others the plesiaster is absent and its place taken by a euaster, which only 
differs from the plesiaster in the fact that its actines all proceed from a common centre. 
While the spiraster can thus be traced through a perfect gradational series into the 
aster, the converse is not true, and I know of no instance but one [Placospongia) in 
which sponges characterized by euasters (Stellettidse, Geodiidse) present any indications 
of a tendency for these to revert to the spirastral form ; the modifications of the aster are 
of a completely different kind, being chiefly in the direction of a reduction in the number 
of the actines. 
While some asters thus originate from spirasters, and these again from sigmaspires, 
it is quite possible that there are other asters possessing a different history; thus in 
Placina, the simplest of all Tetractinellida, microcalthrops occur which, unless this sponge 
is to be regarded as a reduced Theneid, not a very likely supposition, must be regarded 
