REPORT ON THE TETRACTINELLIDA. 
419 
Family III. Spirastrellid^, Ridley and Dendy. 
Sjnrastrellidx, Ridley and Dendy. loc. cit., p. 229. 
The megascleres are rhabdi or styles, the microscleres are spirasters or discasters. 
The microscleres form a dense dermal layer. 
Genera. 
Spirastrella (0. Schmidt), Ridley and Dendy, loc. cit., p. 229. 
LaXrunculia, Bocage, Ridley and Dendy, loc. cit., p. 233. 
Podospongia, Bocage, Journ. Sci. Math. Rhys, et nat. Lisbonne, p. 159, pi. x. 
fig. 1, 1869. 
The Relations of the Spintharophora to the Tetractinellida. 
That the Spintharophora are nearly related to the Tetractinellida I do not doubt, 
but am inclined to think that the connection is not as Vosmaer imagines with the 
higher Tetractinellida, but with the lowest, such as Placinidse. If we compare the 
lowest form of the Spintharophora (Astropeplus) with the lowest of the Tetractinellidge 
{Placina), we cannot fail to perceive a close resemblance, but if for Placina we 
substitute the very nearly related Placortis simplex the similarity becomes surprising. 
With the same general kind of canal- and chamber-system, the prevailing spicules of 
Placortis are diactinose asters, or microxeas, O'l mm. in length in Placortis and from 
0‘2 to 0'4 mm. in Astropeplus ; the chief difference between the two forms lies in the 
replacement of the triactinose asters of Placortis by smaller polyactinose asters in 
Astropeplus, and the importance of this difference, though it is not great, is further 
lessened by the fact that the actines of the aster in Astropeplus are inconstant in 
number and are frequently reduced to three. The passage from Placortis to 
Astropeplus is so easy that one cannot help thinking that Astropeplus has been 
derived from some Placinid form by a variation in the number of actines of the 
aster, and a slight modification of the microxea, which is seldom of a quite regular 
form in the Placinidse. The microxeas of Astropeplus may readily pass into megoxeas, 
and, given these two forms, asters and oxeas, the different spicules of the Spintharophora 
can readily be derived. In Epallax, which probably lies near the root of the 
AxineUidse, we find , asters similar to those of Astropeplus, and oxeas, which in 
accordance with the mode of growth of the sponge have acquired greatly increased 
size both in length and breadth. The tendency to pass into strongyles or 
strongyloxeas is manifested by these spicules, and the strongyloxeas and styles of 
the more differentiated AxineUidse are thus foreshadowed. The canal -system and 
