On an orthogenetic series of growth forms in certain Tetraxonid Sponge Spicules

In my memoir on “ The Tetraxonid Sponge-Spicule : A Study in Evolution ” [1921], I have dealt with the principal lines of spicule evolution met with amongst the Tetraxonida, tracing them back to the primitive tetract, whose symmetry is that of the principal axes of a regular tetrahedron. I have, further, shown how variation in the number of the primary rays gives rise in the genus Dercitopsis to forms with two, three and five rays respectively, while in other genera a further increase gives rise to many-rayed “ asters,” the increase in the number of rays being accompanied by diminution in the size both of the rays themselves and of the entire spicule. At the time when the paper referred to was written I had had no occasion to study in detail certain aberrant types of spicule to which Sollas [1888, p. lxiii] applied the term “ streptaster,” and which he defined as asters “ in which the actines do not proceed from a centre, but from a longer or shorter axis, which is usually spiral.” Sollas distinguished these spicules from true asters (euasters), and showed that they are characteristic of the families Theneidæ and Pachastrellidæ, which he accordingly united in one group, the Streptastrosa. The same observer divided his streptasters into three categories, as follows :— (1) Spirasters, consisting of a spire of one or more turns, produced on the outer side into several spines. (2) Metasters, consisting of a spire of less than a single revolution, with fewer but relatively longer spines than the spirasters. (3) Plesiasters, in which the spines or actines proceed from a very short, straight axis, so that they almost appear to radiate from a common centre.