The porifera and coelentera . )icuies which compose theskeleton of the Tetraxonida are divisible into megascleres andmicroscleres—two categories which in the order Tetractinellida aresharply distinct from one another, diflering not only in size and SPONGES. Il;. SS. Transvi^rse section across an excurrent canal and surrounding choanosome of Cydoniumtosaster, Soil, e, excurrent canal ; /, flagellated chambers coiinnunicating with it by aphodali-anals ; t, an incurrent Kinal cut across ; s, a sterraster ; o, an oxea cut across. (Alter SoUas,• kalknger Report, X 12i.) function, but also ver}^ fre
The porifera and coelentera . )icuies which compose theskeleton of the Tetraxonida are divisible into megascleres andmicroscleres—two categories which in the order Tetractinellida aresharply distinct from one another, diflering not only in size and SPONGES. Il;. SS. Transvi^rse section across an excurrent canal and surrounding choanosome of Cydoniumtosaster, Soil, e, excurrent canal ; /, flagellated chambers coiinnunicating with it by aphodali-anals ; t, an incurrent Kinal cut across ; s, a sterraster ; o, an oxea cut across. (Alter SoUas,• kalknger Report, X 12i.) function, but also ver}^ frequently incertain forms of microsclere, suchas the commonly occurring asters,conform to types of structure notrepresented among tlie megas-cleres. In this respect we find amarked contrast with the Hexac-tinellida, where all the spicules,even the asters, are variations ofthe one fundamental triaxon type. Forms of the Spicules.—In thefirst place, a distinction must bedrawn l^etween the simple (prim-ary) spicules, on the one hand,and the compound (secondary)spicules or desmas, characteristic ofthe sub-order Lithistida, on theother hand. Since the desma isitself founded, in most instances,upon a primary s{)icule, we maycommence with the discussion o
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