The porifera and coelentera . ^ 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
The porifera and coelentera . ^ 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 ofthe latter. All primary spicules in the morphological characters. Thus0,. Diplodal canal system in Corticium cnnde-lahrum, c, excurrent canal; the incur-rent canal is .shown on the left-hand side,near its commencement in the cortex. (AfterK. E. Schulze, x200.) 132 SPONGES Tetraxonida may be considered ideally—that is to say, from aimrely architectural or geometrical point of view, and withoutprejudice to the question of their actual phylogeny and evolution—as modifications of one of two types: (a) the tetraxan type,characteristic of the megascleres, though not confined to them •and {h) the poli/uxon type, only found among the speaking, the tetraxon type itself could be considered asa modification of the polyaxon, and has probably been derivedfrom it, but for practical purposes it is best to consider the twotypes separately. (a) Tetraxon Type.—The simplest form of tetraxon spicule has fourequal and similar rays meeting at equal angles (Fig. 47, d and ;>).Such a spicule is known as a caWtrops, and though of commonoccurren
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