}S I 



SPONGIAE 



49 



I 



^^m (b) Tetraxial spicules or tetraxons (Fig. 46 ^^). The normal form is 

 ^Waracterised by four equal rays intersecting like the bisectrices of the plane 

 angles of a regulär tetrahedron. Triaxial forms result from the occasional 

 abortion of one of the rays. One of the rays may become elongated or other- 

 wise modified so as to form anchors (triaens) with three simple or furcate hooks 

 (Fig. 46 18-23^^ Three of the rays may be numerously divided or foliately 

 expanded so as to produce forms resembling thumb-tacks {trichotriaens, phyllo- 

 triaens) ; atrophy of the fourth ray in the last-named form reduces the spicule 

 to a delicate siliceous disk (Fig. 46 ^). A peculiar forking of the shaft gives 

 rise to candelabras or amphitriaens, while other modifications may produce 

 umbellate spicules (Fig. 46 ^^), etc. 



Certain skeletal Clements of the Lithistids (Figs. 48-63) may be regarded 

 as irregulär tetraxons {desmoms), in which the extremities of the four rays are 



Fio. 46. 

 Varlous forms of Sponge spicules from the Upper Cretaceous of Haldem, Westphalia ; magnified 25 diameters. 

 1-6, Uniaxial rod.s and needles. 7-9, Uniaxial siliceous elements with coarse axial canals. 10-13, Uniaxial 

 cylinders and spheres. 14, Microspined spicule. 15, Clasp-hook flesh-spicule. 16, Bispatulate tlesh-spicule. 

 17, llegular four-rayed spicule (chevauxde frise). 18-21, Trifld anclior-shaped spicules. 22-23, Anchors with 

 furcate head-rays. 24-25, Irregulär four-rayed skeletal elements. 26, Umbel-shaped spicule. 27, Six-rayed 

 spicule. 28, Polyaxial siliceous disk. 



prolonged in knotty, root-like excrescences, or in which, owing to the un- 

 symmetrical growth, branching or atrophy of one or more of the arms, 

 extremely irregulär forms are produced ; for these a special terminology has 

 been devised by Rauff. 



(c) Hexactinellid spicules (hexadins or triaxons) (Figs. 65-70). The 

 ground-form is an axial cross with six equal arms intersecting at right angles 

 like the axes of a regulär octahedron. Atrophy of one or more of the rays 

 may result in pentaxial, tetraxial, triaxial, or even clavate forms, without their 

 real character becoming entirely obliterated. Bifurcation or other modifica- 

 tions of a number or of all the rays produce beautifully formed siliceous 

 structures highly characteristic of the group IlexadineUida, which resemble 

 candelabras, double-headed anchors, fir-trees, pitch-forks, rosettes, etc. The 

 fusion of juxtaposed hexactins produces more or less symmetrical latticeworks 

 with cubical interstices. 



VOL. I K 



