44 COELENTERATA—SPONGIAE CLASS 1 
triaens) ; atrophy of the fourth ray in the last-named form reduces the spicule 
to a delicate silicious disk (Fig. 51°%). A peculiar forking of the shaft gives 
rise to candelabras or amphitriaens, while other modifications may produce 
umbel-like spicules (Fig. 517°), ete. 
Certain skeletal elements of the Lithistids (Figs. 53-68) may be regarded 
as irregular tetraxons (desmoms), in which the extremities of the four rays are 
prolonged, in knotty, root-like excrescences, or in which, owing to the un- 

Various forms of Sponge spicules from the Upper Cretaceous of Haldem, Westphalia ; magnified 25 diameters. 
1—6, Uniaxial rods and needles. 7—9, Uniaxial silicious elements with coarse axial canals. 10—13, Uniaxial 
cylinders and spheres. 14, Microspined spicule. 15, Clasp-hook flesh-spicule. 16, Bispatulate flesh-spicule. 
17, Regular four-rayed spicule (chevaux de frise). 18—21, Trifid anchor-shaped spicules. 22—23, Anchors with 
furcate head-rays. 24—25, Irregular four-rayed skeletal elements. 26, Uinbel-shaped spicule. 27, Six-rayed 
spicule. 28, Polyaxile silicious disc. 
symmetrical growth, branching, or atrophy of one or more of the arms, extremely 
irregular forms are produced ; for these a special terminology has been devised 
by Rauff. 
(c) Hexactinellid spicules (/eaactins or Triaxons) (Figs. 69-74). The ground- 
form is an axial cross with six equal arms intersecting at right angles like the 
axes of a regular octahedron. Atrophy of one or more of the rays may result 
in pentaxial, tetraxial, triaxial, or even nail-shaped forms, without their real 
character becoming entirely obliterated. Bifurcation or other modifications of 
a number or all of the rays produce those exquisite silicious structures so 
characteristic of the group Hezxactinellida, 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, 
(d) Anaxile or polyaxile bodies of spherical, cylindrical, stellate, or discoidal 
shape, which are not derivable from either of the three ground-forms, occur in 
only a few varieties of recent and fossil silicious sponges. 
Calcarcous skeletal elements are much less complicated, and are generally 
smaller and more perishable than the silicious. Their form is either triaxial 
(triods), tetraxial (tetraxons), or nail-shaped (monavons). The triaxial and 
