44 



COELENTERATA SPONGIAE 



CLASS 1 



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

 to a delicate silicious disk (Fig. 5 1 28 ). A peculiar forking of the shaft gives 

 rise to candelabras or amphitriaens, while other modifications may produce 

 umbel-like spicules (Fig. 5 1' 26 ), etc. 



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- 



Fio. 51. 



Various forms of Sponge spicules from the Upper Cretaceous of Haldevn, Westphalia ; magnified 25 diameters. 

 !:>, Uniaxial rods and needles. 70, Uniaxial silicious elements with coarse axial canals. 1013, Uniaxial 

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

 17, Regular four-rayed spicule (chevaux de frise). 1821, Trifid anchor-shaped spicules. 2223, Anchors with 

 furcate head-rays. 2425, Irregular four-rayed skeletal elements. 20, Umbel-shaped spicule. 27, Six-rayed 

 spicule. 28, Po'lyaxile 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 (Hexactins 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 Hexadinellida, 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. 



('/) 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. 



Calcareous skeletal elements are much less complicated, and are generally 

 smaller and more perishable than the silicious. Their form is either triaxial 

 (triads), tetraxial (tetraxons), or nail-shaped (monaxons). The triaxial and 



