Siliceous Sponges. 



119 



The central node of the spicule may be simple or slightly inflated, 

 but in many sponges of this group it has the form of a hollow 

 octahedron, and such spicules have been termed lantern-spiculcs. 

 This form is present in the genus Ventriculites. Smaller flcsh- 

 spicules of very varied and beautiful forms are present in recent 

 Hexactinellids, but it is very rarely that they are found fossil 

 (Fig. 172^, A). ^ 



In one division of these sponges, known as the Lyssakina, the 

 spicules are either loosely imbedded in the soft tissues or arranged 

 in the form of a definite meshwork, but they are not as a rule 

 cemented together ; in the division of the Dictyonina, however, 



Sponges. 



0ALLEB7 

 X. 



West Side. 



Wall 

 7&8, 

 Table 

 cases 

 11-16 



Fig. 172 —Spicules and fragments of the spicular mesh of fossil hexactmellid 

 sponges. («) Spicule with lantern or octahedral node, showing the axial 

 canals extending from the centre of the node, (b) Detached spicule with 

 compact node, {c) Spicule in which only five rays are developed, {e) 

 Frac^ment of spicular mesh of dictyonine hexactmellid, Sestrodtctyon. 

 (/)" Fragment of spicular mesh with lantern nodes of Cceloptychuim. 

 (ff) Pinule flesh-spicule. (^) Amphidisc flesh-spicule. 

 a, X 66; *, X 40; c, X 40; e, X 47 ; /, X 40 ; g, X 134; h, X 114. 

 the spicular arms of adjoining spicules overlap each other, and 

 are completely fused together in a deposit of silica so as to form 

 a connected skeletal framework with definite quadrate or cubical 

 interspaces (Fig. 172^, /). These sponges, moreover, are fre- 

 quently invested with a perforated exterior siliceous membrane, 

 formed by modified spicules. Another peculiar feature of Lyssakinc 

 Hexactinellid sponges is the development of twists or tufts of 

 very elongate, rod-like spicules, which served to anchor the sponge 

 in the soft mud of the sea-bottom. Frequently these anchoring 

 spi.ular tufts are preserved in the rocks, whilst the body-skeleton 

 of the sponges has completely disappeared. 



