1 68 



PORIFERA. 



usually on a longer or shorter stem, and attached thereby to some 

 foreign body. In some cases, however, the stem is wanting, and 

 the sponge is attached simply by diverging root-fibres. The ex- 

 halant water-canals open at the summit of the sponge by oscula 

 situated within an apical cavity or cloaca, while the inhalant canals 

 open by pores on the lateral surfaces. The skele- 

 ton-spicules (fig. 54, b) are four-armed, the ends 

 of the rays being expanded and furnished with 

 tubercles and intervening depressions, by means of 

 which they are interlocked with the rays of adjoining 

 spicules so as to produce a rigid framework. As 

 pointed out by Sollas, there is a close agreement in 

 form between the spicules of Siphonia and those of 

 the recent genus Discodermia. The genus Jerea is 

 very closely allied to Siphonia, but there is no general 

 cloacal chamber ; while in the related genus Hallirhoa 

 the body of the sponge is lobed. 



In the Tertiary period, lastly, the remains of 

 Lithistids are for the most part of infrequent oc- 

 currence, though numerous sponges belonging to 

 this order have been detected in Miocene strata 

 in Algeria. 



Order 6. Hexactinellid^e. — In this group of 

 the siliceous Sponges, the skeleton is composed of 

 six-armed flinty spicules, the rays of which are at 

 Fig. 58.— Siphonia right angles to each other (fig. 59, b). In the 

 LkSsdd^ponge" 115 centre of each spicule are three canals cutting each 

 other at right angles, and forming an axial six-rayed 

 tube. In some Hexactinellid Sponges, the spicules are simply 

 united by the soft tissues. More commonly, the spicules are 

 fused with one another by the ends of corresponding rays, or are 

 united by means of amorphous silica, so as to form a trellis-work 

 of rectangular or polyhedral meshes, the individual spicules of 

 which may be only recognisable by the persistence of their axial 

 canals (fig. 63). The " flesh-spicules " are fundamentally six-armed, 

 but may give off secondary branches so as to form a rosette. 



The Hexactinellid Sponges may be divided into two groups ac- 

 cording as the skeletal spicules are fused with one another or are 

 free. In the group of the Dictyonina, comprising such genera as 

 Eiiplectella, Ve?itriculites, Tremadictyon, Cceloptychium (fig. 59), and 

 many others, the skeleton is composed of six-armed spicules which 

 became fused with one another by a secondary deposit of silica, so 

 as to form a latticed trellis-work. On the other hand, in the group 

 of the Lyssakina, comprising such genera as Stauractinella and 

 Hyalostelia, the skeletal spicules are interlaced, and are usually 



