154 



PORIFERA. 



i. In certain sponges (such as the Common Bath Sponges) the 

 skeleton (fig. 48, a) is wholly composed of netted horny fibres, with- 

 out proper " spicules." The substance composing the fibres in such 



Fig. 48. — Forms of the skeleton in the fibrous Sponges. A, Horny, non-spiculate skeleton of 

 the Bath Sponge, enlarged about fifty times ; B, Horny fibre cored with sand-grains ; c. Horny 

 fibre with projecting siliceous spicules (" Echinonematous " Sponge); d, Fibre in which the 

 spongin has been more or less completely replaced by siliceous spicules (" Holorhaphidote " 

 Sponge), b, c, and D are greatly enlarged, and are after Carter. 



types is allied to horn, but not precisely of the same nature, and it 

 is known as " spongin " or " keratode." 



2. In another group of sponges, including most of the commoner 

 forms, the skeleton is more or less extensively composed of siliceous 







a b 



Fig. 49. — Spicules of Sponges. «, Monactinellid skeleton-spicule oiReniera ; b, Monactinellid 

 spicule of Cliona ; c, Monactinellid spicule of Spongilla ; d, Tetractinellid skeleton-spicule of 

 Geodia; e, Skeleton-spicule of a Lithistid Sponge (Jerea) ; /, Flesh-spicule otCribella; £-, Flesh - 

 spicule of Esperia ; h, Flesh-spicule of Hyalonema. All the figures are greatly enlarged. (After 

 Schmidt, Vosmaer, Zittel, &c.) 



needles or "spicula," of various forms (fig. 49). These spicules 

 may be embedded in various ways in a reticulated fibrous skeleton 

 of spongin (fig. 48, c) ; or the horny material may be greatly reduced, 



