ffeteractinellid Sponges. 



121 



Y, Octactinellidse. — The normal spicule of this suborder has Sponget. 

 eight rays, six of which are in a horizontal plane radiating from GALLEBY 

 a common centre, whilst the other two rays form a vertical axis. -v^estSide 

 One or both the vertical rays are often reduced or absent. The wall-casei 

 only representatives of this division are the discoidal forms of 7 & 8, 

 Astr(EOspongia, Romer, from the Silurian. cases" 



VI. Heteractinellidae. — The skeletal spicules are of relatively 11-16. 

 large size, and consist of an indefinite number of rays or arras, 

 varying from six to thirty, radiating from a common centre 

 (Fig. 173«). The body spicules are irregularly arranged in the 

 soft tissues ; those of the dermal layer are interwoven together, 

 and their rays are in part fused with each other. Only fragments 

 of these sponges are as yet known. It may be mentioned that 

 Zittel considers that this and the preceding group are but aberrant 

 Hexactinellid sponges. 



Fig. 173.— DifPerent forms of siliceous heteractinellid and multiaxial spicules 

 and also spicules of fossil calcisponges. («) Spicule of Asleractmella, with 

 twelve rays. {b, c) Dermal spicules of Geodia. (d) Globostellate spicule 

 (e) Microspined globate spicule. (/) Stellate flesh-spicule of tetractinellid 

 sponge, iff) Flattened disc-shaped, probably dermal, spicule, [h) Uiree- 

 rayed equiradiate spicule of calcisponge. (i) Three-rayed sagittate spicule 

 of calcisponge. [J, m) Three-rayed spicules of calcisponge, Tremacystta. 

 [k, I) Four-rayed spicules of calcisponge, Tremacystia. («) Ihree-rayed 

 (tuning-fork) spicule of calcisponge. 



a, X 13; b, X QQ; c, X 66; d, X 66; e, X 66 ; /, X 167 ; 9,^f\ 

 A, X 26 ; i, X lU ; j, X 134; ^, X 80 ; ;, X 80 ; m, X 134 , 

 n, X 134. 



Receptactjlitii)^. — This group of organisms, which includes 

 the genera Receptaculites, Ischadites, and Sphcerospovgia, has been 

 described by Hinde as a family of Hexactinellid sponges, but it 

 has been shown by Rauff that the spicular bodies of which they 

 are composed are probably of carbonate of lime. Their systematic 



