1148 A MONOGRAPH OP THE AUSTRALIAN SPONGES, 



(m), this contains the ordinary triradiate spicules disposed 

 exactly tangentally. It is formed by Mesoderm — a thin wall 

 with sparsely scattered tissue cells — and a coating of flat 

 ectodermal pavement cells on either side. The surface a c b 

 forms part of the outer surface of the Sponge, d ef on the other 

 hand is part of the surface of the pseudosculum. The pores in 

 the outer surface (p) are of course inhalent, they are small, 

 those of the inner surface are not so numerous and much larger 

 (P) they are exhalent. The pseudoscular tube very flexible, 

 and following every current of water acts like a moveable 

 chimney, and evidently greatly assists the flagellate cells in 

 producing a strong current of water through the Sponge. If 

 we consider the Pseudoscular tube as a real gastral cavity and 

 the Ascon tubes as ciliated chambers, we have an ordinary 

 Leuconide or Syllcibide Sponge before us. Inhalent (p) and 

 exhalent ( P) canals are clothed with low epithelium. The inner 

 surface of the tube (g h) is covered by the ordinary flagellate 

 cells. The spicules, regular Triradiates have the shape of low 

 pyramids following absolutely tangentally the curvature of the 

 Ascon tubes. Their points never protrude. 



Fig. 13. — Ascetta Macleayi. R. v. L. Transverse section through the solid 

 peduncle. Osmic acid, Alumn Carmin, DD. Oc. I. In the tubes 

 and pseudoscular wall we find only a single layer of spicules. In 

 the peduncle we meet with a strong cote of three to five layers 

 of spicules with numerous multipolar tissue cells in the 

 mesoderm. The central pai't is destitute of spicules and filled 

 with numerous highly colourable cells (a) which appear 

 spherical in the specimens treated with hardening reagents. 

 They may be amorboid cells ; it appears not unlikely that they 

 are young stages of ova or spermatophores. In which case the 

 peduncle must be considered as a kind of sexual organ or 

 brooding place. (Similar to the formation of ova in the hollow 

 peduncle of Homoderma and the Hydrorhiza of some sessil 

 Hydromedusse.) 



Fig. 14. — Homoderma Sycandra. R. v. L. Growing from an Aplysilla 

 violacea, painted from life in natural size. 



Fig. 15. — Homoderma Sycandra. R. v. L. Longitudinal section combi- 

 nated picture. The same kind of Entodermal flagellate cells 

 throughout the Sponge and the Spongorhiza. Ciliated tubes 

 as in Syconidre. Spongorhiza hairy. Thes ummits of the 

 ciliated tubes crowned by tufts of Acerates. Regular disposition 



