194 



PORIFERA 



fusion of apposed terminal flat expansions of the rays, and in some, 

 possibly all, genera a continuous deposit of calcium carbonate 

 ensheaths the spicular reticulum. Thus they recall the forma- 

 tion of the skeleton on the one hand of the Lithistida and on 

 the other of the Dictyonine Hexactinellida (see pp. 202, 211). 

 " Tuning-forks " may occur in the dermal membrane. 



The Astroscleridae, as known at present, contain a single 

 genus and species, apparently the most isolated in the phylum. 



Astrosdera v.ul- 

 leijnna, Lister. A, the 

 Sponge. X about 3. ^j. 

 The ostia on its distal 

 surface. B, a portion 

 of the skeleton show- 

 ing four polyhedra 

 with radiating crystal- 

 line fibres. C, an 

 ostium ; the surround- 

 ing tissue contains 

 young stages of poly- 

 liedra. (After Lister.) 



Astrosdera wil/ri/ana ^ was brought back from the Loyalty Islands, 

 and from Funafuti of the Ellice group. Its skeleton is both 

 chemically and structurally aberrant. In other Calcarea the 

 calcium carbonate of the skeleton is present as calcite, in 

 Astrosdera as aragonite, and the elements are solid polyhedra, 

 > J. J. Lister in Willcy's Zoolojical Jksulls, pt. iv. 1900, p. 459. 



