126 



Guide to the Invertehrata. 



Sponges, of England have had their skeletons nearly entirely replaced by 



GALLERY iron-peroxide ; very frequently also they are now enclosed in 



Wesfside nodules of solid flint, in part retaining their forms, but their 



Wall-cases interior structures are now merged in the flinty matrix, and only 



7&8, 



Table- ><^^fek ' cl. 



cases 



11-16. 



Fig. 176. — Vertical median section of a litMstid sponge, Siphonia tulipa, Zittel, 

 showing the cloaca (cZ.), the fine incurrent canals {in.), and the excurrent 

 canals {ex.) opening into the cloaca. Upper Greensand : "Warminster, Wilts. 

 (Reduced one-half.) 



Fig. 177. — Doryderma dichotomum, Benett, a raegamorine lithistid sponge. 

 Upper Greensand : "Warminster, Wilts. (Reduced to one-fourth.) 



show the course of the larger excurrent canals and the cloacal 

 cavity. In the sponges from the Upper Chalk of Tlamborough, 

 Yorkshire, the form is usually preserved, and also the main 



