62 COBLES TERAT A SPONGIAE CLASS i 



skin punctured by large, irregularly shaped apertures, uniformly enveloping the 



FIG. 83. 



Camerospongia fungiformis, 

 Goldf. sp. Planer; Oppeln, 

 Silesia. Natural size. 



FIG. 84. 



Cystispongia bursa, Quenst. Cuvieri-Planer (Turonian) ; Salz- 

 gitter, Hanover, a, Sponge, natural size ; b, Dermal layer with 

 underlying skeletal framework ; c, Skeleton, 12/j. 



whole sponge body. Body composed entirely of tubes. Cretaceous and still 

 living. 



Sub-Class 4. CALCISPONGIAE. Calcareous Sponges. 



Skeleton composed of calcareous spicules of three-rayed, foitr-mii<><1, or uniaxial t <//><*. 



The external form of the Calcisponges is quite as variable as that of the 

 silicious sponges, and reminds one particularly of the Lithistida. Like the 

 Lithistids, too, the thick-walled Leucones and Pharetrones have a canal-system 

 consisting of a central cavity into which radial excurrent 

 canals conduct ; while the numerous tributaries of. the latter 

 end in ciliated chambers which are fed by fine incurrent 

 canals. In the Sycones the wall is perforated by simple 

 radial tubes, but in the thin-walled Ascones it is pierced by 

 mere holes. 



The calcareous skeletal elements lie free in the soft parts, 

 sometimes forming but a single layer disposed in the same 

 plane (Ascones) ; sometimes their disposition is more or less 

 distinctly radial, following the canal courses (Sycones) some- 

 times they are irregularly crowded together (Leucones) ; and 

 sometimes they are closely apposed in the form of solid anastomosing fibres 

 (Pharetrones). Regular triaxial spicules are of the most common occurrence, 

 next monaxial spicules, sharpened on both sides, and more rarely four-rayed 

 spicules. 



Owing to the ready solubility of the skeletal elements in calcareous sponges, 

 they are usually but poorly preserved in the fossil state, and are ill-adapted for 

 microscopical investigation. The three-rayed and rod-shaped spicules which are 

 united in fibres are seldom distinctly recognisable as such, since, as a rule, they 

 are either wholly or partially dissolved, and are converted into homogeneous or 

 crystalline fibres of calcite (Fig. 88) ; in these minute threads of calcite may be 



\( : >V -A^_ ; I ^ 



FIG. 8: 



