62 COELEN TERATA—SPONGIAE CLASS I 
skin punctured by large, irregularly shaped apertures, uniformly enveloping the 

Fic. 83. Fic. $4. 
Camerospongia  fungiformis, E Cystispongia bursa, Quenst. Cuvieri-Planer (Turonian) ; Salz- 
Goldf. sp. Planer; Oppeln, gitter, Hanover. a, Sponge, natural size; b, Dermal layer with 
Silesia. Natural size. 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, four-rayed, or uniaxial types. 
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 exeurrent 
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 
Fic. 85. plane (Ascones) ; sometimes their disposition is more or less 
Triaxial skeletalelenents distinctly radial, following the canal courses (Sycones) ; some- 
E Ascon, 99/3. i “ 
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 

