330 Mr. A. Hancock on the Excavating Powers of Sponges, 
The superficial covering of the animal of C. celata, Pl. XIII. 
fig. 3, is able to adhere to the sides of the excavations. If a 
portion of this is carefully removed and placed between plates of 
glass with the external surface uppermost and treated with strong 
nitric acid, large crystalline bodies of a peculiar character are dis- 
covered scattered over it (Pl. XII. fig. 1). These bodies are of a 
pale straw colour, and of the most brilliant lustre and gem-like 
beauty ; the largest measuring ;4,th of an inch across: they are 
mostly irregularly six-sided, depressed, and scale-like ; but stout 
and frequently thickened in the centre, the upper surface being 
covered with numerous, elevated, lozenge-shaped points, each 
generally having an expanded base of a squarish form slightly 
raised above the common surface (fig. 2). These bodies are fre- 
quently congregated into groups, and are occasionally placed 
together side by side. Strong nitric acid does not the least affect 
them after many days’ immersion, the sharp angularity of the 
elevated points remaiming unimpaired, and their brilliancy undi- 
minished. From these facts, and from the manner in which 
these bodies refract light, there can be little doubt that they are 
composed of silex, or some other substance equally dense. Be- 
sides these other crystalline bodies crowd the surface, which bodies 
are as brilliant as theformer, and like them resist strong nitric acid. 
These are mostly minute, being generally ¢oypth of an inch wide ; 
they vary, however, considerably in size, and are occasionally very 
much larger: they are mostly angulated, have an expanded scale- 
like base, and much resemble the lozenge-shaped points of the 
larger bodies. These smaller ones are crowded together into dense 
masses, forming as it were a sort of silicified epithelium; occa- 
sionally they become united by the blending of their expanded 
bases, and then the combined mass has considerable resemniaawe 
to the larger forms before described. 
Similar minute siliceous granules have been observed im all see 
species examined. The allied genus Thoosa, too, has the surface 
provided with siliceous bodies of a very peculiar and novel ap- 
pearance. This genus is unfurnished with spicula in the interior, 
but has occasionally radiating ones supplying the surface, which is 
almost entirely composed of the siliceous bodies just alluded to. 
If a portion of the animal adhering to the chamber-walls of 
Thoosa be removed and placed with the surface uppermost, and 
examined in the microscope as an opake object, it is seen to be 
covered with a whitish semi-pellucid crust of a granular ap- 
pearance; on increasing the power to about 200 diameters, 
this crust is seen to be composed of a multitude of crystalline 
bodies formed of nodules. On examining these bodies by trans- 
mitted light with a still higher power (400 diameters), they are 
observed to rest on a thin membrane distinct from the substance 
