330 Mr. A. Hancock on the Excavating Powei^s of Sponges, 



The superficial covering of the animal of C. celata, PI. XIII. 

 fig. 3, is liable 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 g^^y 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 afiect 

 them after many days' immersion, the sharp angularity of the 

 elevated points remaining 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 the former, and like them resist strong nitric acid. 

 These are mostly minute, being generally g-y^Q^th 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 resemblance 

 to the larger forms before described. 



Similar minute siliceous granules have been observed in all the 

 species examined. The allied genus Tlioosa, 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 



