200 ADDITIONS AND CORRECTIONS. 



flame of a lamp, and then mounted in Canada balsam. The 

 transparent membrane lining the interior of the spicula is con- 

 verted by this means into an opaque carbonaceous film, which 

 becomes distinctly visible to the observer. 



" If the fleshy substance of the sponge be examined by 

 transmitted light, with a power of about 500 linear, another and 

 very curious description of these organs present themselves in 

 the form of minute stellate spicula, the rays varying in num- 

 ber from three to ten or twelve, passing ofl" from a common 

 centre in every direction. The radii are gradually attenuated 

 from their basis towards their points, and tei'minate acutely. 

 These singular spicula aboimd in every part of the yel- 

 low fleshy matter of the sponge, in which they are dispersed 

 without any definite arrangement. 



" If a thin slice be taken from the outer siurface of the crust of 

 the sponge, and examined as a transparent object with a linear 

 power of about 300, numerous short fusiform spicula, with very 

 abrupt terminations, will be observed. They are thickly dis- 

 persed throughout the whole of the encrusted surface, but with- 

 out any appai-ent order of disposition. Upon submitting them 

 to examination with a power of 900 linear, their surfaces were 

 observed to be irregularly tuberculated. These little spicula, 

 from their great abundance in the encrusted surface of the 

 sponge, are justly entitled to be distinguished as the spicula of 

 the crust. They are fomid occasionally, but comparatively 

 rarely, along with the stellate spicula of the fleshy substance, 

 and they also occur thinly dispersed beneath the surface of the 

 great excurrent canals. I did not succeed in detecting any or- 

 ganization in the epidermal membrane of the crust, probably in 

 consequence of its being obscured by the great abundance of 

 minute spicula imJnediat^ly beneath its surface ; but in the 

 membrane lining the interior of the excui'rent canals, there 

 were munerous minute vessels or fibres, disposed in parallel 

 lines in one part of the tissue, while at another spot, they as- 

 sumed the form of an irregular reticulation. These fibres or 

 vessels are exceedingly minute, and it is requisite that they 

 should be observed with a power of at least 800 linear, and 



