65 



initiate in depressed areas, which are sometimes surrounded by slightly 

 elevated marginal rings. These ventral orifices are frequently dis- 

 posed in parallel lines, which occupy the summits of the ridges that 

 intervene between the depressed parts of its surface. 



The great variety that exists in the form of the spicula of this spe- 

 cies is very remarkable. The most common is a simple slightly 

 curved cylindrical spiculum, of equal diameter throughout its whole 

 length, with hemispherical terminations, as represented at fig. 6, a. 

 Occasionally, but not very frequently, there is a single spiculum to 

 be seen, in which there is a gradual attenuation towards one end, but 

 in all the cases that I have observed of this description it does not 

 terminate acutely, but as in the normal form, hemispherically. Among 

 the common forms there are frequently to be observed large tri-radiate 

 and irregularly branched spicula, which have the radii usually termi- 

 nating in a simple point, but they are sometimes bi-furcated, as repre- 

 sented at fig. 5, a. The remaining figures of this group (figs. 3 & 4) 

 represent some of the varieties of these forms which were drawn by 

 the camera lucida, with a power of 100 linear. The central cavities 

 are distinctly visible in all these spicula, (fig. 8) and in the compound 

 ones there is a free communication between all their parts. In some of 

 the spicula the cavity is much larger than in others, and it may be 

 observed with great advantage if a small piece of the sponge is burned 

 to a white ash in the flame of a lamp, and then mounted in Canada 

 balsam. The transparent membrane lining the interior of the spicula 

 is converted 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 500 

 linear, another description of this organ presents itself, which is also 

 very remarkable, in the form of minute stellate spicula, the rays vary- 

 ing in number from three to ten or twelve, passing off from a common 

 centre in every direction. The radii are gradually attenuated from 

 their bases towards their points, and terminate acutely. Four of these 

 spicula, which were sketched from the camera lucida with a power of 

 450 linear, are represented at fig. 2. These singular spicula abound 

 in every part of the yellow fleshy matter of the sponge, in which they 

 are disposed without any definite arrangement. 



If a thin slice be taken from the outer surface of the crust of the 

 sponge, and examined as a transparent object with a power of about 

 300 linear, short fusiform spicula with very abrupt terminations will 

 be observed. They are thickly dispersed throughout the whole of the 

 encrusted surface, but without any apparent order of disposition. — 



TRANS. MIC. SOC. VOL I. p 



