ADDITIONS AND CORRECTIONS. 201 



then it is only in the clearest and most transparent parts of the 

 membrane that they are likely to be seen. 



" The gemmules of this sponge are dispersed in great abmid- 

 ance throughout every part of its substance ; they are of an 

 oval form, the longest diameter being the g^i}-, and the shortest 

 the j^j of an inch. They vary considerably in size, but the 

 above is their average dimensions. WTien seen by direct light 

 with a power of 100 linear, they appear of the same colour as the 

 surroxmding fleshy matter, but when viewed as transparent ob- 

 jects, they assume an iron or slate-gray colour, having their 

 surfaces closely studded with minute papillse, which are pro- 

 duced by the projection of the points of numerous very small 

 spicula, which are imbedded in the crust or shell of the gem- 

 mule, and are disposed in lines radiating from the centre to the 

 circumference of its body. The form of these minute spicula is 

 exceedingly various, but the best developed ones appear conical, 

 having their bases towards the centre of the gemmule, and their 

 apices sUghtly elevating the parts of the outer integument im- 

 mediately above them. The mode of disposition of these spi- 

 cula is best observed when a small portion of the sponge has 

 either been treated with boiling nitric acid, or by incineration 

 in the flame of a lamp. The dissolution of the gemmules is 

 not effected by either of these agents ; and to view them with 

 the greatest effect, they should be gently tritiu-ated with a little 

 water between two pieces of glass, until some of them are 

 broken into small pieces. In these fragments, the spicula may 

 be seen in situ, arranged in the manner I have before described, 

 and cemented together apparently by siliceous matter, which ap- 

 pears to abound in the outer integument of the gemmule. Upon 

 measuring some of those minute spicula in situ, I foimd the 

 average length to be the jjVo o^ ^^ "^^h, or about equivalent to 

 the diameter of a disc of human blood, and their average dia- 

 meter the gjjxTtj of a" inch, so that they are of exceeding minute- 

 ness, as compared with those found on other parts of the same 

 sponge: the rays of the stellate spicula averaging the iX}\)o 

 of an inch in length, by -.j-go^ '" diameter, and those of the 

 skeleton, a diameter of about the i ^l^-o of an inch. In the en- 



