192 THE MICaOSCOPE. 



Spicula having both extremities bent alike — ^bieurvate — have been 

 obtained from Trieste sponge. (See Plate IV. No. 3.) 



Some South Sea sponges have spicula twice bent, and have extre- 

 mities like the flukes of an anchor — ^bicurvate-anchorate ; sometimes 

 the flukes have three pointed ends. (Plate IV. No. 6.) 



The gemmules in fresh-water sponges are generally found in the 

 oldest portions near the base, and each one is protected by a frame- 

 work of bundles of acerate spicula of the flesh, as shown in Plate IV. 

 No. 9 ; but in many marine species, Gfeodia and Pachymatisma, they 

 are principally confined to the crust. In the fresh-water sponges, the 

 amount of animal matter in the gemmules is considerable; but in 

 PachymatistHa, Geodia, and many other marine species, a very small 

 quantity only is ever to be found, the substance of each gemmule being 

 almost entirely composed of minute siliceous spicula ; and if they be 

 viewed when taken fresh from the sponge, and after boiling in acid to 

 remove the animal matter, a slight increase in transparency is the only 

 perceptible diiference of appearance in these two opposite conditions. 



XANTHIDIA. 



In conjunction with the skeletons of the former species, It will be 

 as well to offer a few remarks upon animals long classed with Infusoria, 

 and but rarely found except in the fossil state. There is every reason 

 to believe that the Xanthidia or double-bar animalcules belong to the 

 Desmidiacese. In proof of this it can be shown that their skeletons are 

 composed of horn, and not of silica, as was once supposed. 



The name Xanthidia is derived from a Greek word signifying 

 yellow, that being their prevailing hue. They are found plenteously in 

 a fossil state, imbedded in flint, as many as twenty being detected in a 

 piece the twelfth of an inch in diameter; in fact, it is rare to find a 

 gun-flint without them. When living they may be described as having 

 a round transparent shell, from which proceed spikes varying in shape 

 in different species. One kind was found by Dr. Bailey in the United 

 States of an oval shape, the 288th of an inch in length; and another 

 kind, round in form, by the late Dr. Mantell, at Clapham ; both these 

 kinds were of a beautiful green colour. Specimens of the Branched 

 Xanthidium, found in flint by Dr. Mantell, were from the 300th to the 

 500th of an inch in diameter. (Plate III. No. 4.) Mr. Ralfs says : 

 " That the orbicular spinous bodies so frequent in flint are fossil sporangia 

 of Desmidiacese, cannot, I think, be doubtful, when they are compared 

 with figures of recent ones. Indeed, the late Dr. G. Mantell, who, in 



