24 ]Mr. H.J. Carter on the Ultimate Structure of Spongilla. 



it may be situated on any part of the shaft, or present in variable 

 pluraUty (e,/^). 



In my notes on the organization of the Infusoria I have 

 stated* that the spicule grows from a hair-like extension pro- 

 jecting from either side of a glairy globule, which now is found 

 to be nothing but an inflation, probably filled with refractive 

 fluid. However, as we see that the spicule grows by layers de- 

 posited on the original one, which, therefore, forms the lougitu- 

 dinal canal, and that, when it is fully formed, the inflation is no 

 longer visible, at the same time that the inflated portion is con- 

 tinuous with the outermost layer of the spicule while the latter 

 is growing, it does not seem improbable that the first layer does 

 arise from the linear extension of a globule, and that every suc- 

 ceeding layer is formed in the same way ; (hence, as there will 

 be more layers in the centre than at the circumference, the spi- 

 cule assumes a pointed form at each extremity) ; the inflation 

 subsiding with the extension of each layer, until the final one 

 leaves no inflation at all, and the spicule assumes its ultimate 

 form. The original form of the spicule, therefore, appears to 

 determine its ultimate one, and the spiniferous character of this 

 will therefore depend on the greater or less tendency in all the 

 layers to assume this figure, whatever it may be (fig. 8 h). AYhe- 

 ther the spicule is developed throughout by the same sponge- 

 cell, or whether, after it very much exceeds in area any of the 

 sponge-cells, which is always the case with the large spicule, a 

 plurality ai'e engaged in its ultimate development, or whether it 

 continues to grow in the intercellular substance until it has 

 reached its largest size, I am ignorant ; but I have ascertained, 

 by a series of observations and measurements, that it does not 

 grow after having become denuded of the sponge-substance. 



Again, I am not certain that the inflation, though extremely 

 common, is always concomitant, or even necessary in the forma- 

 tion of the spicule, for many present no trace of it in any stage 

 of their development, from the time they are first \'isible to that 

 in which they have attained their largest size. 



Long before the spicules are formed, however, they are trans- 

 ported from place to place by the sponge-cells individually, and 

 when too large for this, are twisted and turned and grouped 

 together by the general mass, to meet the requirements of the 

 case, with as much instinct as that which characterizes the 

 arrangement of the bits of stick in an ant-hill ; while they ap- 

 pear never to become finally fixed until the substance in which 

 they may be imbedded has altogether lost its vitality. 



Investing Membrane. — As the sponge-substance accumulates 

 vertically, the flat transparent border seems to disappear by 

 * Annals, vol. xviii. p. 118. 



