QUEKETTS LECTURES ON HISTOLOGY. 79 



" A very striking specimen for exhibiting tlie structure of the articula- 

 tions is GonilUna incrassata ; a vertical section of this plant is represented 

 by A in fig. 86. The joints, as there shown, are composed of elongated 

 cells, and from having no lime about them, are soft and flexible, and 

 even of a green colour. A magnified portion of one of the joints is shown 

 at B, and a transverse section at C, both are made up of cells, of which the 

 central ones are much elongated. That the calcareous investment of the 

 Lithophytes is not a mere precipitation from the water, as happens with 

 many of the Characece, is, I think, very evident ; for I have never yet seen 

 any specimen of Coralline in which the part forming the articulation was 

 coated over, nor has any section shown that the calcareous matter is ever 

 present except as a coating to the cell-walls or the spaces between them. 

 In the NuUipores, which have no joints, the cellular structure is of the 

 same nature throughout; there are no elongated cells in the centre, as in 

 the Corallines, consequently it would appear that the articulation is the 

 i-esult of a vital action in some of these cells, whereby they are deprived 

 of the power of selecting a calcareous coating i^i-om the surrounding water, 

 their energies being entirelj' devoted to the function of growth." 



The lectures devoted to the Sponges contain a large number 

 of illustrations of the peculiar forms assumed by the siliceous, 

 calcareous, and cartilaginous matters of which their hard 

 parts are composed. As illustrative of some of the forms 

 assumed by spicula in sponge, we extract the following : — 



" Other spicula are very peculiar, consisting of a central portion, or 

 shaft, the extremities of which are furnished with two or three branches, 

 each of these again subdividing into two or three still smaller branches. 

 These spicula interlace with each other, and produce a sort of coarse net- 

 work ; they are generally found in small sponges, attached to masses of 

 coral ; two specimens of the largest kind are shown at g g, in fig. 14. In 



Fig. 86. 



B A C 



% 





A, a vevtiaxl section of f'oraUina incrassata, showing the joints, b, a portion of one of the 

 joints, c, a transverse section of the same, Iwth magnified 130 fiiameters. 



