\ Esmark ow the Geological History of the Earth. 121 



completed, which the earth required to the full development of 

 its own constitution ; that, after it began, it proceeded by suc- 

 cessive steps from the less to the more perfect formations, end- 

 ing with man as the head of the whole. 



Observations on the Structure and Functions of the Sponge. 

 By R. E. Grant, M. D., F. R. S. E., V. L. S., M. W. S., 

 Honorary Member of the Northern Institution, &c. Com- 

 municated by the A uthor. Concluded from the preceding 

 Volume, p. 351. (With a Plate.) 



JL HE silicious and calcareous spicula above described are group- 

 ed into strong fasciculi, which are disposed around the internal 

 canals of the sponge, in the order best calculated to defend these 

 passages from compression, and from the entrance of extraneous 

 bodies, and likewise to form between the canals certain inter- 

 stitial spaces for the development and exit of the ova. Like the 

 hard parts composing the skeleton in other animals, these earthy 

 spicula are maintained in their relative situations by a tough li- 

 gamentous matter, distinct from the other soft parts of the 

 sponge. In the horny species, however, where the axis is com- 

 posed of cylindrical tubular horny fibres, ramified and continu- 

 ous throughout the whole body, this connecting cartilaginous 

 matter appears to be unnecessary, and, from the examination of 

 dried specimens, it appears to be altogether wanting. The exa- 

 mination of the living properties of the axis in the horny species 

 forms a subject of curious and interesting inquiry, which must 

 be left to those who have opportunities of observing them alive 

 in warmer latitudes, as they do not seem to inhabit the British 

 shores. The dried filaments of the S. Jistular'is^ Lam. when 

 viewed through a powerful microscope, appear to consist of one 

 continuous ramified tube, whose central cavity (PI. II. Fig. 19. 6) 

 is entirely filled with a dark opaque granular matter, which does 

 not consist of spicula, while the sides of the tube (a) are trans- 

 parent and amber coloured like common cat gut. In the S. offi- 

 cinalis^ where the filaments are much finer, the sides of the tube 

 (Fig. 20. a) have the same colour and homogeneous appearance, 

 but the central cavity (h) appears empty. Mr Ellis states, 



