840 MICKOSCOPIC FORMS OF ANIMAL LIFE 



In the fossilised condition in which Eozoon is most commonly 

 found, not only the cavities of the chambers, but the canal systems 

 to their smallest ramifications are filled up by the silicious infiltra- 

 tion which has taken the place of the original sarcode-body, as in the 

 cases already cited, and thus when a piece of this fossil is subjected 

 to the action of dilute acid, by which its calcareous portion is dis- 

 solved away, we obtain an internal cast of its chambers and canal 

 system (fig. 642), which, though altogether dissimilar in arrangement, 

 is essentially analogous in character to the ' internal casts ' repre- 

 sented in figs. 622, 626. This cast presents us, therefore, with a 

 model in hard serpentine of the soft sarcode-body which originally 

 occupied the chambers, and extended itself into the ramifying canals, 

 of the calcareous shell ; and, like that of Polystomella, it affords an 

 even more satisfactory elucidation of the relations of these parts 

 than we could have gained from the study of the living organism. 



FIG. 642. Decalcified portion of Eozoon canadense shell, showing the ser- 

 pentinous internal cast of the chambers, canals, and tubuli of the original, 

 presenting an exact model of the animal substance which originally filled 

 them. 



We see that each of the layers of serpentine, forming the lower part 

 of such a specimen, is made up of a number of coherent segments, 

 which have only undergone a partial separation ; these appear to 

 have extended themselves horizontally without any definite limit, 

 but have here and there developed new segments in a vertical direc- 

 tion, so as to give origin to new layers. In the spaces between these 

 successive layers, which were originally occupied by the calcareous 

 shell, we see the * internal casts ' of the branching canal system, 

 which give us the exact models of the extensions of the sarcode-body 

 that originally passed into them. But this is not all. In specimens 

 in which the Nummuline layer constituting the * proper wall ' of the 

 chambers was originally well preserved, and in which the decalcifying 

 process has been carefully managed (so as not, by too rapid an evolu- 

 tion of carbonic acid gas, to disturb the arrangement of the serpen 

 tinous residuum), that layer is represented by a thin white film 

 covering the exposed surfaces of the segments ; the superficial aspect 



