6-1 THE SPONGES. 



branches is tubular with an aperture at each end. The cliaracter of the 

 branches is well shown in Fig. 1, Plate 8, and in Fig. 9, Plate 7, which 

 represents a detail of the opposite side of the sponge shown in Fig. 1, 

 Plate 8. 



A comparison of different branches shows that before the lips of the 

 original cup fuse, they become folded outward so as to present toward 

 each other parallel faces (Fig. 12, Plate 7). Fusion then takes place 

 between these folds along their outer edges, and later, often imperfectly, ' 

 along their inner edges. In this way hollow ridges or tubular structures 

 open at the opposite ends, and at first communicating with the cavity of 

 the branch, are formed. Such structures, once formed, indicate the line 

 along which the cup lips have concresced (Fig. 9, Plate 7). The fusion 

 between the outer lips of the folds may take place at first in spots, and 

 thus for a time the cavity of the ridge or tube communicates with the 

 exterior through slits or a series of rounded pores, as in the ridge extending 

 between the two concrescing cups of Fig. 12, Plate 7. 



The sponge at its upper end terminates in an expanded cup, which in 

 the different specimens shows different stages in division. The division 

 of the terminal cup (Fig. 12, Plate 7) takes place in the same way as 

 that of the lateral cups: the concrescing lips become apposed along a 

 straight line and then bend outward. 



All along the sponge, extending from the lateral branches on to the 

 axis, are tubular structures or ridges similar to those formed in the division 

 of the cups. One such is shown in Fig. 9, Plate 7, extending at about 

 right angles from the tubular ridge which itself extends between the 

 terminal apertures of the branch. On the lower part of the sponge these 

 structures are often insignificant in size, as if in process of disappearance, 

 but in general they are conspicuous. They extend sometimes from the edge 

 of a flaring cup on to the axis, and again as in the figures they lie at a 

 distance from the cup edges. They are arranged spirally along the axis 

 of the sponge, their long axes coinciding with those of the turns of the 

 sponge. Obviously these structures are relicts left at successive stages of 

 growth by the continually dividing terminal cup. The latest-formed such 

 structure in one specimen is shown in Fig. 12, Plate 7. It here has the 

 character of a two-walled ridge which extends between the two cups in 

 process of division, and about at right angles to the lines along which the 

 lips of these cups ate concrescing. The ridge, as I interpret it, marks 



