22 



SPONGES 



the persons are indistinguishable except by the exhalant vents of 

 the canal system. 



A compact sponge of this kind, if it grows more or less equally 

 in all directions, becomes simply massive (Fig. 39). It may, how- 

 ever, grow very greatly in a horizontal direction, and increase very 

 little, or not at all in height ; this gives a flat incrusting form, in 

 which the oscula may be prominent as elevated cones or tubes, or 

 may be quite inconspicuous (Fig. 32). On the other hand, the 



FIG. 35. 



rhakellia, ventilabrnm, Johnst. A, flabellate 

 specimen. />', cup-shaped specimen. 



Fio. 30. 

 Phakellin temtx (after Agassiz). jj. 



young sponge may grow very rapidly in height, and in this way a 

 large series of forms arises. In the first place, a sponge which 

 grows vertically may become greatly branched and assume a 

 dendritic form (Fig. 34). The numerous oscula are found scattered 

 along the branches, which in their turn may be more or less circular 

 in transverse section, or very flattened. In the second place, rapid 

 growth of the sponge in a vertical direction in height may be com- 

 bined with a horizontal growth which preponderates in, or is 

 restricted to, a particular vertical plane ; the result is a fan-shaped 

 or flabellale form (Fig. 35, A\ a type which may undergo subsequent 

 modifications of great importance. 



