GROWTH OF SPONGES. 1 1 5 



sponges whose ground plan and waterways are hard to trace. 

 Yet it is not difficult to gain some idea of their origin. With 

 the simple primitive cups, the complex forms are connected 

 by a gradual series of steps, and simple cups they all are 

 when very young. 



Several facts help us to understand how the complications 

 arise. The older naturalists called sponges plants, and 

 there is certainly much of the character of plants about 

 them. They are sedentary and passive, and seem to feed 

 easily and well by drawing in water and food-particles, as 

 we have described. It is natural, therefore, that they should 

 bud or branch as plants often do. But these buds, like the 

 suckers round a rose-bush, often acquire some apparent 

 independence, and the sponge looks like many cups, not 

 like one. Moreover, as they grow, the buds may join 

 together and fuse like branches of a tree tied closely together. 

 Thus the structure becomes intricate, while it must also be 

 noted that, just as trees may be blown out of shape by the 

 wind, or may grow of themselves into various forms, so the 

 sponges are carved by currents, moulded by the substratum 

 on which they grow, or influenced by peculiarities of their 

 own constitution. Again, in the simple cup the internal 

 cavity is continuously lined by characteristically collared 

 ciliated cells, while in the more complex forms these are 

 restricted to numerous little chambers, communicating on 

 the one hand with incurrent, on the other hand with 

 excurrent canals. How do these arise ? As the cup grows, 

 the inner layer increases more rapidly than the outer, and 

 becomes folded into a series of radial chambers ; these retain 

 the characteristic ciliated cells, while the lining of the central 

 cavity into which they all open loses them. But each of 

 the radial chambers grows in a similar way ; each is folded 

 into a series of side aisles which retain the collared ciliated 

 cells, while the cavity of the radial chamber becomes a mere 

 canal. This process of folding is continued, and the common 

 condition with numerous separate ciliated chambers results. 



Again, in the simple cup the middle stratum is a very 

 simple structure ; always, indeed, it seems to owe its units to 

 contributions from the inner layer. In more complex forms 

 it acquires more prominence, and its cells become more 

 numerous and varied. Some make spicules of lime, or flint. 



