118 THE SEA SHOKE 



common cavity, communicate with wider tubes which join together 

 until, finally, they terminate in a large opening seen on the 

 exterior of the sponge. Hence it will be seen that the water 

 entering the minute pores of the surface has to circulate through 

 a complicated system of channels and spaces, some of which are 

 lined with the ciliated cells that urge the current onwards before 

 it is expelled through the large hole. Further, imagine a number 

 of such structures as we have described growing side by side, their 

 masses coalescing into one whole, their inner tubes and spaces 

 united into one complex system by numerous inter-communications, 

 and having several large holes for the exit of the circulating water, 

 and you then have some idea of the general nature of many of 



the more complex sponges to 

 be found on our shores (see 

 fig. 66). 



.. ,. But even this is not all, for 



Ji Ji /^-jl_/y'* as yet we have been regarding 

 n / V^ il\f \r\\ II ^ e sponges as consisting of 



animal matter only, whereas 

 nearly all of them possess some 

 kind of internal skeleton for 

 the support of the soft, gela- 

 tinous animal substance. The 

 skeleton consists of matter 

 secreted by certain cells from 

 material in the water and food. 

 FIG. 67. HORNY NETWORK OF A and is either horny, calcareous, 

 SPONGE, MAGNIFIED or siliceous. The horny skele- 



ton is formed of a network of 



fibres of a somewhat silky character, and often, as in the case 

 of the toilet sponges, highly elastic ; but it is sometimes so brittle 

 that the sponge mass is easily broken when bent. The fibres of 

 this framework support not only the outer wall of the sponge, but 

 also the walls of all the internal tubes and spaces, which are often 

 of so soft a nature that they would collapse without its aid. 



The other forms of skeletons consist of minute bodies of 

 carbonate of lime or of silica, respectively, which assume certain 

 definite shapes, resembling stars, anchors, hooks, pins, spindles, &c., 

 and are known as spicules. Such spicules are usually present in 

 those sponges that have horny skeletons, but in others they form 

 the entire skeleton. 



