26 NATURAL HISTORY OF OUR SHORES 



were, the curved base going first ; then on the reverse 

 stroke they strike boldly, thus causing the current of 

 water to flow always in the same direction viz. inward. 



Now, the simple sponges, as Sycandra, do not need to 

 extend laterally, but the form is complete, and size limit is 

 reached, when they are of the form figured ; but most 

 other sponges do. A complex sponge, such as Halichondria, 

 is, as it were, a fused- together mass of Sycandra-like 

 arrangements. The canals, chambers, and cavities be- 

 come altered in arrangement to meet the shape, the short 

 canals opening into longer ones, which reach the surface, 

 and appear as pores. The inner part of the canals open 

 into cavities, which terminate in the osculce. A glance at 

 Fig. 11 will explain this. 



If a piece of healthy living sponge is put in a glass of 

 clear sea-water, and a very little powdered carmine is 

 added, then the currents will become apparent, moving 

 from all sides to the pores, and then, sometimes spasmodic- 

 ally, the expulsion of the water which has travelled through 

 will be seen at the osculaB. 



At certain seasons cells which have been modified into 

 eggs and fertilised (both male and female elements are 

 present in each sponge) are ejected with the waste water 

 from the osculce, and develop into little sac-shaped organ- 

 isms, closely similar to the young of many other animals. 

 These swim freely in the sea for a while, and then, settling 

 down, and affixing to a rock or other object, soon develop 

 into a form like a little Sycandra, and then, in the case of 

 more complex sponges, extend, and multiply parts in all 

 directions, until the large mass is built up. 

 --^Now we shall get back to the shore. If the tide has well 

 receded, and there are deep rock crevices or grottos ac- 

 cessible, a search within these will reveal many other kinds 

 of sponges, possibly ridged, snow-white, crisp-feeling lumps 

 of the calcisponge, Leuconia nivea ; large harsh-feeling 



