WONDERS OF SPONGE STRUCTURE. 7 



Every now and then the current stops and all the 

 tiny orifices are closed, without even a mark to show 

 where they had been. Presently it begins again, and 

 then it will be seen that the former orifices are not 

 reopened, but that fresh apertures are developed as they 

 are wanted. 



Now we may ask ourselves how these larger apertures 

 are kept open, and to answer the question we must call 

 chemistry to our aid. In some sponges we can use the 

 blow-pipe, but as a general rule some strong acid or 

 caustic alkali will destroy the whole of the animal mat- 

 ter. If the residuum be examined with the microscope, 

 a vast number of glassy spicules will be seen, varying in 

 shape, size, and colour with the kind of sponge. 



Some of them look exactly as if they were made of 

 pink and white sugar-candy, and all children to whom I 

 have shown them have expressed regret at their inability 

 to eat such tempting objects. Those of the ordinary 

 sponge have the most striking resemblance to the 

 " crow's-feet," or " caltrops," which were once used to 

 impede the progress of cavalry. It is said that these 

 spiculse vary according to the substance on which the 

 sponge is fixed, as well as in the species ; but I have had 

 no opportunity of testing this theory, and content myself 

 with mentioning it. 



The structure of the ordinary marine and fresh-water 

 sponges having been carefully studied, zoologists set 

 about the very necessary task of classifying them. Con- 

 sidering the nature of the subject, this was no easy task 



