198 Objects for the Microscope. 



CHAPTER XL 



SPICULES OF SPONGES. 



SPICULES OF SPONGE. 



THESE slides, although useful, and to a certain extent 

 interesting, are very far from what is wanted to illustrate 

 the nature of a sponge. They are isolated siliceous spicula 

 of the horny skeleton of the sponge; very various in form, 

 but all for the same purpose of strengthening the framework 

 of the animal. 



Sponges in their living state are by no means like the 

 dried specimens sold for domestic purposes ; these are but 

 the dead form, the mere skeleton of what was once a living 

 creature. When alive it possesses a firm, fleshy substance, 

 composed of cells about 1 -7000th of an inch in diameter ; 

 the horny skeleton is developed in the inter-cellular sub- 

 stance, and within cells of horny matter these spicula are 

 secreted. 



Sponges present a great variety in their external ap- 

 pearance ; some being soft as jelly, whilst others are as 

 hard as flint; some very large, and others exceedingly 

 minute. The nature of the body closely resembles that of 

 the Foraminifera and Amcebse, having no distinct organs, 

 and capable of assimilating food in all its parts. There is 

 a current flowing in and out through the whole sponge, 

 entering the small apertures or oscuia, and being expelled 

 by the animal through the large apertures or oscuia. The 

 channels through which the currents are drawn and expelled 

 are furnished with ciliated cells, which promote the circula- 

 tion of the water from whence the sponge derives its needful 

 supply of oxygen and food for the maintenance of its life. 



