Sponges (Spongiae). 



For a long time it was doubtful whether these creatures , 

 which, in their fully developed state, are firmly fixed and see- 

 mingly immoveable, were plants or animals. Late research has 

 finally shown that sponges are animals, and that their resem- 

 blance to plants depends upon a few entirely superficial cha- 

 racteristics. 



Sponges consist of an inner fibrous skeleton, surrounded and 

 hidden by a living body of a gelatinous substance. The skele- 

 ton is secreted from the gelatinous mass, and consists either of 

 a web of elastic horny fibres (the Bath-sponge and correlati- 

 ves) , or of innumerable microscopic formations of a determi- 

 ned shape (calcareous sponges) which compose the skeleton ; 

 these shapes vary in a wonderful manner, and are very orna- 

 mental, resembling anchors , stars , balls , hooks , cylinders , 

 and so on. 



Each species of sponge is distinguished by the shapes of the 

 particles that form the skeleton, and therefore the special study 

 of these shapes is of great importance in the systematication 

 of sponges. 



The sponge best known to the unscientific, the common lath- 

 sponge Euspongia, has, when the animal is alive, a very different 

 .appearance to that which it bears when brought to market. The 

 mass of elastic horny fibre with which we wash ourselves is 

 really only the skeleton of the sponge, and is, in the living 

 animal, covered by a soft jelly-like substance. This substance 

 contains a large quantity of pores which can open and shut , 

 allowing the sea-water to flow into the body of the sponge in 

 all directions. The water is kept flowing through these chan- 

 nels by the action of ciliated hairs, and leaves them by fewer 

 but larger openings. The infusoria and other small organisms 

 ^contained in the water are retained and absorbed as nourishment 

 by the inner cells of the animal. 



