484 CHARACTERS OF INVERTEBRATE ANIMALS 



SPONGES (Porifera) 



Sponges were long considered to be of vegetable nature, an 

 idea that was only finally upset by the study of their minute 

 structure. Except to a naturalist the word " sponge " merely 

 suggests a bath-sponge, which is in reality the horny skeleton 

 of a colonial species. To gain a clear idea of the structure of 

 the group it is necessary to consider the simpler cases presented 

 by some of the solitary forms. In the simplest of these the 

 body is shaped like a cup or vase, fixed at one end and open 

 at the other. It is tempting to draw a comparison with Hydra> 

 or better with Protohydra (p. 480), which is simply a tube open 

 at one end. Such a comparison, however, is beset with diffi- 

 culties, for while in Protohydra the aperture is clearly a mouth 

 through which food is introduced, it will be found by watching 

 the living sponge that currents of water are constantly flowing 

 out of the corresponding opening, here technically known as 

 the osculum (L. for little mouth). And, further, the body- wall of 

 the sponge is perforated by numerous small holes through which 

 water-currents set into the central digestive cavity. These cur- 

 rents are the result of ciliary action, and by their means the 

 animal is provided with the food and oxygen it requires, while 

 the various products of waste are swept out to the exterior 

 through the osculum. This mode of life is associated with the 

 sedentary or fixed habit of sponges, which is the chief reason 

 for the old mistake of considering these creatures of vegetable 

 nature. 



Microscopic examination shows that the thin body- wall of 

 the simple sponge consists of an external ectoderm made up 

 of a layer of flattened cells, a middle supporting layer, and an 

 internal layer of entoderm composed of collar-cells. Each of 

 these cells is provided with a whip-like projection of protoplasm 

 {flagellum) (see p. 471), at the base of which is a collar-like 

 projection. By the lashing movements which these threads 

 execute the water-currents upon which the life of the animal 

 depends are produced. The middle supporting layer is com- 

 parable to the similarly -placed gelatinous layer of a jelly-fish, 

 and it contains numerous cells of various kinds which have 

 been derived from the ectoderm and entoderm. Some of these 



