iqo; 



British Field Zoology 



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the sponge of commerce there arc no spicules, and the skeleton consists 

 solely of the horny cement.) In Spongilla these spicules are simple in 

 form, — like needles slightly curved and pointed at both ends, — but in 

 other genera they assume forms of endless variety, so that naturalists 

 classify sponges in genera and species founded entirely on the form of 

 the spicules found in each. Indeed, it is Qnly in the skeletons (spicules) 

 that sponges differ from one another ; the living portion which clothes 

 and forms the skeleton is practically the same in all genera. This is 

 convenient in one way, for we need only study one type in order to 

 understand the structure and physiology of the entire group. 



Fig. 17. — Spongillafluviatilis. 



To the naked eye the living mass of the sponge — considered apart 

 from its skeleton — appears as a semi-transparent whitish jelly, and 

 even in the microscope under a low power nothing more is to be seen. 

 It is only by the use of reagents and very high-power objectives that 

 all the details can be seen. The mass is then found to consist of three 

 elements — (1) amceba-like cells; (2) collared cells; and (3) connective 

 jelly. This last is merely a formless film, pervading and holding 

 together not only the two kinds of cells, but the spicules and skeleton 

 generally. The collared cells are found lining the cavities in the interior 

 of the sponge. The amoeboid cells arc beneath them and distributed 

 throughout the mass. In other words, the two kinds of cell are arranged 

 in a certain definite order, one kind on the surface, the other kind 

 beneath the surface. Those biologists who regard the sponges as 

 Mctazoa, look upon the arrangement of the cells as two distinct layers, 



