XATVVlE STUDY IN SCHOOLS. 



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differentiated, into cells of other forms from those of the primitive gastrula. 

 They form muscle cells, nerve cells, and reproductive cells, all of which taken 

 together, constitute what is known as sponge flesh. 



Skeletons of Sponges. 



Early in the history of young sponges, we find that they develop skele- 

 tons. That is, after the sponge flesh has beccme foamed to some extent, 

 and the growth of the sponges has piocecded, upwaid to such an extent, 

 as to require the support of a skeleton, we find one forming. Then certain 

 rells are set apart, and in place of protoplasmic material, produce, in the horny 



Fig. 107. 



Diagram of young sponge. 



Nponges, a horn-like material, which increases in thickness with the age of 

 the sponge; hence older sponges are of a firmer structure than when young. 

 In the tube sponge, in which the fiber of the skeleton is quite laige, we 

 find one of the best species for study. 



The general color of the skeleton is amber, light when young, but dark- 

 ing with age. The fibers are cylindrical, well rounded and seldom flattened, 

 even at the points of jointure. They are hollow, and at the extreme tips of 

 the new growths are composed of a single, soft membrane which is yellow in 

 color and opaque. This soon becomes covered with a layer of pale, amber- 

 colored, horn-like matter, which is nearly transparent, and successive layers of 

 this horny matter are deposited : thus the older growths not only beccme 

 larger in diameter, but darker in color on account of the continuous thicken- 

 ing of the eleposits. See figs. 108 and 109 where I have given an enlarged 

 cut of the fiber, a, being the new in A, C and B in fig. ION. and v t]w 



