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SYSTEMATIC ZOOLOGY FOR TEACHERS. 



its origin in some even more slender forms than it at present shows. From 

 this we may judge that the small types of the single cylinder was the first 

 form. 



Of course both forms may have become evolved at separate times, and 

 both produced their own types of sponges. In this case, I think we have 

 reason for believing that the spiculigenous sponges originated from cup forms, 

 and the horny sponges from tubes. 



Before leaving the subject of encrusting sponges, I will say that a num- 



Fig. 103. 



Fig. 104. 



Fig. 105. 



s 



Giant Cup Sponge. 



Green Cup Sponge. 



Gray Cup Sponge. 



ber of species of these sponges are provided with spicules. Among these 

 is the beautiful green encrusting sponge, ( see plate X) the spicules of which 

 are spherical in form with points projecting out in all directions over the 

 entire surface. Thus the spicules resemble somewhat the pollen grains of 

 some plants, like the asters, for example. At first sight it might appear 

 that these encrusting sponges which have spicules were evolved directly from 

 the fleshy encrusting species. But I do not think that this is the case, 

 for we find that the young of this particular green sponge is partly free 

 growing in sheets before it adheres to rocks, showing that it had its ori- 

 gin in some species which grew upright. 



Now returning once more to the tube sponge, we find that this species 

 is not only inclined to produce filaments, (see fig. 105) but this incli- 

 nation is even more pronounced in the filamentous sponge, this tendency in 

 this last named species is well illustrated in the fine specimen figured on 

 page 175. The filamentous spouge grows in swift tide ways and in shal- 

 low water which is probably the cause of the elongated filaments. In swift 

 tide ways and at greater depths, thus where there is considerably less force 



