STRUCTURE AND ECONOMY OF TETUEA. 4:11 



of other silicious sponges, that these structures possess precise 

 geometrical forms. I am not prepared at present to enter into 

 details, for the subject is one beset with many difficulties, 

 but I may state generally — 1st, That all silicious spicula, with 

 continuous margins and pointed, rounded, or apparently trun- 

 cated extremities, are, in fact, oblong ellipsoids or ovoids, 

 more or less compressed or not in the direction of their con- 

 jugate diameter, and more or less spirally twisted or not 

 around their transverse diameters ; 2d, That all the forms of 

 unbranched spicula are reducible to ellipsoids or ovoids, or to 

 linear combinations of these ; and, 3d, That the radiations of the 

 branched spicula are not only portions of ellipsoids or ovoids, 

 but are arranged in reference to one another at definite 

 angles. 



No one, as far as I am aware, has hitherto investigated 

 the mode of development of the silicious spicula. Dr. Grant 

 in Hcdichondria, and Mr. Carter in Spongilla, have observed 

 them forming in the embryo sponge, but the combination of 

 structures by which a spiculum is laid down has not been 

 noted. Dr. Grant, indeed, appears to consider the spicula 

 crystalline productions ; but the general existence of a cavity 

 in their interior, the animal matter which they contain, the 

 peculiar forms which it is generally admitted they present — 

 all tend to prove their organic origin, and clearly to distinguish 

 them from raphides or ordinary crystals. 



As the formation of bone, shell, tooth, hair, and other cal- 

 careous and horny structures, has now been ascertained to 

 depend on the deposit of calcareous salts and horny matter in 

 nucleated particles peculiarly figured and arranged, it becomes 

 important to determine whether the silicious spicula of the 

 sponges are developed in a similar manner. I have not been 

 able hitherto to procure any observations bearing directly on 

 this question, but the general considerations and structural 

 arrangements appear to me to indicate clearly a law of de- 



