408 STRUCTURE AND ECONOMY OF TETIIEA. 



the group to which it belongs, consists of silicious spicula 

 bound together by horny matter. 



The spicula of which the skeleton is essentially composed 

 present the following forms : — 



1. Tubular, elongated, somewhat flattened, pointed, or 

 truncated spicula, slightly bent or twisted, with an average 

 length of one line and a half, but of various smaller sizes. 



2. A similar form, but terminated at one end in a spheri- 

 cal head, and very rare. 



3. Tubular, elongated, cylindrical, straight spicula, one line 

 and a half in average length. 



4. Very slender spicula, of uniform thickness, short, with 

 both ends bent towards one another. 



5. Similar spicula, with their ends bent in opposite direc- 

 tions ; sigmoidal. 



6. Tri-radiate spicula in two forms. 



7. Six-radiate spicula. 



8. Stellate spicula. 



Viewed as a whole, the skeleton may be divided into a 

 central and a peripheral portion. 



The central portion may be considered as consisting of two 

 parts, as it is arranged in the nucleus, and among the radiat- 

 ing masses. 



The nuclear portion consists of single spicula, and bundles 

 of spicula of the 1st, 2d, 3d, 4th, and 5th orders, bound together 

 and arranged so as to form a mesh-work to support the walls 

 of the areolar water-passages of the nucleus. 



The radiating part consists of thick trunk-like bundles of 

 the same kinds of spicula, springing from, and tied to, the 

 nuclear part. These trunks ramify as they radiate, so as to 

 be resolved at or near the surface into numerous nearly uni- 

 form bundles, which in the recently-formed rind pass through 

 between the six-radiate spicula, and elevate, to a greater or 

 less extent, the free surface ; but when the rind is of older 



