STKUCTUKE AND ECONOMY OF TETIIEA. 409 



date, or has disappeared, they project more or less from the 

 surface, the individual spicula becoming denuded, and diverg- 

 ing in various or definite directions from one another. 



The peripheral portion of the skeleton is peculiar to the 

 rind, and only exists therefore on the lemon-yellow and light- 

 grey portions of the surface. 



It consists essentially of the tri-radiate and six-radi- 

 ate spicula ; but intermingled with the extremities of the 

 terminal bundles of the central part of the skeleton, the 

 spicida of which pass into the rind towards or through its 

 free surface. 



The shafts of the tri-radiate spicula pass through the rind, 

 deep into the radiating masses of the sponge, packed into the 

 centre or attached to the surface of certain of the terminal 

 bundles of the central portion of the skeleton ; from the 

 spicula of which they may be distinguished by their tapering 

 form, peculiar twist, and greater opacity. 



The radiating branches of the tri-radiate or six-radiate 

 spicula are imbedded in the substance or on the surface of 

 the recently-formed rind. But in rind of older date many 

 of them project, as if their shafts had been pushed outwards, 

 so that their radiations are suspended horizontally at a 

 greater or less distance from the free surface of the sponge. 



AVhen a section of the rind is made towards the centre of 

 the sponge, the radiations of these spicula may be observed 

 in profile, arranged parallel to and at various distances from 

 the free surface. 



When the free surface of a recently-formed portion of the 

 rind is strongly illuminated by a condenser, and examined by 

 a 1-inch or 2-inch objective, the radiating branches of the 

 superimposed spicula may be brought into view in succession, 

 from the free surface inwards, by slowly altering the focus of 

 the instrument. 



I have endeavoured to ascertain whether the six-radiate 



